Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
Thanks,
Simon.

On 14 Dec, 12:39, sm_jamieson wrote:
Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
Thanks,
Simon.

How do you know you need a ridge beam? Rafters are self-supporting and
there may be little or no load at the ridge

dg

On 14 Dec, 13:12, dg wrote:
On 14 Dec, 12:39, sm_jamieson wrote:

Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
Thanks,
Simon.

How do you know you need a ridge beam? Rafters are self-supporting and
there may be little or no load at the ridge

dg

Sorry, I didn't say - there will be a dormer on one side.
I do wonder about lateral forces in this situation - maybe a thin
timber I beam would not be suitable.
Simon.

In article
,
sm_jamieson wrote:
Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.

It's more usual to install steel purlins - that's where the load is.

--
*Toilet stolen from police station. Cops have nothing to go on.

Dave Plowman London SW
To e-mail, change noise into sound.

On 14 Dec, 13:42, "Dave Plowman (News)" wrote:
In article
,
sm_jamieson wrote:

Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.

It's more usual to install steel purlins - that's where the load is.

--
*Toilet stolen from police station. Cops have nothing to go on.

Dave Plowman London SW
To e-mail, change noise into sound.

Purlins need to be minimized (stairs running up parallel to the roof
slope), thus a ridge beam would be required to take some of the load
(why not, it's in dead space !), and the purlins only need to stop the
rafter sag. Roof pitch is quite steep at 40 degrees.
In the main section hopefully dwarf wall at 1/3 height and a ridge
beam will be enough. Purlins at 2/3 height are the awkward ones.
Simon.

sm_jamieson wrote:
On 14 Dec, 13:42, "Dave Plowman (News)" wrote:
In article
,
sm_jamieson wrote:

Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
It's more usual to install steel purlins - that's where the load is.

--
*Toilet stolen from police station. Cops have nothing to go on.

Dave Plowman London SW
To e-mail, change noise into sound.

Purlins need to be minimized (stairs running up parallel to the roof
slope), thus a ridge beam would be required to take some of the load
(why not, it's in dead space !), and the purlins only need to stop the
rafter sag. Roof pitch is quite steep at 40 degrees.
In the main section hopefully dwarf wall at 1/3 height and a ridge
beam will be enough. Purlins at 2/3 height are the awkward ones.
Simon.

Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Unless you go for warren braced (W braced) trusses, or some other method
of connecting the joist to the rafter in the roof truss, all the
strength is actually IN the rafter and the joist itself. Any other
bracing is for stability only and carries very little load, including
the ridge.

Howerver if the rafter to joist integrity is poor,so the rafters can
slide outwards at the base - a fairly common thing where upstairs
conversions under the eaves have been done and tie beams cut
through,leading to wall spreading - then a steel ridge running
gable-to-gable can act as a primary support. At this point teh roof
ceases to be a triangulated truss structure ands simply becomes a bunch
of poles leaned against a structural ride pole.The wight of the roof
instead of being carried as a downward an outward force on the rafter
bases, is transferred to a downward force on the ridge and a downward
and INWARD force on the rafter ends.

If you also add in *structural* steel purlins and carry those downwards
via posts to the joists, you can end up with a completely different
loading again.

That adds load to the joists, which are no longer in pure tension, but
also being subjected to a bending force. You might use such vertical
ties if the original ceiling was not strong enough to support rooms
above - you trasnfer loads to the roof timbers that way, which are
generally massive enough to take tiles or slates..they can cope.

In all cases, however, *unless* the purlins or the ridge is/are *very*
strong, *and* supported at the gable ends or by a central chimney etc,
they do not carry any major loads at all. They are there for stability.
Increasing their size is a waste of time.

On 14 Dec, 14:54, The Natural Philosopher wrote:
sm_jamieson wrote:
On 14 Dec, 13:42, "Dave Plowman (News)" wrote:
In article
,
sm_jamieson wrote:

Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
It's more usual to install steel purlins - that's where the load is.

--
*Toilet stolen from police station. Cops have nothing to go on.

Dave Plowman London SW
To e-mail, change noise into sound.

Purlins need to be minimized (stairs running up parallel to the roof
slope), thus a ridge beam would be required to take some of the load
(why not, it's in dead space !), and the purlins only need to stop the
rafter sag. Roof pitch is quite steep at 40 degrees.
In the main section hopefully dwarf wall at 1/3 height and a ridge
beam will be enough. Purlins at 2/3 height are the awkward ones.
Simon.

Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Unless you go for warren braced (W braced) trusses, or some other method
of connecting the joist to the rafter in the roof truss, all the
strength is actually IN the rafter and the joist itself. Any other
bracing is for stability only and carries very little load, including
the ridge.

Howerver if the rafter to joist integrity is poor,so the rafters can
slide outwards at the base - a fairly common thing where upstairs
conversions under the eaves have been done and tie beams cut
through,leading to wall spreading - then a steel ridge running
gable-to-gable can act as a primary support. At this point teh roof
ceases to be a triangulated truss structure ands simply becomes a bunch
of poles leaned against a structural ride pole.The wight of the roof
instead of being carried as a downward an outward force on the rafter
bases, is transferred to a downward force on the ridge and a downward
and INWARD force on the rafter ends.

If you also add in *structural* steel purlins and carry those downwards
via posts to the joists, you can end up with a completely different
loading again.

That adds load to the joists, which are no longer in pure tension, but
also being subjected to a bending force. You might use such vertical
ties if the original ceiling was not strong enough to support rooms
above - you trasnfer loads to the roof timbers that way, which are
generally massive enough to take tiles or slates..they can cope.

In all cases, however, *unless* the purlins or the ridge is/are *very*
strong, *and* supported at the gable ends or by a central chimney etc,
they do not carry any major loads at all. They are there for stability.
Increasing their size is a waste of time.

