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unheated barn

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Guest User
6/16/2015
I have an unheated barn (not at all weather tight) with a shed roof of plywood/OSB (mixed) covered with roll roofing. The roll roofing is failing in places so I want to cover the whole thing with metal panels. Do I need to be concerned with condensation rotting the purlins? We're thinking of putting blocks under them to let water run down. What is the recommended size and frequency of purlins? The roof is 55' x 25' and the slope is 1.5/12. Also the roof is a bit saggy in spots; I've done some rebuilding and wonder how absolutely flat it needs to be. Thanks for all help and advice! I'm doing this myself and would like to get it right the first time.
Eric Novotny
An informed customer is the Best Customer!
6/16/2015
If the building isn't conditioned, it shouldn't be any more likely to form condensation than it was before with the roll roofing. Purlin size depends on span and snow load on the roof. Doesn't need to be laser straight and you can straighten it out with the purlins.
Guest User
6/16/2015
Thank you. I was wondering about condensation in the small space between the rolled roof deck and the metal. Is that an issue?
Eric Novotny
An informed customer is the Best Customer!
6/23/2015
Diane, It should not be. The metal is vapor impermeable as is the rolled roofing. As long as water isn't getting between those two via bulk transfer, there shouldn't be any ability for water vapor to get in there either. Good luck.

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