Flat warm roof - insulation/deck confusion :(

[oly]

Single story flat roof extension with some skylights. Trying to understand the best way to create the warm deck but getting confusing advice from manufacturers of insulation/membrane...

I've got lots of 120mm kingspan foil/foil.

I believe I need deck of 18mm OSB or WBP?

May need vapour control (1000 gauge?)

Another layer of decking?

What order do they go all down?

Cheers for any advice!

 

[jeds]

A warm deck roof is basically the same as a standard flat roof except there is no ventilation and the insulation is on top of the deck rather than within the roof structure. The insulation being on top of the deck means the deck stays warm thus 'warm deck' roof. The other essential ingredient is a vapour control layer, to prevent vapour from penetrating the insulation, which must be on the warm side. So you might lay; rafters, decking, vapour layer, insulation, another top deck and then the weathering layer. Strictly speaking the top deck isn't necessary and you could put the weathering layer directly on the insulation but that isn't very practical for walking on.

 

[nickjb]

Mine is exactly as jeds describes: rafters, decking, vapour layer, insulation, another top deck and then the weathering layer. As mentioned the top decking isn't essential but it does make it easier to lay the weathering layer and allow you to walk around up there. I used 12mm WPB as the ply/kingspan sandwich is pretty stiff.