Can a cut roof be at 600mm centres?

soldsold

Hi,

In an attempt to save costs and reduce thermal bridging I have thought about having the roof timbers at 600mm centres instead of 400mm.

I know this is a very general question, and I know truss roofs are usually 600mm and cut roofs are usually 400, my question is if any of you have opinions on whether I can use rafters at 600 centres with a cut roof.

The structure will be - from inside to out:

Plasterboard
Horizontal 50x38mm counter batten (to give an insulated service cavity)
15mm osb with taped joints (for airtightness)
225x44 rafter at 600mm centres (for depth for insulation)
60mm softboard OR 50x44 horizontal counter batten (for depth of insulation)
Solitex membrane (for windtightness and rain barrier)
vertical slating battens (to allow water to run off membrane)
horizontal slating battens

Anyone with strong opinions as to why the above structure wont work? It looks to me like the roof should have enough strength...

Thanks,

Steve

mickdw

600mm centres are quoted in the design tables for rafters so yes they can be used like that but you will have to check the design.
you wont be saving costs as a step up in rather size will most likely be required unless you have oversized rafters already

soldsold

Thanks for the reply.

Would the racking strength from all the "extra" counter battens and OSB board count at all?

archtech

Sounds like a roof that it would be worth while getting a structural engineer to do some calculations on.

not sure if taping the osb board alone with achieve the required air tightness. It might be worth while getting the air test done before you proceed with the construction of your services void.

Mellor

The rafters will have to be up sized. 600mm is still standard.
The racking strength won't assist here as it is lateral, the load isn't.

Check IS444 tables (includied on homebond) for required size of rafter

soldsold

Thanks. I have a homebond manual so will check.

My roofer says the brazilian slates I am planning to use will crack from the the slating battens bouncing as the roofers move around if I go with 600 centres so that might rule it out anyway.