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.

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