Venting two bathroom exhausts through one roof hood

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Flyover

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I have two bathrooms with exhausts that are maybe 6 feet apart. I believe they are the same type of exhausts. I need to duct them up through the roof and I'm wondering what I can and cannot do, both to save money and to reduce the number of holes I need to cut in my roof. I've read a few things saying they need to be vented separately, but I'm not 100% clear on this.

I guess I'm looking for someone who knows to tell me "Yes, you always need to vent them separately" or "You can vent them together with ducts shaped like an upside-down Y" or "You can only do it if XYZ".

If you need more info I'll provide as much as I can.
 

bud16415

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I’m no expert and don’t know what code says, but my first thought would be will the flow be all in one direction or will some of the flow back feed down the other bathrooms fan.


I suppose you could wire both switches to turn on both fans like a 3way switch and two lights and that would always provide a positive flow, or you could put dampers (check valve) so that flow could not back flow.


We can wait and see what the pros say.
 

nealtw

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I saw a wiring diagram where were they both ran with two timer switches wired in parallel. It would be the same as doing a line fan in the attic.
After that I would say no, separate controls separate ducts.
 

Flyover

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I’m no expert and don’t know what code says, but my first thought would be will the flow be all in one direction or will some of the flow back feed down the other bathrooms fan.
That's what it warned about in the sources I read. I just wasn't sure if this can be fully mitigated by the "design" of the ductwork, like if the two ducts joined at an acute angle right under the hood. Maybe is it even possible for that final "upside-down Y" shaped joint to keep the intakes separate, with a divider down the middle inside of it, so it's effectively two separate ducts right next to each other that happen to terminate out of the same hood? But I don't know if they make a product like that. I don't think I have the ability to fabricate one.

Again, my biggest concern is about putting more holes -- i.e. more potential for leaks -- in my roof than I absolutely need to.
 

oldognewtrick

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The roof vents are easy to install and if you follow directions of the installation won't be a leaking issue. Why take the chance of having to redo it because it didn't work right the first time.
 

pjones

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Install two lines into one vent and you risk potential condensation issues. It’s more likely you will get a leak from installing two exhausts through one vent then you will from installing two roof vents.

Roof vents at designed to keep the water out. wont leak if installed right. If you are unsure how to do it then I suggest hiring a roofer.
 

pjones

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As mentioned above you also will get exhaust air back feeding into the other washroom. It is best to keep them separate. To don’t want the issues that go along with the shortcut you are suggesting
 

bud16415

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That's what it warned about in the sources I read. I just wasn't sure if this can be fully mitigated by the "design" of the ductwork, like if the two ducts joined at an acute angle right under the hood. Maybe is it even possible for that final "upside-down Y" shaped joint to keep the intakes separate, with a divider down the middle inside of it, so it's effectively two separate ducts right next to each other that happen to terminate out of the same hood? But I don't know if they make a product like that. I don't think I have the ability to fabricate one.

Again, my biggest concern is about putting more holes -- i.e. more potential for leaks -- in my roof than I absolutely need to.
If I were going to do it at the inside roof area I would install a T running to the vent and then each leg of the T I would place one of these. https://www.homedepot.com/p/VENTS-US-4-in-Galvanized-Back-Draft-Damper-with-Rubber-Seal-KOM-100-U/205715168

Then attach your two runs to these.


This might be a method for someone that has a roof vent and wants to add a second without involving getting on the roof and making new holes.


If you don’t have the first one there and don’t have a location to vent out the side wall someplace and are going to have to involve roof work you might as well put in two.


Another method would be an attic mounted unit. https://www.familyhandyman.com/bathroom/exhaust-fan/use-an-in-line-fan-to-vent-two-bathrooms/


Again not a pro here and trying to be helpful is all.
 

Flyover

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Thanks all, these are great comments. It sounds like my worries about roof leaks are overblown, so I guess if I eliminate that concern I'm willing to do the two separate vents.
 

Jeff Handy

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Venting a fan out the roof is common, but in OH they will be occasionally buried in snow, maybe even iced over depending on where they end up.

Raccoon and squirrels sometimes like to try to rip them apart to get into comfy quarters.

And ice and snow can form a ledge under it, then snowmelt or winter rain can back up and defeat the flashing.
The flapper inside can also freeze up.

I always like to go out the side of the house if possible.
There is less chance of the vent being blocked.
And roof vents are harder to maintain or inspect for trouble.

And because a lot of the duct run will be horizontal, there is less chance of condensation running down the vent and dripping out of the fan, or staining the ceiling around it.

I am not a fan of venting out of the soffit either.
 
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Flyover

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And roof vents are harder to maintain or inspect for trouble.
Why's that? On my house I'm pretty sure the side would be slightly more difficult to access -- but is there some other aspect of roof vents that makes them harder to maintain?
 

Jeff Handy

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The siding surface of most houses is much easier to access with a ladder than having to go climbing up onto a roof.

And if you place a bath fan vent on the roof, but lower down close to the soffits so you can get to it easier, it is now in the zone most likely to experience ice dams or leaking problems.
 

Flyover

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The siding surface of most houses is much easier to access with a ladder than having to go climbing up onto a roof.

And if you place a bath fan vent on the roof, but lower down close to the soffits so you can get to it easier, it is now in the zone most likely to experience ice dams or leaking problems.
Below is a rough approximation of what my house looks like from an aerial view. You see the driveway in the upper right; that is where I can easily place my 8' ladder and climb onto my roof. From there I can just walk over to the vent at Position 2 on the roof (where I labeled it with a red "2"). If my vent was at Position 1 on the side of the house (labeled with a red "1"), it'd probably be at least 12' and maybe as much as 15' above the soft, uneven ground there. I'm a lot less sure about being able to access that safely and easily from my 8' ladder.

Screen Shot 2019-11-22 at 1.31.30 PM.png

That's why I was wondering if there's something about, I dunno, mounting vents horizontally or something, that made them easier to maintain. Or if it was just an access issue. Because for me I think the roof would be easier to access, at least given the ladder I have.
 

Jeff Handy

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If you have a house you should have an extension ladder.

You can buy cheap new ones, or super cheap but stronger used ones on Craiglist or from local ads or eBay.
Or garage sales.

16 feet, 20 feet, up to at least 32 feet.
They store very compactly.

You can lay a piece of old carpet or old welcome mat to give support on soft grass.

And you can buy ladder leveler legs that adjust each leg for un-level ground.

Or do what I do, just kick a depression in the dirt to lower down the high leg level.

For more severe uneven level, dig a little quick hole with a hand trowel under the high ladder leg, takes about 20 seconds.
When done, put the dirt back in the hole, green side up.

And a vent mounted on the side of the house would be mounted vertically.
The vent line in the attic would connect to it horizontally.
 

Flyover

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I hear ya, but you're asking me to buy (and store) a whole new piece of equipment. I just bought my 8' ladder, at auction, at it was still $80! (Granted, it's a really nice fiberglass Louisville Ladder.) Seeing as it gets me up onto my roof fairly easily...

And a vent mounted on the side of the house would be mounted vertically.
The vent line in the attic would connect to it horizontally.
Right, good catch.
 

Jeff Handy

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Borrow an extension ladder from a neighbor.
Pay them back with a few beers and a burger off the grill.
 

oldognewtrick

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The issue with mounting it on a gable end is the ability to provide flashing not the upper section of the vent. If you have vinyl siding, it may be tricky if you've never done siding before. Any type of hard surface will require the top part of the flange to be tucked under the siding material. Simply caulking the top will eventually fail and allow water being the vent.
 
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