Air gap and choice of material?

[Taberg]

Hello,

I am planning to insulate the roof of my garage and have some questions about the size of the air gap and choice of materials. The idea is to lay hardboard with spacers against the roof decking for ventilation towards the eaves. The question is how large the air gap needs to be, as I have read everything from 25mm to 45mm, and whether a specific quality is required for the hardboard?

The roof is 28 m2 and has a slope of 6 degrees.

 

[larsbj]

25 mm is enough, but larger provides better circulation during new construction, it's usually 45 mm.

Regular masonite works, but I believe it should actually be oil-hardened.