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.

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

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.

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.

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

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

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,...