Indeed.
So ...
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ?
Cheers,
Simon.

sm_jamieson wrote:
On 14 Dec, 14:54, The Natural Philosopher wrote:
sm_jamieson wrote:
On 14 Dec, 13:42, "Dave Plowman (News)" wrote:
In article
,
sm_jamieson wrote:
Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
It's more usual to install steel purlins - that's where the load is.
--
*Toilet stolen from police station. Cops have nothing to go on.
Dave Plowman London SW
To e-mail, change noise into sound.
Purlins need to be minimized (stairs running up parallel to the roof
slope), thus a ridge beam would be required to take some of the load
(why not, it's in dead space !), and the purlins only need to stop the
rafter sag. Roof pitch is quite steep at 40 degrees.
In the main section hopefully dwarf wall at 1/3 height and a ridge
beam will be enough. Purlins at 2/3 height are the awkward ones.
Simon.
Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Unless you go for warren braced (W braced) trusses, or some other method
of connecting the joist to the rafter in the roof truss, all the
strength is actually IN the rafter and the joist itself. Any other
bracing is for stability only and carries very little load, including
the ridge.

Howerver if the rafter to joist integrity is poor,so the rafters can
slide outwards at the base - a fairly common thing where upstairs
conversions under the eaves have been done and tie beams cut
through,leading to wall spreading - then a steel ridge running
gable-to-gable can act as a primary support. At this point teh roof
ceases to be a triangulated truss structure ands simply becomes a bunch
of poles leaned against a structural ride pole.The wight of the roof
instead of being carried as a downward an outward force on the rafter
bases, is transferred to a downward force on the ridge and a downward
and INWARD force on the rafter ends.

If you also add in *structural* steel purlins and carry those downwards
via posts to the joists, you can end up with a completely different
loading again.

That adds load to the joists, which are no longer in pure tension, but
also being subjected to a bending force. You might use such vertical
ties if the original ceiling was not strong enough to support rooms
above - you trasnfer loads to the roof timbers that way, which are
generally massive enough to take tiles or slates..they can cope.

In all cases, however, *unless* the purlins or the ridge is/are *very*
strong, *and* supported at the gable ends or by a central chimney etc,
they do not carry any major loads at all. They are there for stability.
Increasing their size is a waste of time.

Indeed.
So ...
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ?

What makes you think you need to?

NOT a rhetorical question, but one that goes right to the heart of the
matter.

I haven't got it straight as to what is going on.

If the structure has been messed around a lot, get in some structural
engineers, who have liability insurance and who do these calcs every day.

If it hasn't, why are you doing it at all?

Faced with a sound standard loft with a steepish pitched roof, all I
would do would be to bolt cross pieces at head height plus to the
rafters to make a ceiling, and take some of the lateral stress out of
the eaves. Whilst not ideal you could nail strips on top of them at the
edges to stabilize the thing, purlin wise.

Or better still, put insulation between rafters and then line with
structural ply, before boarding over. That does all the job that purlins
do and its only 19mm thick, plus 12mm plasterboard etc.

If the floor is weak, add bigger joists alongside or on top of existing,
and plate out with chip/ply or real boards..and reduce the need for
ridge support by BOLTING them HARD to the rafters at the ends.











Cheers,
Simon.

On 14 Dec, 15:57, The Natural Philosopher wrote:
sm_jamieson wrote:
On 14 Dec, 14:54, The Natural Philosopher wrote:
sm_jamieson wrote:
On 14 Dec, 13:42, "Dave Plowman (News)" wrote:
In article
,
sm_jamieson wrote:
Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
It's more usual to install steel purlins - that's where the load is.
--
*Toilet stolen from police station. Cops have nothing to go on.
Dave Plowman London SW
To e-mail, change noise into sound.
Purlins need to be minimized (stairs running up parallel to the roof
slope), thus a ridge beam would be required to take some of the load
(why not, it's in dead space !), and the purlins only need to stop the
rafter sag. Roof pitch is quite steep at 40 degrees.
In the main section hopefully dwarf wall at 1/3 height and a ridge
beam will be enough. Purlins at 2/3 height are the awkward ones.
Simon.
Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Unless you go for warren braced (W braced) trusses, or some other method
of connecting the joist to the rafter in the roof truss, all the
strength is actually IN the rafter and the joist itself. Any other
bracing is for stability only and carries very little load, including
the ridge.

Howerver if the rafter to joist integrity is poor,so the rafters can
slide outwards at the base - a fairly common thing where upstairs
conversions under the eaves have been done and tie beams cut
through,leading to wall spreading - then a steel ridge running
gable-to-gable can act as a primary support. At this point teh roof
ceases to be a triangulated truss structure ands simply becomes a bunch
of poles leaned against a structural ride pole.The wight of the roof
instead of being carried as a downward an outward force on the rafter
bases, is transferred to a downward force on the ridge and a downward
and INWARD force on the rafter ends.

If you also add in *structural* steel purlins and carry those downwards
via posts to the joists, you can end up with a completely different
loading again.

That adds load to the joists, which are no longer in pure tension, but
also being subjected to a bending force. You might use such vertical
ties if the original ceiling was not strong enough to support rooms
above - you trasnfer loads to the roof timbers that way, which are
generally massive enough to take tiles or slates..they can cope.

In all cases, however, *unless* the purlins or the ridge is/are *very*
strong, *and* supported at the gable ends or by a central chimney etc,
they do not carry any major loads at all. They are there for stability.
Increasing their size is a waste of time.

Indeed.
So ...
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ?

What makes you think you need to?

NOT a rhetorical question, but one that goes right to the heart of the
matter.

I haven't got it straight as to what is going on.

If the structure has been messed around a lot, get in some structural
engineers, who have liability insurance and who do these calcs every day.

If it hasn't, why are you doing it at all?

