roof design query / regs

[sm_jamieson]

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.

[Hugo Nebula]

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

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.

[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?

dg

[sm_jamieson]

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.