Right. 1992 part A tells me my shallow pitch roof can be just purlins
and sheathing (if I'm wrong then the diagrams are misleading but please
someone tell me). This fits in well, since my roof is smaller in this
direction anyway. Assuming 0.75kN/m2 loading (snow etc), SC4 timber can
do 4 metres at 63x200mm spaced every 900mm.
My question is:
Can I therefore use e.g. 63x100 every 450mm. This would have an effect
on point loading, but could I do this ... or similar ?
TIA,
Simon.

On 17 Oct 2005 09:44:37 -0700, a particular chimpanzee named
randomly hit the keyboard and produced:

Right. 1992 part A tells me my shallow pitch roof can be just purlins
and sheathing (if I'm wrong then the diagrams are misleading but please
someone tell me). This fits in well, since my roof is smaller in this
direction anyway. Assuming 0.75kN/m2 loading (snow etc), SC4 timber can
do 4 metres at 63x200mm spaced every 900mm.
My question is:
Can I therefore use e.g. 63x100 every 450mm. This would have an effect
on point loading, but could I do this ... or similar ?

No. Bending force is proportional to the square of the span, and
deflection is proportional to its cube, so you can't simply halve the
load to double the span.

I don't know whether you're assuming only imposed loading, but you'll
need to include for the dead loading. This will include the weight of
the tiles or roof covering and the plasterboard ceiling under. All of
which could amount to more than 0.75kN/m². Unless you're going to use
a Tescos style crinkly tin roof, you'll have to counter-batten over
the purlins to tile the roof.

All of which sounds like an awful lot of messing about to save a few
quid on a bit of steel for a purlin.
--
Hugo Nebula
"If no-one on the internet wants a piece of this,
just how far from the pack have you strayed?"

I think you are confusing your purlins and your rafters, and sheathing
and laths.

The tables and diagrams in the approved docs are quite self
explanatory. What are you trying to acheive?

dg

I think you are confusing your purlins and your rafters, and sheathing
and laths.
The tables and diagrams in the approved docs are quite self
explanatory. What are you trying to acheive?
No, purlins are defined as beams that span across the roof slope,
rafters down. A purlin is not necessarily supporting rafters. The
diagram in the 1992 regs for roof 10-30 degree pitch, shows only
purlins and sheathing. There is no table for rafters at this pitch - it
is like a flat roof.
All I am trying to acheive, is to design my roof ! The table I was
looking at had separate tables for imposed loads 0.75 and 1.0, and
columns for dead loads up to 0.75.
I appreciate the bending force rule though, I had forgotten that !
Cheers,
Simon.