Faced with a sound standard loft with a steepish pitched roof, all I
would do would be to bolt cross pieces at head height plus to the
rafters to make a ceiling, and take some of the lateral stress out of
the eaves. Whilst not ideal you could nail strips on top of them at the
edges to stabilize the thing, purlin wise.

Or better still, put insulation between rafters and then line with
structural ply, before boarding over. That does all the job that purlins
do and its only 19mm thick, plus 12mm plasterboard etc.

If the floor is weak, add bigger joists alongside or on top of existing,
and plate out with chip/ply or real boards..and reduce the need for
ridge support by BOLTING them HARD to the rafters at the ends.

Cheers,
Simon.

I'm designing a dormer loft conversion. I omitted that off my original
post I think.
The room is to be open plan from front to back, i.e. to cantilever
support as in John Rumms conversion.
Simon.

sm_jamieson wrote:
On 14 Dec, 15:57, The Natural Philosopher wrote:
sm_jamieson wrote:
On 14 Dec, 14:54, The Natural Philosopher wrote:
sm_jamieson wrote:
On 14 Dec, 13:42, "Dave Plowman (News)" wrote:
In article
,
sm_jamieson wrote:
Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
It's more usual to install steel purlins - that's where the load is.
--
*Toilet stolen from police station. Cops have nothing to go on.
Dave Plowman London SW
To e-mail, change noise into sound.
Purlins need to be minimized (stairs running up parallel to the roof
slope), thus a ridge beam would be required to take some of the load
(why not, it's in dead space !), and the purlins only need to stop the
rafter sag. Roof pitch is quite steep at 40 degrees.
In the main section hopefully dwarf wall at 1/3 height and a ridge
beam will be enough. Purlins at 2/3 height are the awkward ones.
Simon.
Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.
Unless you go for warren braced (W braced) trusses, or some other method
of connecting the joist to the rafter in the roof truss, all the
strength is actually IN the rafter and the joist itself. Any other
bracing is for stability only and carries very little load, including
the ridge.
Howerver if the rafter to joist integrity is poor,so the rafters can
slide outwards at the base - a fairly common thing where upstairs
conversions under the eaves have been done and tie beams cut
through,leading to wall spreading - then a steel ridge running
gable-to-gable can act as a primary support. At this point teh roof
ceases to be a triangulated truss structure ands simply becomes a bunch
of poles leaned against a structural ride pole.The wight of the roof
instead of being carried as a downward an outward force on the rafter
bases, is transferred to a downward force on the ridge and a downward
and INWARD force on the rafter ends.
If you also add in *structural* steel purlins and carry those downwards
via posts to the joists, you can end up with a completely different
loading again.
That adds load to the joists, which are no longer in pure tension, but
also being subjected to a bending force. You might use such vertical
ties if the original ceiling was not strong enough to support rooms
above - you trasnfer loads to the roof timbers that way, which are
generally massive enough to take tiles or slates..they can cope.
In all cases, however, *unless* the purlins or the ridge is/are *very*
strong, *and* supported at the gable ends or by a central chimney etc,
they do not carry any major loads at all. They are there for stability.
Increasing their size is a waste of time.
Indeed.
So ...
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ?
What makes you think you need to?

NOT a rhetorical question, but one that goes right to the heart of the
matter.

I haven't got it straight as to what is going on.

If the structure has been messed around a lot, get in some structural
engineers, who have liability insurance and who do these calcs every day.

If it hasn't, why are you doing it at all?

Faced with a sound standard loft with a steepish pitched roof, all I
would do would be to bolt cross pieces at head height plus to the
rafters to make a ceiling, and take some of the lateral stress out of
the eaves. Whilst not ideal you could nail strips on top of them at the
edges to stabilize the thing, purlin wise.

Or better still, put insulation between rafters and then line with
structural ply, before boarding over. That does all the job that purlins
do and its only 19mm thick, plus 12mm plasterboard etc.

If the floor is weak, add bigger joists alongside or on top of existing,
and plate out with chip/ply or real boards..and reduce the need for
ridge support by BOLTING them HARD to the rafters at the ends.

Cheers,
Simon.

I'm designing a dormer loft conversion. I omitted that off my original
post I think.
The room is to be open plan from front to back, i.e. to cantilever
support as in John Rumms conversion.
Simon.


So what have you removed to necessitate beefing up the structure?

Even punching a dormer out hardly weakens anything.

and surely you don't mean a cantilever..I have yet to see a cantilever
structure in any house..well a few Elizabethan ones where the top storey
overhangs the lower maybe.

The message
from The Natural Philosopher contains these words:

Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.

If all that was needed to be done was to stabilise the rafters a dinky
bit of 4" x 2" (likely the same section as the rafters) nailed flat
against the rafters would do.

I am quite sure that if I nailed a few bits of 4 x 2 across my rafters
and then cut out the purlins most of the 20 or 30 tons of stone on my 30
degree roof would promptly join me inside the house. Rafters are only 4"
x 3" (or 4 x 2 where they have been replaced) but the clear span is
never more than about 4 feet (which is perhaps half what you might
expect for a lightweight slate roof).

--
Roger Chapman

In article ,
The Natural Philosopher wrote:
Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that.

Perhaps you'd explain why my loft conversion involved the fitting of steel
purlins if they have so little work to do?

--
*When everything's coming your way, you're in the wrong lane *

Dave Plowman London SW
To e-mail, change noise into sound.

On 14 Dec, 17:44, "Dave Plowman (News)" wrote:
In article ,
The Natural Philosopher wrote:

Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that.

Perhaps you'd explain why my loft conversion involved the fitting of steel
purlins if they have so little work to do?

--
*When everything's coming your way, you're in the wrong lane *

Dave Plowman London SW
To e-mail, change noise into sound.

