[John Rumm]

Some of the recent discussions that involved the topic of roof
construction, seemed to wander off a various tangents, mostly based on
differing use of terminology it seemed.

Hence I though having a hymn sheet for us all to sing from might be handy:

http://wiki.diyfaq.org.uk/index.php?...f_construction

Have look and see if there is anything that you think might be useful in
future. Feel free to tweak or post comments here if you prefer.

(if you fancy doing a section on use of trusses, feel free!)


--
Cheers,

John.

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

[The Natural Philosopher]

John Rumm wrote:
Some of the recent discussions that involved the topic of roof
construction, seemed to wander off a various tangents, mostly based on
differing use of terminology it seemed.

Hence I though having a hymn sheet for us all to sing from might be handy:

http://wiki.diyfaq.org.uk/index.php?...f_construction

Have look and see if there is anything that you think might be useful in
future. Feel free to tweak or post comments here if you prefer.

(if you fancy doing a section on use of trusses, feel free!)


I think you have a real problem with terminology.

There are two traditional rooves.

One as John Rumm described, where the loads are taken in the ridge and
purlins, and the joists do nowt., and one where there is a true truss,
where the rafters are nailed to the joists, and the ridge and purlins do
almost **** all.

That is still a truss. Any triangulated structure - even one triangle -
is a truss.

I suspect that YOU mean a typical warren or W type truss is what you
call a truss.

The moment you put a binder across a couple of rafters, you have a sort
of truss.

This does suggest a neat way of doing a loft conversion with a 'massive
propped purlin' type roof.

Convert it to truss.

Simply add rafters - alongside or below existing - and cross link them
at new ceiling height with 'loft ceiling joists' bolted through as
binders for real rigidity..nails work, but bolts and spider plates are
better.

Then bolt new joists (if needed fior floor rigidity: otherwise use
existing)) at floor level to new rafters. Hey presto - you have a center
span braced truss and you can remove the purlins altogether apart from
lightweight stuff to stabilise the rafters laterally. Since you will be
lining the rafters, after insulation that could be no more than sheets
of ply screwed to the rafter undersides.

It may actually turn out that existing rafters and joists are adequate -
the purlin function being replaced by spreaders at ceiling height to
stop the rafters bowing inwards. And existing joists enough top prevent
outward loads on the upper walls at the rafter eave level.

[John Rumm]

The Natural Philosopher wrote:

http://wiki.diyfaq.org.uk/index.php?...f_construction

Have look and see if there is anything that you think might be useful
in future. Feel free to tweak or post comments here if you prefer.

(if you fancy doing a section on use of trusses, feel free!)


I think you have a real problem with terminology.

Well partly the purpose of the article - save spending too much time
debating what things we are talking about...

There are two traditional rooves.

There are a good deal more in reality! (king post designs etc)

One as John Rumm described, where the loads are taken in the ridge and
purlins, and the joists do nowt., and one where there is a true truss,
where the rafters are nailed to the joists, and the ridge and purlins do
almost **** all.

On a traditional roof as I have drawn here, the ridge takes no load at
all, the purlins do however take significant load. Ridge loads only tend
to become significant if you unbalance the roof with large one sided
dormer etc.

However even on this sort of roof, there is an element of triangulation
even if the joists are not fixed to the rafters or each other, since
they are fixed to the wall plate - as are the rafters. This naturally
places the joists under a degree of tension.

That is still a truss. Any triangulated structure - even one triangle -
is a truss.

This is true, but does not really sit well with modern builders naming
conventions, where "truss" is a fairly universal shorthand for a
prefabricated item.

I suspect that YOU mean a typical warren or W type truss is what you
call a truss.

Indeed - or any other prefab truss such as fink, attic etc.

In this particular case I was using truss as in a modern prefabricated
item, typically with thin section wood, much triangulation, and joints
formed with staple or spike plates etc.

The moment you put a binder across a couple of rafters, you have a sort
of truss.

yup agreed - in the engineering sense of the word truss.

This does suggest a neat way of doing a loft conversion with a 'massive
propped purlin' type roof.

Convert it to truss.

Simply add rafters - alongside or below existing - and cross link them
at new ceiling height with 'loft ceiling joists' bolted through as
binders for real rigidity..nails work, but bolts and spider plates are
better.

