"The Natural Philosopher" wrote in message
...
wrote:
On 10 Apr, 12:52, Rob Horton wrote:

I have a house that was built in 1984. The roof trusses are constructed
from wood that is 34mm (1 3/8") by 95mm (3 3/4"). I have no idea what
the rating of the wood is. Each truss is spaced about 600mm (centre to
centre) apart. The ceiling joist part of the truss spans 7.2 metres.
Some of the wood has been spliced together using what looks like
perforated metal sheets banged into each side of the wooden beams. The
roof trusses are of a W type, fink, construction.

I want to partly board the roof area using Knauf spaceboard (chipboard
with polyfoam insulation underneath) so that It can be effectively used
for storage.

Where can I find information about the load carrying capacity for my
roof space? Something like kg/m2?

Thanks.


If those really span 7.2m I dont think I'd put anything on them.


They are W "Warren" braced. Ultimate breaking strain probably around 2-5
tons on any given beam. Unacceptable deflection? probably a fair bit
less. Probably half a ton on any beam. Adding a chipboard floor to
spread the load will probably result in a floor not much worse than any
normal room.
NT


I can't find any reference to the figures above in the truss searches I've
done to date - do you have a reference perhaps?

I've been watching this thread with interest, as next week we move into a
70s bungalow with a very similar roof structure. Our trusses are spanning
7.7 metres, and made up from the same 95x35 timber. The only real difference
is that our trusses are on a slightly narrower 500mm spacing. There are
internal block walls under most of the trusses, but interestingly at one end
of the building there is a 3 metre section with no support - and no apparent
ceiling bowing. There is an existing water tank, sitting on 100x50 timbers
nailed(!) to the vertical members, parallel to three of the truss bases, at
about a foot up - presumably to gain a little extra head. This is only
partly over an internal wall, and does not seem to have distorted the
structure - although I need to check this more carefully once we are in.

Our new neighbours (who have been there some twenty years) have boarded out
their loft, and store a fair bit of "stuff", and say they have had no
problems to date.

I would like to add extra insulation to the 100mm or so that is there now,
and was thinking of adding parallel joists above the present bases of the
trusses, to give space for another 150mm of glass fibre. These joists would
be something like 100 or 125x35m, sitting on blocks on the wall plates, and
bolted to the truss vertical(ish) members. The centre of each joist would
have to have a bolted joint, as 7.5 metre timber is (obviously) difficult to
get, and impossible to get into the roof space anyway. I could then board
out the centre 3.5m or so for access and storage.

My rough calculations seem to show that the cost of the timber & glass fibre
would be a bit less than the equivalent insulation value of Kingspan or
similar, and the timber/glass combination would make subsequent access to
services easier in the future. I would also assume that effectively doubling
the truss bases, albeit at a slightly higher level, should make the
ceiling/loft floor more rigid.

Has anyone done anything like this to date, or have any thoughts?

Charles F