I would tend to have to agree with the dissenters - a common 1950's
terraced council house roof structure involved heavyweight purlins
running from gable wall to gable wall supporting lightweight rafters.
This is equivalent to a long span roof with a number of trusses.

The essence is that the roof weight is transferred to the trusses/
gable walls by the purlins. We only have to look at the roof
structure of old buildings to see that the purlins were of significant
size, the actual roof timbers light in comparison and the structure
supporting the purlins massive too.

Rob

Roger wrote:
The message
from The Natural Philosopher contains these words:

Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.

So where else does it go?

Unless your purlins are actually PROPPED all the weight of the roof is
transmitted via the rafter ends. There *is* no other point of support.

Purlins are simply there to either stabilise the rafters, or if above
them, to transmit te roof weight TO the rafters.




If all that was needed to be done was to stabilise the rafters a dinky
bit of 4" x 2" (likely the same section as the rafters) nailed flat
against the rafters would do.


Thats geberally waht is used IME.


I am quite sure that if I nailed a few bits of 4 x 2 across my rafters
and then cut out the purlins most of the 20 or 30 tons of stone on my 30
degree roof would promptly join me inside the house. Rafters are only 4"
x 3" (or 4 x 2 where they have been replaced) but the clear span is
never more than about 4 feet (which is perhaps half what you might
expect for a lightweight slate roof).


Well I guess wasted a lot of time doing an engineering degree then.

Look. purlins are only there in mast cases to spread the load from the
roof weight TO the rafters

http://en.wikipedia.org/wiki/Purlins

Definitions of Purlins on the Web:

* A horizontal structural member spanning between beams or trusses
to *support a roof deck*. In slope glazing, purlins are the horizontal
framing members.
http://www.inspectorwebhosting.com/c...erms-m-q-2.htm

* The horizontal members of the roof structure used to *support and
attach the roof panels.*
www.ameren.com/Environment/adc_ev_glossary.asp

* Timber members spanning over trusses to *support cladding or
between trusses to support loose timbers.*
www.wolf-system.co.uk/glossary.asp

* are the horizontal roofing members, which connect bents.
www.trilliumdell.com/vocabulary/

* Horizontal beam along the length of a roof, *resting on
principals and supporting the common rafters or boards.(
http://www.bbc.co.uk/homes/property/...glossary.shtml

* In architecture or structural engineering, a purlin (or purline)
is a longitudinal structural member in a roof. Purlins support the loads
from the roof deck or sheathing and are supported by the principal
rafters and/or the building walls. ...
en.wikipedia.org/wiki/Purlins



In every case it is clearly specified that these rely ON the principal
rafters and the joists (binders) to support the weight. They act merely
to transmit the weight *to* them.

If your rafters are VERY widely spaced, they need to be substantial.

But apart from a 'starved horse' looking roof,removing them wont cause
the roof to fall down generally.

I repeat, in a conventional trussed roof, any longitudinal members are
there to stabilise the rafters in the correct (laterla) position, and
act as local beams to transmit localised stresses to them, in the case
of the ridge and exterior elements - like the tile battens.

All the weight of the roof transfers to compression in the rafters and
tension ion the binders, the triangulation so defined forming a net
downward load on the binder ends at the eaves.

There is some net bending force on the rafters as well, but a long
purlin does very little to affect this unless its supported at the
ends,. If less than substantial rafters are used, or widely spaced one,
the better approach is W braves from the off binder centers to midway up
the rafters, or vertical posts between the rafter centers and the binders.

Or more relevantly when using the roof space as a room, you can add
another set of binders at 'loft ceiling' level bolted to the rafters.

Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that.

Perhaps you'd explain why my loft conversion involved the fitting of steel
purlins if they have so little work to do?


No idea. Possibly because whoever did it didn't do the sums, and just
decided to play safe in an ignorant sort of way. Its pretty common. I
have yet to meet a builder that really understood engineering
mathematics. The guys that did my house didn;t even understand triogonmetry.

They had to lay out the rafters at the right angle (52 degrees for
thatch) on the ground and measure across the span to get them cut to the
right length. Using a large protractor,...

robgraham wrote:
On 14 Dec, 17:44, "Dave Plowman (News)" wrote:
In article ,
The Natural Philosopher wrote:

Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that.
Perhaps you'd explain why my loft conversion involved the fitting of steel
purlins if they have so little work to do?

--
*When everything's coming your way, you're in the wrong lane *

Dave Plowman London SW
To e-mail, change noise into sound.

I would tend to have to agree with the dissenters - a common 1950's
terraced council house roof structure involved heavyweight purlins
running from gable wall to gable wall supporting lightweight rafters.
This is equivalent to a long span roof with a number of trusses.

The essence is that the roof weight is transferred to the trusses/
gable walls by the purlins. We only have to look at the roof
structure of old buildings to see that the purlins were of significant
size, the actual roof timbers light in comparison and the structure
supporting the purlins massive too.


Mmm.

That is not strictly a trussed roof at all, then.

And shockingly bad design.

I guess you could get away with it over short roof spans. Spans that are
e.g. less than the cross span from front to back.

But you still need binders to hold the rafter ends in, unless you have a
massive ridge also, and then there is a distinct possibility of the
rafters sliding down the ridge board..you are relying on nails alone to
hold them up.

At this point it really isn't a truss roof at all. you are in fact using
the purlins and ridge as the main structural elements as simple beams.
and nailing rafters over the top simply to transfer load from the tiles
to the purlins and ridge.

It seems to me to be a very wasteful way to do it.




Rob

In article ,
The Natural Philosopher wrote:
Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.

So where else does it go?

Unless your purlins are actually PROPPED all the weight of the roof is
transmitted via the rafter ends. There *is* no other point of support.

Think you need to look at applied mechanics...

--
*The older you get, the better you realize you were.

