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:
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 fuck 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
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
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.
Well partly the purpose of the article - save spending too much time
debating what things we are talking about...
There are a good deal more in reality! (king post designs etc)
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
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.
This is true, but does not really sit well with modern builders naming
conventions, where "truss" is a fairly universal shorthand for a
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.
yup agreed - in the engineering sense of the word truss.
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
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.
Its rare to find existing joists deep enough to support a floor unless
you are talking about converting only a tiny floor area.
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.
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
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.
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.
Interestingly I have seen some '30s properties the south east that use
planked sarking - although with clay tiles over. No soffits either on
Yup, good plan... let me know if you have any good links.
Well, if you have to compromise the purlins with dormer holes..it uses
Hate thos modern trusses. Impossible to use nay of the space..
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.
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..
from The Natural Philosopher contains these words:
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.
In your dreams.
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 don't think so. Why not do the calculations?
Normally if there is a dormer going through the purlin it is easier to
replace the purlin with a dwarf wall.
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
Even 8x3" won't span 6m though as a plain joist... something a truss can
do without too much difficulty.
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
from The Natural Philosopher contains these words:
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.