help regarding warm flat roof layers

[giovanni1988]

Hi,

currently building an extension 6mx4m. Planning for the roof and thus far ive followed drawings given to me by architect to the mm. However changing the roof surface from mineral felt to fiberglass.

With the felt we were going for the celtotex td4000 roof deck with 5.5mm ply which is suitably for felt but now I would have to put 18mm osb on top to fiberglass and it will work out very very expensive (celotex td4000 is £80 per sheet).

so basically what other option do I have? I need to know the layers step by step.

1.plywood on top of furrings/joists (what thickness, type)
2.vapor control layer (wheres the best place to buy a good quality type like foil faced
3. Insulation (what type? Origionally was 120mm thick, with 120mm celotex be fine?)
4. Osb3 board 18mm thick
5.fiberglass.

how orrect is this? And could anybody fill in the queries I have so I can get a good nights kip tonight

Thanks

 

[StevieV1]

Hi there

Ive just done a flat (crown) roof like this:

1 EPDM roof
2. 18mm ply
3. Firrings
4. Roof joists with 120mm Celotex in between and 50mm min airgap between top of insulation and bottom of ply deck
5. 30mm Celotex under joists sealed with foil tape and acting as VCL.

I was going to get a fibreglass roof but cam across a few issues re loud banging noises associated with thermal expansion/contraction and possible cracking leading to water ingress.

You can buy VCL quite cheaply from Screwfix. Ive bought some green stuff they do in 2.4m rolls for the walls of my extension.