Unvented flat roof over unconditioned space

[Wayne Whitney]

Hello,

I'm building a new, detached, residential garage in a Mediterranean climate (Berkeley, CA). Is there any need to ventilate a flat roof over unconditioned space in this context?

Without any space conditioning, I don't see how the roof assembly could develop a temperature gradient that would lead to condensation. But perhaps I am missing something? I will have a gypsum board ceiling on the interior, so the joist bays will be enclosed.

Thanks, Wayne Whitney

[PeterD]

Check with the building inspector's office. Considering the location there are probably many meaningless requirements that you will have to meet, based on the concept that it must be 'green' and even though you don't plan on it, it may be used some day for other purposes.

[jloomis]

One reason I would ventilate a roof that is being sheathed is that it may some day be conditioned. Also, the temperature inside may be warmer than the exterior temp. and thus a condensation will occur. I live in Northern Calif. and do ventilate whenever possible since I have seen the effects of no ventilation and problems with ceiling members and sheathing damage due to water and moisture. johnloomisconstruction.com

Hello,

I'm building a new, detached, residential garage in a Mediterranean climate (Berkeley, CA). Is there any need to ventilate a flat roof over unconditioned space in this context?

Without any space conditioning, I don't see how the roof assembly could develop a temperature gradient that would lead to condensation. But perhaps I am missing something? I will have a gypsum board ceiling on the interior, so the joist bays will be enclosed.

Thanks, Wayne Whitney

[v8z]

Longevity of the roofing materials would be a consideration - with no venting the elevated temps would significantly reduce the life of the roofing.

Unless you're planning a poly membrane over foam, which is what I would recommend on a flat roof anyway.