Warm roof on log cabin

[Welshopen]

firstly, hello everyone. For sometime I have used these forums for info but never needed to post.

I have purchased and today, built a 3Mx4M log cabin purchased from Tuin.
It has an apex pitched roof.
As I write this the roof just has the T & G roof boards, ready for insulation, felt shingles etc.
The advice from the company who sells these is to lay the PIR on top of the roof boards and then, using long clout nails pin the roof shingles through the PIR and into the roof boards.
Now. Every where I have read advises to add a vapour barrier. And to add it under the PIR(warm side), on top of the roof boards.
This is ok but if I’m clouting 3 nails in each shingle, of which I estimate 60 plus on each side of the roof, isn’t the vapour barrier going to become useless? And possibly more concerning, potentially allow for water vapour to get through and trapped?
A friend advised to get breathable felt instead but I wasn’t sure if this was going to help, except I do like the thought of the breathable felt acting as a backup if any water should compromise the felt.

Can anyone help shed some light on this as I’m stuck!

Joe

 

[^woody^]

You don't need a vapour barrier nor a breathable membrane.

The problem I see, is trying to fix shingles through insulation and into the timber below it. The fixings will pivot as the insulation will not hold them rigidly above the deck board.

I'd suggest that another board of at least 12mm thickness is required above the insulation and the shingles are fixed directly to this.

 

[Welshopen]

Thanks for your reply, Woody.

No need for either. Ok wasn’t expecting that!

I see what you mean re the fixings. I suppose I could fix some 12mm OSB into the support rafters and walls. Then, as you say pin into the OSB.

Why is the vapour barrier not needed? Everything I read implies it’s a must!

Thanks

Joe

 

[Notch7]

Do you mean bitumin shingles, not real ones?

 

[^woody^]

Why is the vapour barrier not needed?

Because that type of roof does not need one, and the condensation risk of a log cabin is different to other types of structures.

Basically a vapour barrier won't be doing anything.

 

[Welshopen]

Do you mean bitumin shingles, not real ones?

Yes, bitumin shingles

 

[Welshopen]

Because that type of roof does not need one, and the condensation risk of a log cabin is different to other types of structures.

Basically a vapour barrier won't be doing anything.

I see, thanks again. I’ll get some OSB and sandwich the PIR. Then fix the felt shingles to that!

Thanks

 

[Notch7]

I think Id do as Woody suggests, overboard with a board, say 18mm osb, then nail into that.

You could tape the insulation joints which would form a vapour barrier -PIR insulation has a foil face which is a vapour barrier.

In any case any vapour can get through bitumin shingles -they must be a bit gappy.

 

[Welshopen]

I think Id do as Woody suggests, overboard with a board, say 18mm osb, then nail into that.

You could tape the insulation joints which would form a vapour barrier -PIR insulation has a foil face which is a vapour barrier.

In any case any vapour can get through bitumin shingles -they must be a bit gappy.

Nice one, will do!

Mate has some breathable felt, guess that can’t hurt as a backup for any possible leeks. It’s a low pitch.