Tin Roof with osb and breathable membrane

[tim79]

Thinking about stopping the condensation beneath a tin roof there is lots of threads on here but mostly is about insulation but I don't want to insulate I just want to stop the condensation.

I have a timber framed mono pitch roof with flitch beams. Quick sketch attached.

Could I OSB on top of the rafters then roll out breathable membrane directly on top and then screw the corrugated tin through the highs through the membrane into the OSB. Would this be an issue with the membrane not being breathable enough to cope with the condensation as it will be laying directly on top of the OSB? Would it be an issue where the tin is screwed down as it will penetrate the membrane.

Or will I have to double/triple batten it all giving air gap beneath and above the membrane allowing the water to go downhill to the bottom and then cross batten to take the tin on top? I can't put the membrane onto the joists directly as it will sag creating pockets as the rafters run horizontal and then the third batten is because the corrugated tin runs vertical.

Hope that makes sense!

Any thoughts?

 

[freddiemercurystwin]

You would create a ventilated cavity between the OSB and the metal roof, so you would a breathable membrane on top of the OSB, then counter battens running parallel with your roofing sheets, then battens on top of the OSB and then the metal deck, then the gap between the metal and the membrane should be vented to the open air at either end of the roof. In principle this but ignore the fact that it's a pitched roof and insulated:

Any condensation occurring on the underside of the metal deck either drips onto the membrane and out into the gutter or evaporates into the air moving through the cavity.

 

[tim79]

So the airflow between the membrane and osb is enough to allow the membrane to breathe. I was worried with the OSB and the membrane touching that would make the membrane not able to breathe and cause it to easily penetrate water.

Normal OSB is okay?
Normal Breathable membrane?

I better get some tape and have a go at that.

Thank you

 

[freddiemercurystwin]

Provided it is properly vented at both ends of the roof, it should work. Any roofing membrane off ebay should be OK, it's a shed afterall. Normal OSB provided you don't take long to get the finishes on ie it doesn't get soaked in the rain.