Venting bathroom through a flat roof
I am interested in learning if there is guidance or code that would cover venting a bath fan straight up through a flat roof. Our state’s Weatherization Field Guide indicates that vent terminations shall not be allowed on flat roofs. My interpretation is that you are not allowed to place a fan vent termination on a flat roof. It also indicates that on sloped roof the termination should be elevated 18″. There seems to be a lack of what is an acceptable venting configuration for a bath fan when you are forced to go through a flat roof. I am in MD so 18″ seems like a safe distance to just run PVC and put a goose neck (180) on the end with screening and a damper somewhere inline. Your thoughts are appreciated.
Thank you,
Matt
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Replies
Matt,
I'm not familiar with the provisions in your weatherization field guide. Penetrations through low-slope (flat) roofs can be problematic, for at least two reasons: they can leak, and they can get buried by snow.
If the bath exhaust fan is used regularly, it will help melt snow. In Maryland, in any case, I'm guessing that deep snow is rare.
All of that said, commercial roofers install exhaust fan terminations through flat roofs all the time. Make sure that your roofer is familiar with flashing methods for making this type of penetration leak-free.
Here is a link to a termination from Broan -- but you probably prefer a product with a smaller diameter:
http://www.broan.com/products/product/61467229-d150-426c-aaf8-7eb812bae43b
Your proposed termination -- a PVC pipe with a 180-degree gooseneck -- should work, but it will introduce a lot of static pressure to the duct system. If you go that route, it makes sense to oversize the duct.