Then bolt new joists (if needed fior floor rigidity: otherwise use
existing)) at floor level to new rafters. Hey presto - you have a center
span braced truss and you can remove the purlins altogether apart from
lightweight stuff to stabilise the rafters laterally. Since you will be
lining the rafters, after insulation that could be no more than sheets
of ply screwed to the rafter undersides.

I see what you are saying, but am not sure it buys you anything since
the existing structure is already self supporting, all you typically
need do in these cases is make the floor stronger and possibly add a
cosmetic ceiling.

Having said that, this is exactly the approach that is used in reverse
for converting modern trussed lofts - essentially building a traditional
joinery support system inside the existing space, so that much of the
prefab truss can be cut away to make space.

One common complexity is that is it not as easy to span significant
distances with a joinery roof (even under tension a 25' long 4x2" is
going to sag in the middle), as can be done with a heavily cross braced
truss. Hence when one is trying to eliminate the many braces of the
truss you end up needing to duplicate the mid span supporting function
of the spine wall in some way.

It may actually turn out that existing rafters and joists are adequate -
the purlin function being replaced by spreaders at ceiling height to
stop the rafters bowing inwards. And existing joists enough top prevent
outward loads on the upper walls at the rafter eave level.

Its rare to find existing joists deep enough to support a floor unless
you are talking about converting only a tiny floor area.


--
Cheers,

John.

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

[Nige Danton]

John Rumm wrote:
http://wiki.diyfaq.org.uk/index.php?...f_construction

Would describing it as a traditional rafter and purlin construction
help to differentiate it from the prefabricated trussed roof designs
commonly found in modern dwellings? Also do you think it would add any
value to put a hip on one end? That would allow jack rafters to be
identified too. Perhaps also locate and identify fascia and barge
boards together with tilting fillets. Just a thought.

--
Nige Danton

[robgraham]

On 18 Dec, 00:33, John Rumm wrote:
Some of the recent discussions that involved the topic of roof
construction, seemed to wander off a various tangents, mostly based on
differing use of terminology it seemed.

Hence I though having a hymn sheet for us all to sing from might be handy:

http://wiki.diyfaq.org.uk/index.php?...f_construction

Have look and see if there is anything that you think might be useful in
future. Feel free to tweak or post comments here if you prefer.

(if you fancy doing a section on use of trusses, feel free!)

--
Cheers,

John.

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

John
This I fear could be in danger of growing topsy-like as there are so
many solutions to rooves, albeit a rafter is a rafter and a joist is a
joist.

For instance the traditional Scottish roof does not contains purlins,
has a simple tie across the rafters at typically 2/3rds height and has
wooden planks as the sarking with nailed on slates.

As I suggest your difficulty will be addressing all roof styles and I
wonder whether a bit of googling might locate other sites which have
details of specific roof designs rather than attempt to do it all on
one page here.

Rob

[John Rumm]

robgraham wrote:

John
This I fear could be in danger of growing topsy-like as there are so
many solutions to rooves, albeit a rafter is a rafter and a joist is a
joist.

Yup, that problem did occur to me... ;-)

I just wanted to cover the basic terminology, to do a "complete" job
would require writing a book.

For instance the traditional Scottish roof does not contains purlins,
has a simple tie across the rafters at typically 2/3rds height and has
wooden planks as the sarking with nailed on slates.

Interestingly I have seen some '30s properties the south east that use
planked sarking - although with clay tiles over. No soffits either on
that one.

As I suggest your difficulty will be addressing all roof styles and I
wonder whether a bit of googling might locate other sites which have
details of specific roof designs rather than attempt to do it all on

Yup, good plan... let me know if you have any good links.

--
Cheers,

John.

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

[John Rumm]

Nige Danton wrote:

John Rumm wrote:
http://wiki.diyfaq.org.uk/index.php?...f_construction

Would describing it as a traditional rafter and purlin construction
help to differentiate it from the prefabricated trussed roof designs
commonly found in modern dwellings? Also do you think it would add any

Could be... I might stick a piccie of a modern truss in as well for
comparison.

value to put a hip on one end? That would allow jack rafters to be
identified too. Perhaps also locate and identify fascia and barge
boards together with tilting fillets. Just a thought.

I will see how much drawing time I have ;-)

--
Cheers,

John.

