50mm air gap on cold pitched roof

[Chanmenie]

How critical is the recommended 50mm air gap on a cold pitched roof.

my rafters are 197mm and I’d like to use 150mm PIR but that would only give 47mm air gap. 
I assume I could in theory pack the rafters with say 6mm ply but that’s a lot of work.

 

[MikeGrahamT21]

I wouldn’t be stressing over 3mm, 47mm is ample for an air gap.

[ETC]

50 minutes ago, MikeGrahamT21 said:

I wouldn’t be stressing over 3mm, 47mm is ample for an air gap.

Agree.

[Temp]

If you use a vapour permeable membrane I don't think you need a 50mm ventilated void. I believe you only need a 25mm void to allow the  membrane to drape.

 

The drape is to ensure water can't become trapped above tile battens causing them to rot. The drape forms a gap under the batten so dirt and water can run down to the gutter.

[Thorfun]

15 minutes ago, Temp said:

If you use a vapour permeable membrane I don't think you need a 50mm ventilated void. I believe you only need a 25mm void to allow the  membrane to drape.

 

The drape is to ensure water can't become trapped above tile battens causing them to rot. The drape forms a gap under the batten so dirt and water can run down to the gutter.

+1 to this. we used Proctor Roofshield and after consultation with our BCO she said that the air gap can be above the membrane. so we used counter battens and battens on the roof for the tiles and fully filled the rafters with mineral wool (well, almost, 197mm rafters and 190mm mineral wool).

 

just make sure your BCO is ok with it first.

[Temp]

7 minutes ago, Thorfun said:

+1 to this. we used Proctor Roofshield and after consultation with our BCO she said that the air gap can be above the membrane. so we used counter battens and battens on the roof for the tiles and fully filled the rafters with mineral wool (well, almost, 197mm rafters and 190mm mineral wool).

 

just make sure your BCO is ok with it first.

 

Yup. If you fully fill between the rafters the membrane ends up flat. You need the counter battens to raise the tile battens off the flat membrane and allow any water to run down.

[Chanmenie]

That makes sense thanks guys, I’ll check with BCO