Correct Layers for Cabin Roof and Should I Also use a Membrane? Information, help and tips on many areas of roofing including trusses, tiling, venting, insulation etc....

[Jay Bee]

I'm building a log cabin and I've progressed up to eve height. The roof is next. According to the plan, the layers for the roof, viewed from the inside out are:

1)19mm spruce boards
2) Celotex 50mm foil/foam/foil insulation
3) OSB
4) Bitumen tiles.

I can't help feeling that a membrane or two is missing from the mix. Should I add one and if so, where and what type?