This was discussed a bit in the BR section but I think I need some advice from roofing experts. Basically I'm aware of the overall design for a warm roof but I cannot find any specifics on how to affix the layers together properly.
Here is the plan of our cabin:
So when I google "warm roof design" I get this sort of thing:
So far so good. But they don't discuss how the layers are actually joined and kept in place, to deal with strong winds, etc.
I have two specific queries with my builders:
So the builder voiced concerns it could get "sucked off" by winds. However he is a builder who does some roofing, not a roofer.
I hope this is detailed enough - any questions just ask. Build is already in progress so the main truss/purlin setup is already in place. By the way, the purlins and sub-deck are exposed on the inside as part of the design - no ceiling.
Here is the plan of our cabin:
- I've coloured the trusses (actually they are more like gables) orange, they have centres ~2.9m
- I've coloured the purlins blue, they have centres ~0.84m. They are 70x145mm timbers.
- The roof has a pitch of 15 degrees
- On top of the purlins go nice 18mm thick, 86mm wide boards.
- My roof covering is made of felt shingles.
- We plan to use PIR insulation board somewhere 50-150mm (working to calculate required thickness)
So when I google "warm roof design" I get this sort of thing:
So far so good. But they don't discuss how the layers are actually joined and kept in place, to deal with strong winds, etc.
I have two specific queries with my builders:
- How thick can the insulation be for the "optional timber deck" to be optional? We've seen some suggestions anything over 50mm PIR would not be structurally sound to affix shingles through PIR to the sub deck even though over-long clout nails do exist.
- Can I just add a timber deck on top of PIR board screwed through to my sub-roof? I've had people say that is quite normal but my builder is dubious that it would be strong enough and wants to build a frame on top of the sub-deck (say using 3x2 at 600mm centres for 75mm PIR, 4x2 for 100mm PIR). This is going to be a fairly big job and use a lot of timber which isn't cheap, as well as increasing roof weight and introducing thermal bridging.
So the builder voiced concerns it could get "sucked off" by winds. However he is a builder who does some roofing, not a roofer.
I hope this is detailed enough - any questions just ask. Build is already in progress so the main truss/purlin setup is already in place. By the way, the purlins and sub-deck are exposed on the inside as part of the design - no ceiling.