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I have a roof with two gable vents as well as slanted vents that look a lot like these one: enter image description here

I do not want to install an extra vent specifically for bathroom so I was wondering if I could use one of the existing top ones. It looks like from underneath there is a circular hole that I can push a duct in. The roof vents don't leak so they are solid even with really crazy rain, hale snow etc.

Can I install on existing vent? If yes, do I have seal it after pushing the vent in so that no humidity will return back down to the attic? What's the best way to go about doing this?

  • Why not use a soffit vent? They are nearly zero impact, and don't affect your roof line. I see some potential issues with trying to use one of these vents, namely if you block it off to ONLY vent the bathroom, you've restricted venting to the room, if you don't restrict it to just the bathroom, you could actually siphon the humid air back into your attic, and needless to say that would be bad. – BrownRedHawk Sep 11 '15 at 12:42
  • The roof of the upper house is about 600 sq ft with two large gable vents and two roof vents. I figured blocking one of the vents won't make much of a difference. Probably it would increase flow on the gables and the other remaining roof vent. Aren't soffits a bad idea since they are considered to be intakes? – Michael Sep 11 '15 at 15:32
  • Typically, vents are there for a reason, and unless you have someone especially familiar with roof construction, I wouldn't block an existing vent. Re. Soffits, typically, you block off a few feet of the soffit vent on either side so it cannot draft/siphon the moist air in. It seems odd to me that you'd have gable vents, central roof vents AND soffit vents. That combination would seem to work against one another. I would check and see which are truly open. – BrownRedHawk Sep 11 '15 at 15:42
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    What if I were to install a louvered vent on the gable side of the house? Cutting a hole on the gable should be easier than soffit (less crawling) – Michael Sep 13 '15 at 1:47
  • Seems reasonable – BrownRedHawk Sep 13 '15 at 1:48
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We've got several things going on here:

  1. This is a ridge-mounted, Slant-Back Roof Vent -- it's used to ventilate Attic air to the outside. It should not be used for double-duty for a Bath Fan exhaust. Warm moist Bathroom air will condensate on its underside and can cause moisture damage & mold on the roof sheathing.
  2. As a Slant-Back Vent, the original photo shows it installed backwards. The slanted side is installed to the top to shed rain water; unseen in this photo is the screened bottom side (see attached photo).
  3. As previously mentioned, most building codes require the Bath exhaust to be vented directly to the outdoors. Attic locations should use insulated duct to reduce condensation problems. The exhaust ducts can discharge through the roof, soffit or gable siding -- but a dampered Vent Hood should be installed as well.
  4. If this installation is to be done by an average Homeowner or Handyman, I typically recommend a siding-mounted vent. Retrofit roof vent installations require proper interlacing with roof shingles (not just gobs of tar); a poorly done job will result in a roof leak.enter image description here
  • It never fails to amaze me how many people, including those who should know better, do not understand that flashing is all about water flow. Installing this type of vent backwards, where the slant-back is actually part of the water deflection, almost guarantees moisture incursion into the attic space. – Peter Rowell Sep 10 '17 at 18:36
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Agree that effectively blocking off an existing roof vent is not a good idea for the long term health of the roof. In my experience ventilation is often lacking in that regards already.

Also the pipe should be as short as possible; the longer it is the colder it is at the end and the more water vapour will condense on the inside of the pipe rather than being piped outside.

Shortest route out is always best.

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It depends on the hole size under the vent cover. Most of those holes aren't even half the size of the cover. So, enlarging the hole for the bath pipe & larger would have no detriment. Additionally, the roof's or attic's natural flow will ensure that no humidity will blow back or remain inside at all & it'll actually assist the bath fan.

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I ran a main bath vent to a roof vent for many years after finding the duct terminated in a previously unvented attic. I simply hung the end of the flex duct up under the vent, and did not seal it in any way. It worked fine, and was certainly an improvement over the earlier configuration, with the following caveats:

  • You will see moisture accumulation on the roof deck, which could accelerate decay. Ideally, point the duct directly into the plastic or metal roof vent and avoid wood framing components.
  • You will probably see premature curling or other degradation of the shingles around the vent due to moisture and freeze-thaw cycles.
  • This solution doesn't include a secondary backdraft preventer such as a purpose-built roof or wall vent would typically provide. You're relying solely on the one included with the fan housing.
  • This is certainly not best practice or code-compliant. Do it with that in mind, you home improvement renegade.

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