Roof construction

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:
formatting link
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!)
Reply to
John Rumm
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 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.
Reply to
The Natural Philosopher
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 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.
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.
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 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.
Its rare to find existing joists deep enough to support a floor unless you are talking about converting only a tiny floor area.
Reply to
John Rumm
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
Reply to
Nige Danton
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
Reply to
robgraham
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 that one.
Yup, good plan... let me know if you have any good links.
Reply to
John Rumm
Could be... I might stick a piccie of a modern truss in as well for comparison.
I will see how much drawing time I have ;-)
Reply to
John Rumm
You're also going the have to address the tricky semantics of whether it is 'rooves' or 'roofs' !! :>)
Rob
Reply to
robgraham
Well, if you have to compromise the purlins with dormer holes..it uses less space.
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..
Reply to
The Natural Philosopher
The message 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.
Reply to
roger
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?
Reply to
The Natural Philosopher
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 an afternoon.
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 support
Reply to
John Rumm
The message 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.
Reply to
roger

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