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

[robgraham]

On 18 Dec, 09:54, John Rumm wrote:
robgraham wrote:
John
This I fear could be in danger of growing topsy-like as there are so
many solutions to rooves, albeit a rafter is a rafter and a joist is a
joist.

Yup, that problem did occur to me... ;-)

I just wanted to cover the basic terminology, to do a "complete" job
would require writing a book.

For instance the traditional Scottish roof does not contains purlins,
has a simple tie across the rafters at typically 2/3rds height and has
wooden planks as the sarking with nailed on slates.

Interestingly I have seen some '30s properties the south east that use
planked sarking - although with clay tiles over. No soffits either on
that one.

As I suggest your difficulty will be addressing all roof styles and I
wonder whether a bit of googling might locate other sites which have
details of specific roof designs rather than attempt to do it all on

Yup, good plan... let me know if you have any good links.

--
Cheers,

John.

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

You're also going the have to address the tricky semantics of whether
it is 'rooves' or 'roofs' !! :)

Rob

[The Natural Philosopher]

John Rumm wrote:
The Natural Philosopher wrote:

http://wiki.diyfaq.org.uk/index.php?...f_construction

Have look and see if there is anything that you think might be useful
in future. Feel free to tweak or post comments here if you prefer.

(if you fancy doing a section on use of trusses, feel free!)


I think you have a real problem with terminology.

Well partly the purpose of the article - save spending too much time
debating what things we are talking about...

There are two traditional rooves.

There are a good deal more in reality! (king post designs etc)

One as John Rumm described, where the loads are taken in the ridge and
purlins, and the joists do nowt., and one where there is a true truss,
where the rafters are nailed to the joists, and the ridge and purlins
do almost **** all.

On a traditional roof as I have drawn here, the ridge takes no load at
all, the purlins do however take significant load. Ridge loads only tend
to become significant if you unbalance the roof with large one sided
dormer etc.

However even on this sort of roof, there is an element of triangulation
even if the joists are not fixed to the rafters or each other, since
they are fixed to the wall plate - as are the rafters. This naturally
places the joists under a degree of tension.

That is still a truss. Any triangulated structure - even one triangle
- is a truss.

This is true, but does not really sit well with modern builders naming
conventions, where "truss" is a fairly universal shorthand for a
prefabricated item.

I suspect that YOU mean a typical warren or W type truss is what you
call a truss.

Indeed - or any other prefab truss such as fink, attic etc.

In this particular case I was using truss as in a modern prefabricated
item, typically with thin section wood, much triangulation, and joints
formed with staple or spike plates etc.

The moment you put a binder across a couple of rafters, you have a
sort of truss.

yup agreed - in the engineering sense of the word truss.

This does suggest a neat way of doing a loft conversion with a
'massive propped purlin' type roof.

Convert it to truss.

Simply add rafters - alongside or below existing - and cross link them
at new ceiling height with 'loft ceiling joists' bolted through as
binders for real rigidity..nails work, but bolts and spider plates are
better.

Then bolt new joists (if needed fior floor rigidity: otherwise use
existing)) at floor level to new rafters. Hey presto - you have a
center span braced truss and you can remove the purlins altogether
apart from lightweight stuff to stabilise the rafters laterally. Since
you will be lining the rafters, after insulation that could be no more
than sheets of ply screwed to the rafter undersides.

I see what you are saying, but am not sure it buys you anything since
the existing structure is already self supporting, all you typically
need do in these cases is make the floor stronger and possibly add a
cosmetic ceiling.


Well, if you have to compromise the purlins with dormer holes..it uses
less space.

Having said that, this is exactly the approach that is used in reverse
for converting modern trussed lofts - essentially building a traditional
joinery support system inside the existing space, so that much of the
prefab truss can be cut away to make space.


Hate thos modern trusses. Impossible to use nay of the space..

One common complexity is that is it not as easy to span significant
distances with a joinery roof (even under tension a 25' long 4x2" is
going to sag in the middle), as can be done with a heavily cross braced
truss. Hence when one is trying to eliminate the many braces of the
truss you end up needing to duplicate the mid span supporting function
of the spine wall in some way.

Or using more substantial timbers.

