Thermal Ply for flat roof OR internal insulation

[macadonald]

Hi guys,

I'm busy creating the shoppign list of materials for my rear extension and am curious whether there are benefits to using thermal ply for the flat roof of the extension or should I rather use structural ply with kingspan or celotex slab insulation between the joist?

Or do you use both?

Thanks

 

[macadonald]

Anyone?

 

[noseall]

The simplest and best way is to use the warm deck method.

100mm celotex or similar on top of the joists with a further say 25mm pushed up tight against the 100mm - between the joists. You can buy a ply deck with this stuff glued to it.

Upside is this method alleviates venting.

Downside is deep fascias.

 

[macadonald]

Thanks NoSeal, It's a flat roof with parapet wall, so if I'm right there won't be any fascias. Appreciate the advice.