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
The Natural Philosopher wrote:
Perhaps you'd explain why my loft conversion involved the fitting of
steel purlins if they have so little work to do?


No idea. Possibly because whoever did it didn't do the sums, and just
decided to play safe in an ignorant sort of way. Its pretty common. I
have yet to meet a builder that really understood engineering
mathematics.

I'd love you to say that face to face with the architect and structural
engineer involved. ;-) I've got reams of calculations proving the stresses
involved.

You're quite simply wrong.

--
*I wish the buck stopped here. I could use a few.

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.

So where else does it go?

Unless your purlins are actually PROPPED all the weight of the roof is
transmitted via the rafter ends. There *is* no other point of support.

Think you need to look at applied mechanics...

Er. no. Thats what my masters is in.

It seems that the 'purlins' in this case ARE propped - at the gable ends.

And the roof is not a trussed roof at all.

On 14 Dec, 16:31, sm_jamieson wrote:
On 14 Dec, 15:57, The Natural Philosopher wrote:



sm_jamieson wrote:
On 14 Dec, 14:54, The Natural Philosopher wrote:
sm_jamieson wrote:
On 14 Dec, 13:42, "Dave Plowman (News)" wrote:
In article
,
sm_jamieson wrote:
Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
It's more usual to install steel purlins - that's where the load is.
--
*Toilet stolen from police station. Cops have nothing to go on.
Dave Plowman London SW
To e-mail, change noise into sound.
Purlins need to be minimized (stairs running up parallel to the roof
slope), thus a ridge beam would be required to take some of the load
(why not, it's in dead space !), and the purlins only need to stop the
rafter sag. Roof pitch is quite steep at 40 degrees.
In the main section hopefully dwarf wall at 1/3 height and a ridge
beam will be enough. Purlins at 2/3 height are the awkward ones.
Simon.
Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Unless you go for warren braced (W braced) trusses, or some other method
of connecting the joist to the rafter in the roof truss, all the
strength is actually IN the rafter and the joist itself. Any other
bracing is for stability only and carries very little load, including
the ridge.

Howerver if the rafter to joist integrity is poor,so the rafters can
slide outwards at the base - a fairly common thing where upstairs
conversions under the eaves have been done and tie beams cut
through,leading to wall spreading - then a steel ridge running
gable-to-gable can act as a primary support. At this point teh roof
ceases to be a triangulated truss structure ands simply becomes a bunch
of poles leaned against a structural ride pole.The wight of the roof
instead of being carried as a downward an outward force on the rafter
bases, is transferred to a downward force on the ridge and a downward
and INWARD force on the rafter ends.

If you also add in *structural* steel purlins and carry those downwards
via posts to the joists, you can end up with a completely different
loading again.

That adds load to the joists, which are no longer in pure tension, but
also being subjected to a bending force. You might use such vertical
ties if the original ceiling was not strong enough to support rooms
above - you trasnfer loads to the roof timbers that way, which are
generally massive enough to take tiles or slates..they can cope.

In all cases, however, *unless* the purlins or the ridge is/are *very*
strong, *and* supported at the gable ends or by a central chimney etc,
they do not carry any major loads at all. They are there for stability.
Increasing their size is a waste of time.

Indeed.
So ...
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ?

What makes you think you need to?

NOT a rhetorical question, but one that goes right to the heart of the
matter.

I haven't got it straight as to what is going on.

If the structure has been messed around a lot, get in some structural
engineers, who have liability insurance and who do these calcs every day.

If it hasn't, why are you doing it at all?

Faced with a sound standard loft with a steepish pitched roof, all I
would do would be to bolt cross pieces at head height plus to the
rafters to make a ceiling, and take some of the lateral stress out of
the eaves. Whilst not ideal you could nail strips on top of them at the
edges to stabilize the thing, purlin wise.

Or better still, put insulation between rafters and then line with
structural ply, before boarding over. That does all the job that purlins
do and its only 19mm thick, plus 12mm plasterboard etc.

If the floor is weak, add bigger joists alongside or on top of existing,
and plate out with chip/ply or real boards..and reduce the need for
ridge support by BOLTING them HARD to the rafters at the ends.

Cheers,
Simon.

I'm designing a dormer loft conversion. I omitted that off my original
post I think.
The room is to be open plan from front to back, i.e. to cantilever
support as in John Rumms conversion.
Simon.

Flippin' heck.
I thought the standard 1930s terraced/semi cut roof with load bearing
purlins supported at either end
was a very well known quantity. These are usually 3x2 rafters and, say
9" purlins. Often propped in
the middle to a central spine wall.
I am surpised it has caused so much debate !
I will ask my questions about timber I-beams in a different way !
Thanks folks,
Simon.

The message
from The Natural Philosopher contains these words:

Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.

So where else does it go?

Onto the purlin. The rafter still has to function as a beam even though
it is by no means horizontal. Even if it wasn't nailed to the purlin it
would still load it and for the load to be in equilibrium at that point
the forces must balance.

Unless your purlins are actually PROPPED all the weight of the roof is
transmitted via the rafter ends. There *is* no other point of support.

No clear what you mean by propped in this context. Purlins are generally
simply supported at both ends and rely on their size to carry the load
transferred to them.

Purlins are simply there to either stabilise the rafters, or if above
them, to transmit te roof weight TO the rafters.

You might think so but that doesn't make it a fact.

If all that was needed to be done was to stabilise the rafters a dinky
bit of 4" x 2" (likely the same section as the rafters) nailed flat
against the rafters would do.

Thats geberally waht is used IME.

YMMV but I have never yet seen a proper roof with the horizontal members
fixed flat rather than upright (roofing laths excepted). To do so would
waste some of the strength of the beam. If the only purpose was to
stabilise the rafters using off-cuts for noggins would be more than
adequate.