IIRC my roof is 7x3 at least. - maybe 8x3

and there is seldom a need to span 25' in one go..thats a huge room NOT
to have a dividing wall in its omewhere..if its joist you mean. If
rafters, the answer is to put a binder 8 ft above the ceiling. That
effectively props the rafter span without intruding below.






It may actually turn out that existing rafters and joists are adequate
- the purlin function being replaced by spreaders at ceiling height to
stop the rafters bowing inwards. And existing joists enough top
prevent outward loads on the upper walls at the rafter eave level.

Its rare to find existing joists deep enough to support a floor unless
you are talking about converting only a tiny floor area.



I suppose it all depends on the crappiness of the original design.

Faced with a naff warren truss roof, I think I woold simply build 'A'
frames inside, of more substantial timbers and cut out the braces..

[The Natural Philosopher]

robgraham wrote:
On 18 Dec, 09:54, John Rumm wrote:
robgraham wrote:
John
This I fear could be in danger of growing topsy-like as there are so
many solutions to rooves, albeit a rafter is a rafter and a joist is a
joist.
Yup, that problem did occur to me... ;-)

I just wanted to cover the basic terminology, to do a "complete" job
would require writing a book.

For instance the traditional Scottish roof does not contains purlins,
has a simple tie across the rafters at typically 2/3rds height and has
wooden planks as the sarking with nailed on slates.
Interestingly I have seen some '30s properties the south east that use
planked sarking - although with clay tiles over. No soffits either on
that one.

As I suggest your difficulty will be addressing all roof styles and I
wonder whether a bit of googling might locate other sites which have
details of specific roof designs rather than attempt to do it all on
Yup, good plan... let me know if you have any good links.

--
Cheers,

John.

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

You're also going the have to address the tricky semantics of whether
it is 'rooves' or 'roofs' !! :)


Either are acceptable.
Rob

[John Rumm]

robgraham wrote:

You're also going the have to address the tricky semantics of whether
it is 'rooves' or 'roofs' !! :)

I will leave that one for a wiki edit war ;-)

--
Cheers,

John.

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

[roger]

The message
from The Natural Philosopher contains these words:

The moment you put a binder across a couple of rafters, you have a sort
of truss.

This does suggest a neat way of doing a loft conversion with a 'massive
propped purlin' type roof.

Convert it to truss.

Simply add rafters - alongside or below existing - and cross link them
at new ceiling height with 'loft ceiling joists' bolted through as
binders for real rigidity..nails work, but bolts and spider plates are
better.

Then bolt new joists (if needed fior floor rigidity: otherwise use
existing)) at floor level to new rafters. Hey presto - you have a center
span braced truss and you can remove the purlins altogether apart from
lightweight stuff to stabilise the rafters laterally. Since you will be
lining the rafters, after insulation that could be no more than sheets
of ply screwed to the rafter undersides.

If the bottom of the A is tied then I don't doubt that ceiling height
braces would provide some rigidity if the purlins were removed but I
doubt very much whether it would be sufficient to prevent the roof
collapsing even in still conditions. Wind loading would case the
windward side to bow in and the leeward to bow out if the roof did
survive still conditions and on top of that any roof would require an
adequate safety factor over and above the most extreme conditions which
might include a foot or more of snow as well as galeforce wind.

I am not at home atm so haven't the time to even take a look at John's
hymn sheet much less expand on what I have written above.

It may actually turn out that existing rafters and joists are adequate -
the purlin function being replaced by spreaders at ceiling height to
stop the rafters bowing inwards. And existing joists enough top prevent
outward loads on the upper walls at the rafter eave level.

In your dreams.

--
Roger Chapman

[The Natural Philosopher]

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

The moment you put a binder across a couple of rafters, you have a sort
of truss.

This does suggest a neat way of doing a loft conversion with a 'massive
propped purlin' type roof.

Convert it to truss.

Simply add rafters - alongside or below existing - and cross link them
at new ceiling height with 'loft ceiling joists' bolted through as
binders for real rigidity..nails work, but bolts and spider plates are
better.

Then bolt new joists (if needed fior floor rigidity: otherwise use
existing)) at floor level to new rafters. Hey presto - you have a center
span braced truss and you can remove the purlins altogether apart from
lightweight stuff to stabilise the rafters laterally. Since you will be
lining the rafters, after insulation that could be no more than sheets
of ply screwed to the rafter undersides.

