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  1. #1
    Join Date
    Feb 2007
    Ontario, Canada
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    Question Bathroom Exhaust - Where to Vent (Modified Mansard Roof)

    My parents are getting their roof redone this fall. It's a variation of a mansard roof - basically flattish roof (pitched towards a drain in the middle), and second floor walls hidden by not-quite-vertical shingled roof sections.

    I suggested this would be a good time to rectify a bathroom fan venting issue (builder didn't install them back in '70s since there's a window in bathroom, and at some point a fan was installed but just vents in the foot-high ventilated attic/roof cavity - yuck). Roofer is more than willing to cut in and seal whatever vent cap the client wants but the question is:

    Would you go up through the flat(ish) roof (high enough to clear typical snowload)? Or vent it sideways (away from windows I suppose) through sloped shingle section, which might mean a longer duct?

    Fan is a 110 cfm pretty standard unit, 4" duct if it makes any difference.

    Thanks for any advice you can offer. Archived posts and Hvac-Calc already saved them from installing an oversized central air unit two years ago (3 quotes but contractors were all sizing it on square footage alone, half to one ton bigger than what the calc came up with).

  2. #2
    Join Date
    Dec 2007
    Cedar Grove, Wi-Sheboygan
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    For a bathroom vent your best bet would be to avoid a thru the roof penatration if at all possible, and vent the bath thru a side wall of the house would be alot better then anything coming up thru your roof, It will be one less place to have any leaks to worry about and you will never have to worry about any snow or ice to cover the vent. Sounds to me your parents have a flat roof which right there is cause enough to not want to have anything coming up thru that roof as it doesn't take much to have water to start puddling especially if the roofers did not do there job right from the start. So If it were me I would vent thru the side wall of the house easy to do and in the end the best location for anyhome not just with a flat roof.

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