12mm or 18mm OSB on roof.

[f0rmula]

I’m building a garage with a trussed roof. Trusses will be at 600mm centres. Slate covering.

Rather than having the breathable felt underneath the slates exposed to the inside of the garage, I’m covering the trusses with OSB3, then putting felt on top of the OSB. I was worried I’d damage the felt or dislodge a slate if I’m swinging wood around in the garage, so wanted to protect it a little.

Will 12mm OSB3 be sufficient, or might it sag over time? Should I use 18mm?

Cheers for any replies.

 

[freddiemercurystwin]

I'd be more concerned about condensation, whilst OSB is not airtight it will severely restrict the effectiveness of you breathable membrane, which incidentally is also required to have a void beneath to allow it to breath.

 

[f0rmula]

Thanks for the reply. Maybe if I drill some ventilation holes in the OSB, or suspend the membrane on additional battens?

Or maybe try a membrane that isn’t breathable? I have a DPM from a garage base that isn’t permeable. The garage has sufficient ventilation at the eaves, so I’m not using the roof for ventilation.

If neither of those options is viable though, how could I protect the underside of the slate/membrane from within the garage? Or do people not normally bother? I worry because I broke a slate on the house roof when poking a disassembled bed through the loft hatch.

 

[f0rmula]

Ah, so non-breathable membranes do exist. Could I just use something like this?

http://www.roofingsuperstore.co.uk/browse/pitched-roofing/non-breathable.html