If the bottom of the A is tied then I don't doubt that ceiling height
braces would provide some rigidity if the purlins were removed but I
doubt very much whether it would be sufficient to prevent the roof
collapsing even in still conditions. Wind loading would case the
windward side to bow in and the leeward to bow out if the roof did
survive still conditions and on top of that any roof would require an
adequate safety factor over and above the most extreme conditions which
might include a foot or more of snow as well as galeforce wind.


Good points, but thats why the rafter sizes may need to go up a bit.

Also, do NOT neglect the weight of a roof. The side loads are not nearly
as great as the downforces except in a serious hurricane - which will
tend to rip the tiles off first anyway.



I am not at home atm so haven't the time to even take a look at John's
hymn sheet much less expand on what I have written above.

It may actually turn out that existing rafters and joists are adequate -
the purlin function being replaced by spreaders at ceiling height to
stop the rafters bowing inwards. And existing joists enough top prevent
outward loads on the upper walls at the rafter eave level.

In your dreams.

I don't think so. Why not do the calculations?

[John Rumm]

The Natural Philosopher wrote:

I see what you are saying, but am not sure it buys you anything since
the existing structure is already self supporting, all you typically
need do in these cases is make the floor stronger and possibly add a
cosmetic ceiling.


Well, if you have to compromise the purlins with dormer holes..it uses
less space.

Normally if there is a dormer going through the purlin it is easier to
replace the purlin with a dwarf wall.

Having said that, this is exactly the approach that is used in reverse
for converting modern trussed lofts - essentially building a
traditional joinery support system inside the existing space, so that
much of the prefab truss can be cut away to make space.


Hate thos modern trusses. Impossible to use nay of the space..

Can't say I am a fan of em either. Still if I was knocking out little
boxes made from ticky tacky for sale to the general public, I can see
the attraction of knocking out a complete roof framing in the space of
an afternoon.

One common complexity is that is it not as easy to span significant
distances with a joinery roof (even under tension a 25' long 4x2" is
going to sag in the middle), as can be done with a heavily cross
braced truss. Hence when one is trying to eliminate the many braces of
the truss you end up needing to duplicate the mid span supporting
function of the spine wall in some way.

Or using more substantial timbers.

IIRC my roof is 7x3 at least. - maybe 8x3

Even 8x3" won't span 6m though as a plain joist... something a truss can
do without too much difficulty.

and there is seldom a need to span 25' in one go..thats a huge room NOT
to have a dividing wall in its omewhere..if its joist you mean. If
rafters, the answer is to put a binder 8 ft above the ceiling. That
effectively props the rafter span without intruding below.

Many modern houses don't have internal load bearing walls (or more to
the point, no foundation under the centre wall if there is one), and a
prefab truss would normally span the house without need of intermediate
support



--
Cheers,

John.

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

[roger]

The message
from The Natural Philosopher contains these words:

I am not at home atm so haven't the time to even take a look at John's
hymn sheet much less expand on what I have written above.

It may actually turn out that existing rafters and joists are adequate -
the purlin function being replaced by spreaders at ceiling height to
stop the rafters bowing inwards. And existing joists enough top prevent
outward loads on the upper walls at the rafter eave level.

In your dreams.

I don't think so. Why not do the calculations?

Time for one thing but I am so rusty on the nitty-gritty that I would
not trust any figures I came up with.

FWIW I think wind is a bigger factor than you make out but like so many
aspects of a roof it depends very much on other factors which also have
a considerable outcome on the calculations. Angle of roof makes a
tremendous difference both to the compressive load on the rafters and to
the wind force on the surface of the roof. Wind is also a complex force
that probably can't be properly catered for without knowing how a roof
behaves in a wind tunnel. Then there is the weight of the roof varying
from almost bugger all for corrugated sheets to roofs* like mine with an
average of perhaps 3" thickness of stone.

*BTW roofs might be pronounced rooves but AFAICT spelling it that way is
not standard English.

--
Roger Chapman

[John Rumm]

Nige Danton wrote:

Also do you think it would add any
value to put a hip on one end? That would allow jack rafters to be
identified too. Perhaps also locate and identify fascia and barge
boards together with tilting fillets. Just a thought.

OK, added new bits for hipped roof designs.

http://wiki.diyfaq.org.uk/index.php?...on#Hipped_roof

--
Cheers,

John.

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