I am quite sure that if I nailed a few bits of 4 x 2 across my rafters
and then cut out the purlins most of the 20 or 30 tons of stone on my 30
degree roof would promptly join me inside the house. Rafters are only 4"
x 3" (or 4 x 2 where they have been replaced) but the clear span is
never more than about 4 feet (which is perhaps half what you might
expect for a lightweight slate roof).

Well I guess wasted a lot of time doing an engineering degree then.

Beginning to look that way. :-)

Look. purlins are only there in mast cases to spread the load from the
roof weight TO the rafters

No, that is what the roofing laths do. On traditional roofs there is
little if any load to transfer from one rafter to the next except where
a rafter has been cut and the gap bridged by a trimmer, and in that case
it is the trimmer that does the transfer.

http://en.wikipedia.org/wiki/Purlins

You unaccountably overlooked this paragraph:

"In timber roof construction prior to the introduction of trusses, under
purlins were used to support rafters over longer spans than the rafters
alone could span. Under purlins were typically propped off internal
walls. For example, an 8"x 4" under purlin would support the center of a
row of 6"x 2" rafters that in turn would support 3"x 2" roof purlins to
which the roof cladding was fixed.

Not the sort of construction you would expect in a domestic environment
in the UK and and the wording seems alien to me but if you dispensed
with the sheet roof cladding and substituted tiles or slates then the
'roof purlins' would also go and you would be essentially left with the
meat of the above paragraph which is that "under purlins were used to
support rafters over longer spans than the rafters alone could span".


Definitions of Purlins on the Web:

* A horizontal structural member spanning between beams or trusses
to *support a roof deck*. In slope glazing, purlins are the horizontal
framing members.
http://www.inspectorwebhosting.com/c...erms-m-q-2.htm

* The horizontal members of the roof structure used to *support and
attach the roof panels.*
www.ameren.com/Environment/adc_ev_glossary.asp

* Timber members spanning over trusses to *support cladding or
between trusses to support loose timbers.*
www.wolf-system.co.uk/glossary.asp

* are the horizontal roofing members, which connect bents.
www.trilliumdell.com/vocabulary/

* Horizontal beam along the length of a roof, *resting on
principals and supporting the common rafters or boards.(
http://www.bbc.co.uk/homes/property/...glossary.shtml

* In architecture or structural engineering, a purlin (or purline)
is a longitudinal structural member in a roof. Purlins support the loads
from the roof deck or sheathing and are supported by the principal
rafters and/or the building walls. ...
en.wikipedia.org/wiki/Purlins

In every case it is clearly specified that these rely ON the principal
rafters and the joists (binders) to support the weight. They act merely
to transmit the weight *to* them.

They certainly don't all say that. (See the quote from Wikipedia above
for one instance). I haven't bothered to follow up all of them but those
that don't say the purlins support the rafters seem to be referring to
sheet clad commercial roofs which require a horizontal beam much more
substantial than a roofing lath for the initial fixing.

Principal rafters would seem to be substitutes for walls. :-)

"a diagonal member of a roof principal, usually forming part of a truss
and supporting the purlins on which the common rafters rest."

If your rafters are VERY widely spaced, they need to be substantial.

If the purlins are widely spaced the rafters need to be substantial.

But apart from a 'starved horse' looking roof,removing them wont cause
the roof to fall down generally.

I repeat, in a conventional trussed roof, any longitudinal members are
there to stabilise the rafters in the correct (laterla) position, and
act as local beams to transmit localised stresses to them, in the case
of the ridge and exterior elements - like the tile battens.

That is not repeating what you said before if you mean a modern truss
roof but you are now moving the goalposts to apply only to modern
trussed roofs but you are still wrong. A roof is close to being a
uniformly distributed load. I would remind you what you said up thread:

*******
"Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Unless you go for warren braced (W braced) trusses, or some other method
of connecting the joist to the rafter in the roof truss, all the
strength is actually IN the rafter and the joist itself. Any other
bracing is for stability only and carries very little load, including
the ridge."
******

All the weight of the roof transfers to compression in the rafters and
tension ion the binders, the triangulation so defined forming a net
downward load on the binder ends at the eaves.

There is some net bending force on the rafters as well, but a long
purlin does very little to affect this unless its supported at the
ends,. If less than substantial rafters are used, or widely spaced one,
the better approach is W braves from the off binder centers to midway up
the rafters, or vertical posts between the rafter centers and the binders.

Of course you can build a roof with substantial enough rafters not to
need purlins but the volume of timber used is considerable. The modern
trussed roof is the space age way round this problem but personally I
think you are pushing the definition of purlin well past its breaking
point to even call the the stabilising strips that stop roof trusses
falling over 'purlins'.

Or more relevantly when using the roof space as a room, you can add
another set of binders at 'loft ceiling' level bolted to the rafters.

Which doesn't do anywhere near as much as a purlin would to stop the
rafters bending under load.

--
Roger Chapman

In article ,
The Natural Philosopher wrote:
Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.

So where else does it go?

Unless your purlins are actually PROPPED all the weight of the roof is
transmitted via the rafter ends. There *is* no other point of support.

Think you need to look at applied mechanics...

Er. no. Thats what my masters is in.

It seems that the 'purlins' in this case ARE propped - at the gable ends.

And the roof is not a trussed roof at all.

Of course not. In my case - a loft conversion - the whole reason was to
get rid of any intruding bracing. And the way you do this in essence is to
add a steel purlin from gable to gable for each roof span.

If you're talking about roof trusses then all a horizontal beam does is
tie them together - but then it's not really a purlin.

--
*"I am " is reportedly the shortest sentence in the English language. *

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.
So where else does it go?
Unless your purlins are actually PROPPED all the weight of the roof is
transmitted via the rafter ends. There *is* no other point of support.
Think you need to look at applied mechanics...

