Pitched warm roof for rear extension - design detail/rafter centres

[HughF]

I'm making good progress with the sketched designs for the rear extension, a simple 3 and a bit x 5 and a bit mono-pitched affair in traditional construction. My wife would like exposed ceiling rafters, which I'm quite OK with, so I'm looking at doing these in glulam, mainly because regularised timber is all fast grown, easy warp, easy twist, easy split stuff which will warp/twist/split and then look naff.

 

My question: If I make this a warm roof, and use a structural sub-deck (18 or 22mm OSB), could I use something like a 250x90 glulam and put them at 600 or even 900 centres? I would batten between the rafters for a service void for pendant lights and then cover with 12mm PB and a skim.

 

Insulation would then go above, followed by battens and double roman tiles to match the house, standard warm roof construction.

 

I suppose this is really a question for a SE, but I haven't considered employing one for this small job, yet...

[HughF]

Nope, change of plan…. Going to make the exposed beams as decorative items (probably 12mm veneer’d ply) and fit them afterwards, over the fire protection of the PB.

 

After speaking to my builder at the weekend he said he’d done a job with exposed beams before and the regs compliance for fire proofing was a pain.

 

Wife is happy so that’s all the matters.

[Temp]

One thing to watch is the roof pitch. Sometimes a thick roof means the pitch has to be too low to get the top end and it's flashing under the first floor windows.

 

 

[HughF]

Yep, when I had a toolbox chat with my builder at the weekend we discussed raising the upstairs windowsills but I don’t think that’s required. I mentioned that I wanted to change these upstairs windows anyway as I want to change them for 3G with thermally broken sills.

 

I’ll do some drawings and figure out the pitch/height