Flat roof replacement - further advice please! Information, help and advice on maintaining, installing and repairing a flat roof. Post your questions and view answers here

[niallmoran]

Evening folks

We've just bought an old cottage with a bigish (approx 35m2) flat roof extension that needs replacement (sagging boards, old felt covering etc) and the roofing contractor that did a pre purchase check quoted £5k + vat. for a new 5 layer felt roof with coolag insulation.

I thought that this seemed quite high so am planning on replacing it myself in the summer. After lots of reading i've decided on a fibreglass roof (mainly cause i'm used to GRP boats!) and want to build a warm roof construction.

My shopping list includes all the fibreglass material (approx 500 quids worth), CSR3 18mm decking but i'm struggling to find out what insulation board i should use (and where to source it!) for the warm roof construction as it can get a bit chilly in Dundee!

I've come across composite board that includes a 6mm ply top but i'm unsure how to use this....do i still need my 18mm deck? How do i attach it to the joists? Do i need noggins?

Can anybody point me in the right direction with this? I'd be most grateful for any help :D

Cheers
Niall

[Gryfiss]

You need a Polyisosanurate Insulation Board (PIR for short) which are available from Kingspan or Celotex. Thickness of the board will depend on the U Value you are looking to achieve but I would suggest that 100mm should suffice in order to achieve 0.25 U Value. Ask the chosen supplier to carry out a calculation for you. Your roof build up should then consist of 18mm plywood deck, Vapour control layer, PIR Insulation and finally your 3 layer built up felt system. You should also ensure that there is a 'fall' on the roof to disperse water to your gutters/outlets. This would need to be a finished roof fall of 1:80 or around 12.5mm every metre. Hope this helps.

[niallmoran]

thats great gryfiss - many thanks :D

would it be basically the same with a grp waterproofing layer?

[Gryfiss]

Certainly would. The only difference would be in the type of finish to the insulation board. Different waterproofing membranes are compatible with different finishes on the topside of the insulation. Again, speak to the insualtion manufacturer who will guide you as to which type you need.