Er. no. Thats what my masters is in.

It seems that the 'purlins' in this case ARE propped - at the gable ends.

And the roof is not a trussed roof at all.

Of course not. In my case - a loft conversion - the whole reason was to
get rid of any intruding bracing.

Did you have W bracing?


And the way you do this in essence is to
add a steel purlin from gable to gable for each roof span.


Assuming you HAVE gables at all ;-)

If you're talking about roof trusses then all a horizontal beam does is
tie them together - but then it's not really a purlin.

Well in fact the definitions I found rather suggest that that is in fact
exactly what a purlin IS. what you describe is something else entirely..

The message
from The Natural Philosopher contains these words:

It seems that the 'purlins' in this case ARE propped - at the gable ends.

Which was the nature of purlins in domestic house construction in this
country until very recently. (Although I don't accept that the
horizontal ties in truss roofs are purlins).

And the roof is not a trussed roof at all.

No idea why you should have thought it was in the first place. There is
nothing in Simons original post to suggest a loft stuffed full of
struts.

And nothing in your early posts to suggest you were confining your
remarks to a trussed roof either.

--
Roger Chapman

Roger wrote:

Or more relevantly when using the roof space as a room, you can add
another set of binders at 'loft ceiling' level bolted to the rafters.

Which doesn't do anywhere near as much as a purlin would to stop the
rafters bending under load.


Actually it does *more*.

Materials in bending are never as good as in compression. The binder
across a rafter pair effectively locates the end points of the binder
precisely. However it leaves a large trapezoid structure underneath. The
maximum sag will be about halfway between the tie point and the eaves.

Which is why the nasty modern trusses use W bracing at about halfway
down the rafters.

With full triangulation any bending stresses are limited to much shorter
lengths of timber.

The only reason I can see to use structural ridges and purlins is when
you have cheap wood no machinery and expensive labour. i.e. its easier
to square off a couple of tree trunks and support them on some masonry
and add a few bits of broomstick over, than make up a few 6x3 rafters.


Today there is no excuse for that waste of wood.

In article ,
The Natural Philosopher wrote:
Dave Plowman (News) wrote:
In article , The Natural
Philosopher wrote:
Dave Plowman (News) wrote:
In article , The Natural
Philosopher wrote:
Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to
be. Very few roofs have a pitch greater than 45 degrees so if the
load is resolved into rightangle components only half or less is
being transmitted directly down the rafter.
So where else does it go? Unless your purlins are actually PROPPED
all the weight of the roof is transmitted via the rafter ends.
There *is* no other point of support.
Think you need to look at applied mechanics...

Er. no. Thats what my masters is in.

It seems that the 'purlins' in this case ARE propped - at the gable
ends.

And the roof is not a trussed roof at all.

Of course not. In my case - a loft conversion - the whole reason was
to get rid of any intruding bracing.

Did you have W bracing?

No. It's a Victorian house. The original attic room had the wall studs
which formed triangulation between the joists and rafters. I wanted to
remove these walls


And the way you do this in essence is to
add a steel purlin from gable to gable for each roof span.


Assuming you HAVE gables at all ;-)

Then you provide steel rafters at either end to take the load of the steel
purlins. Don't you ever watch Grand Designs? ;-)

If you're talking about roof trusses then all a horizontal beam does
is tie them together - but then it's not really a purlin.

Well in fact the definitions I found rather suggest that that is in fact
exactly what a purlin IS. what you describe is something else entirely..

The name purlin is as old as the hills - long before roof trusses as we
know them today. On a simple structure where you don't want individual
joists tying the bottoms of rafters together - like say a small church -
the purlins are tied by joists at either end as well as the gable. And the
rafters mounted on a wall plate with the same arrangement.

--
*Experience is something you don't get until just after you need it.*

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Roger wrote:
The message
from The Natural Philosopher contains these words:

It seems that the 'purlins' in this case ARE propped - at the gable
ends.

Which was the nature of purlins in domestic house construction in this
country until very recently. (Although I don't accept that the
horizontal ties in truss roofs are purlins).

And the roof is not a trussed roof at all.

No idea why you should have thought it was in the first place. There is
nothing in Simons original post to suggest a loft stuffed full of
struts.

And nothing in your early posts to suggest you were confining your
remarks to a trussed roof either.

A trussed roof will have very much smaller joists and rafters anyway - so
totally unsuitable for an attic room. Which is why converting a modern
house is so much more expensive than an older one - you effectively have
to replace the lot.

--
*I want it all and I want it delivered

Dave Plowman London SW
To e-mail, change noise into sound.

The message
from The Natural Philosopher contains these words:

Or more relevantly when using the roof space as a room, you can add
another set of binders at 'loft ceiling' level bolted to the rafters.

Which doesn't do anywhere near as much as a purlin would to stop the
rafters bending under load.

Actually it does *more*.

Materials in bending are never as good as in compression. The binder
across a rafter pair effectively locates the end points of the binder
precisely. However it leaves a large trapezoid structure underneath. The
maximum sag will be about halfway between the tie point and the eaves.

How that structure behaves will depend on the stability of the footing
in the eaves. Take a triangular truss with a 'binder' half way up and
the binder will be in compression, not tension as you suggested earlier.
Take an A shape structure and the binder can be in compression, neutral,
or in tension depending on how much movement there is in the feet as a
result of the uniformly distributed roof load. Should the binder be in
tension then far from strengthening the roof structure it is weakening
it by increasing the bending moment on the rafter. It might be
convenient to view each joint as a pin joint but treating the binder
location that way gives a false impression of the stresses involved.

On the other hand a purlin will always provide support for the rafters
resting on it. (If the ends are not supported it is not a purlin).

Which is why the nasty modern trusses use W bracing at about halfway
down the rafters.

