Design for my shallow-pitched warm roof

[d000hg]

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:

• 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:

1. 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.
2. 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.

I wondered if the fact we have quite large spacings is a concern or not. If you picture a 2400x1200 sheet of 18mm OSB it is not going to have too many fixing points:

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.

 

[Leofric]

• 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.

Roof construction seems odd - the purlins are closer together than the joists What exactly are the 'trusses' or what you call the 'gables'

 

[d000hg]

Indeed it is a little unusual. The trusses/gables are basically solid log walls going right up to the apex of the roof, the purlins fit into them. This side elevation of an internal wall should make it clearer.

As I said the purlins are big (145x70) so it's almost like they are the rafters. They are cut on an angle so they are flush with the angle of the roof.

This design must have a name, but I wouldn't know what. I think I've seen it on some smaller brick buildings like garages before.

 

[Notch7]

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

I fail to see how wind could suck off the insulation if you fit 18mm OSB to it which is screwed through to the 145 x 70 purlins.
Once its screwed down with 6mm x 150mm CSK screws for 100mm insulation, itll be rock solid

The fascia will need to cover the ends off the insulation, so all round the perimeter the OSB can be screwed down.

If you are concerned go for 18mm structural ply instead which has greater pull out strength.

 

[d000hg]

I think his concern was the outer OSB would not be screwed through everywhere, only on the perimeter, since we don't have typical 600mm truss centres. But everything I've read and everyone I ask seems to reckon it's fine.
I was just recommended by a solar installer that BC would be worth running it past since they'll have to sign off on it, which makes good sense.

You're suggesting structural ply for the outer deck instead of OSB... that's an interesting idea, I hadn't considered different materials. Costs more but certainly a lot less than all that framing!

The guy is happy to do what we want but though this isn't his expert area he knows more than me so I want to make absolutely sure before we get him doing something he has doubts about.

Thanks.

 

[Notch7]

I think his concern was the outer OSB would not be screwed through everywhere, only on the perimeter, since we don't have typical 600mm truss centres. But everything I've read and everyone I ask seems to reckon it's fine

Its not ideal because you can only fix to the purlins, but theres enough to get plenty of fixings, I wouldve thought.

The only alternative I can think of is to lay your insulation down, then screw down 50 x 25mm battens at say 400mm centres -then screw the 18mm OSB to the battens using as many screws as you like.

 

[datarebal]

Glue

 

[d000hg]

Its not ideal because you can only fix to the purlins, but theres enough to get plenty of fixings, I wouldve thought.

The only alternative I can think of is to lay your insulation down, then screw down 50 x 25mm battens at say 400mm centres -then screw the 18mm OSB to the battens using as many screws as you like.

Would the air gap be any kind of issue, or not since it's "outside"?

Glue

The problem of the internet for advice when you don't know much is you can't tell if things are serious or silly suggestions at times!

 

[Notch7]

The problem of the internet for advice when you don't know much is you can't tell if things are serious or silly suggestions at times!

Glued warm roofs are becoming morr common -no metal fixings creating thermal bridges.
But you need different PIR and Im not sure you could glue to solid timber.

 

[Leofric]

This design must have a name

- hmm,
If it is any help ,helical fixings can be used for fixing through thick insulation.

 

[datarebal]

The problem of the internet for advice when you don't know much is you can't tell if things are serious or silly suggestions at times!

i am not joking

 

[d000hg]

i am not joking

No problem