Renegade105 Posted February 26, 2023 Share Posted February 26, 2023 Hi, part of our house has a flat roof…and as you can see timbers have already been placed. I think roof build up is: Timber 18mm Marine Ply Membrane 120mm insulation 18mm OSB3 GRP how do I connect this on top of the ICF and keep AirTight? help is appreciated, or examples. I can’t be. The first Link to comment Share on other sites More sharing options...
ToughButterCup Posted February 26, 2023 Share Posted February 26, 2023 The top layer of our flat roof uses Evalon VSK , not GRP . @SteamyTea's advice put us off GRP. We lapped it over the top of the ICF and down the side a good way (200 mm?). Then put Tyvek waterproof rainscreen overlapping the VSK, sealed with tape. All covered with Siberian larch cladding. Link to comment Share on other sites More sharing options...
Radian Posted February 26, 2023 Share Posted February 26, 2023 1 hour ago, ToughButterCup said: @SteamyTea's advice put us off GRP. Feck, where was he when I got our flat roof GRP'd? Link to comment Share on other sites More sharing options...
SteamyTea Posted February 27, 2023 Share Posted February 27, 2023 8 hours ago, Radian said: 9 hours ago, ToughButterCup said: @SteamyTea's advice put us off GRP. Feck, where was he when I got our flat roof GRP'd? Don't know, may have been in Weymouth. Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 27, 2023 Share Posted February 27, 2023 You need to stick the ply down with D4 glue. All perimeter and joins. When it’s all in trim off excess glue and seal up all edges and joins with a polyurethane sealant. If it was mine I would sprayfoam the underneath of the ply with a closed cell foam. You will need the insulation layer on top of the ply to extend over the concrete core to the timbers that form the blocking behind the facia board. Link to comment Share on other sites More sharing options...
Iceverge Posted February 27, 2023 Share Posted February 27, 2023 Tricky detail. Important to get right. I would use the ply as the main air control layer. You could use the polythene above it but it's always easier to seal solid to solid materials You have 3 types of junctions. Ply to Ply = airtight tape. One suitable for external use like Siga wigluv. Ply to ICF.= Airtight tape as above. Alternatively mesh like you would use to tank a shower and then airtight paint over the top. Ply to concrete. Either an airtight expanding foam like FM330 or generous amounts of mastic squeezed between the boards and the top of the wall as they are laid. 120mm insulation is the bare minimum. U value about 0.18. I guess it is done to facilitate 200mm screws. Could you add another 50mm board (offset from the first) and use the pricier 250mm screws. Alternatively make a hybrid roof. 100mm of rockwool inside the ply should give a U value of 0.13 with no risk of condensation provided you get your Airtightness/ vapour control layer right. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now