At the point where a roof full of rafters would have had a purlin
(assuming it didn't require two).

With full triangulation any bending stresses are limited to much shorter
lengths of timber.

The only reason I can see to use structural ridges and purlins is when
you have cheap wood no machinery and expensive labour. i.e. its easier
to square off a couple of tree trunks and support them on some masonry
and add a few bits of broomstick over, than make up a few 6x3 rafters.


Today there is no excuse for that waste of wood.

Still a choice between waste of wood and waste of loft space.

--
Roger Chapman

The Natural Philosopher wrote:

I'm designing a dormer loft conversion. I omitted that off my original
post I think.
The room is to be open plan from front to back, i.e. to cantilever
support as in John Rumms conversion.
Simon.


So what have you removed to necessitate beefing up the structure?

Even punching a dormer out hardly weakens anything.

Its not so much weakening that is the problem. With a substantial dormer
(which basically amounted to one entire side of the roof in the case of
mine), you are removing several tones of weight of roof tiles. So the
ridge is no longer counter balanced by roughly equal loadings from
either side.

and surely you don't mean a cantilever..I have yet to see a cantilever
structure in any house..well a few Elizabethan ones where the top storey
overhangs the lower maybe.

My conversion did away with the need for a ridge beam, by having a
central wall dividing two rooms in the roof. The wall was not directly
under the ridge however but about a meter behind it, so the flat roof
joists of the dormer sailed over it to meet the slope of the remaining
front roof. These were hence supporting the front roof in cantilever:

http://www.internode.co.uk/loft/images/roofsupport.jpg

The supporting wall is just out of shot to the left of the above photo.

--
Cheers,

John.

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| John Rumm - john(at)internode(dot)co(dot)uk |
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Roger wrote:

Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.

I would go along with that... The purlins were about 7x4" on my roof.

If all that was needed to be done was to stabilise the rafters a dinky
bit of 4" x 2" (likely the same section as the rafters) nailed flat
against the rafters would do.

I am quite sure that if I nailed a few bits of 4 x 2 across my rafters
and then cut out the purlins most of the 20 or 30 tons of stone on my 30
degree roof would promptly join me inside the house. Rafters are only 4"
x 3" (or 4 x 2 where they have been replaced) but the clear span is
never more than about 4 feet (which is perhaps half what you might
expect for a lightweight slate roof).

Having actually done this I can tell you what happens! ;-)

On the hipped section of roof I was removing, we had de-tiled it when we
took the strut supporting the purlin out. The whole centre roof section
and purlin promptly dropped about 3 to 4" as the rafters sagged just
under their own load and that of the purlin. Had the tiles still been on
the roof, I expect the results would have been far more dramatic!

--
Cheers,

John.

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| Internode Ltd - http://www.internode.co.uk |
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| John Rumm - john(at)internode(dot)co(dot)uk |
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Roger wrote:

So where else does it go?

Onto the purlin. The rafter still has to function as a beam even though
it is by no means horizontal. Even if it wasn't nailed to the purlin it
would still load it and for the load to be in equilibrium at that point
the forces must balance.

Unless your purlins are actually PROPPED all the weight of the roof is
transmitted via the rafter ends. There *is* no other point of support.

No clear what you mean by propped in this context. Purlins are generally
simply supported at both ends and rely on their size to carry the load
transferred to them.

On mine, there were three purlins on each of three sides of the hipped
roof. The front and back ones were rested on a corbelled out section of
brickwork on the party wall, and nailed to third purlin at the other end
with a compound mitre. There were then three struts - each at mid span
of each purlin that were approximately at right angles to the pitch of
the rafters, that took the purlin load down to the spine wall across the
middle of the house.

Purlins are simply there to either stabilise the rafters, or if above
them, to transmit te roof weight TO the rafters.

You might think so but that doesn't make it a fact.

On my roof they carried a substantial proportion of the roof load.

If all that was needed to be done was to stabilise the rafters a dinky
bit of 4" x 2" (likely the same section as the rafters) nailed flat
against the rafters would do.

Thats geberally waht is used IME.

YMMV but I have never yet seen a proper roof with the horizontal members
fixed flat rather than upright (roofing laths excepted). To do so would
waste some of the strength of the beam. If the only purpose was to
stabilise the rafters using off-cuts for noggins would be more than
adequate.

This is the sort of thing you often see on trussed roofs - often wide
flatish timbers (4x1") are run (often at a jaunty angle) across the
rafters to add lateral restraint but not much else. I would hesitate to
call those purlins though.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
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| John Rumm - john(at)internode(dot)co(dot)uk |
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replying to me9, Laura.... help.. wrote:
I have just read yr post re roof.. i have a L shaped conversion on a valley
roof. I believe one of those walls has been removed then filled with studwork
the house is twisting like billeo and dropped. What the hell do i do now ...

--
for full context, visit https://www.homeownershub.com/uk-diy...am-443467-.htm

I'm assuming the original posting date on the thread this is joined to has
nothing to do with the person with the new problem. Get a structural
engineer in together with a rep from the insurance company you are with and
look suitable pleading I guess.
Brian

--
----- -
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Laura.... help.." m
wrote in message ...
replying to me9, Laura.... help.. wrote:
I have just read yr post re roof.. i have a L shaped conversion on a
valley
roof. I believe one of those walls has been removed then filled with
studwork
the house is twisting like billeo and dropped. What the hell do i do now
...

--
for full context, visit
https://www.homeownershub.com/uk-diy...am-443467-.htm

On Thursday, 1 February 2018 18:44:06 UTC, Laura.... help.. wrote:

I have just read yr post re roof.. i have a L shaped conversion on a valley
roof. I believe one of those walls has been removed then filled with studwork
the house is twisting like billeo and dropped. What the hell do i do now ...

talk to your building insurer if you have one.


NT