Advice on how to connect flex duct from bath fan to this roof vent

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Old 11-28-20, 10:39 PM
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Advice on how to connect flex duct from bath fan to this roof vent

Hi, I recently had a new roof put on and requested the roofers add a roof jack above the bathroom so that I could install a bath fan where there was none previously. They put a 6" T-top vent on the roof, and from inside the attic it looks like this:

https://i.imgur.com/CMYrTYk.mp4

I was expecting to see something (metal duct pipe) protruding down into the attic space that the flex duct could be attached to. How would you connect the flex duct to this opening? Using what connectors and attached how? Thanks in advance for any advice.
 
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Old 11-28-20, 11:04 PM
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Well they didn't do you any favors there. You would almost need to fasten something like this to the underside of your roof deck, using short screws and silicone.

https://www.homedepot.com/p/Master-F...LRF2/100324454

Then you could attach your duct to it with a clamp. But that's not standard by any means. Just one idea that could make it work.
 
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Old 11-28-20, 11:22 PM
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Thanks so much for answering. Yeah, It would have been nice if the roofers had attached a take-off to the vent hood before fastening it to the roof, but they didn't, so now I'm trying to figure out how to attach one after the fact, or some other solution. Something like the part you linked to, screwed to the plywood decking and sealed with silicone, is definitely an option. I'm wondering if I enlarged the hole in the plywood by a couple of inches, could I attach something like this to the metal flashing that is part of the vent? https://www.lowes.com/pd/IMPERIAL-7-...Off/1000228293

My worry about screwing something with a flange onto the plywood is that there would still be potential for the moist air from the exhaust to escape between the plywood and the metal flashing...not sure how airtight it would be and whether that is a concern.
 
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Old 11-29-20, 01:41 AM
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I think they just made more work for you....

I would be tempted to finish cutting roof decking to better match the diameter of the vent stack. Run a couple of 2X4's, between the trusses, spaced so you could run a short section of crimped duct pipe like:
https://www.lowes.com/pd/IMPERIAL-6-...t-Pipe/3134337
up into the vent, smooth end inside vent. Screw the duct pipe to the 2x4's to hold in place. Could build a box around pipe, so you would have support on all four sides. Than attach your flex venting to the lower crimped end.

By doing it this way, you are not risking screwing through roofing and causing a leak. The vent pipe will still be supported enough to stay in place.
 
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Old 11-29-20, 09:44 AM
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Trying2Help, thank you for that idea. I had thought about inserting a short piece of duct pipe up into the vent, but was unsure how to support it. Your explanation is great and clear and I can see how it would work and I think I could do it. There is not much space in my attic and the vent is fairly close to the eave side, making work up there really a pain, but not impossible.

You guys pointing out that the roofers didn't really help me, was also good to know. I may call them and see if they are willing to come back and re-do the install of the vent, putting a take-off on it this time. I kind of doubt it since the roof job was 9 months ago, but it can't hurt to ask.

Thanks again for all the advice XSleeper and Trying2Help.

 
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Old 11-30-20, 08:39 AM
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Hope it all works out for you.
 
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Old 12-03-20, 10:13 AM
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It all worked out! There was not a lot of vertical space under the opening (like 12" or so), which limited what I could do, but also it meant that the duct would be lying on the joists and its weight would be supported.

First I used a Dremel to cut away the plywood that was obstructing the vent hole.



I used a 6-in to 5-in reducer, stuck the 5-in end up into the vent, it fit nice and snug. Then I taped the seam on the inside with aluminum tape. At that point it felt very solid and secure, and i knew it would not be weight-bearing, so I decided it didn't need any other support.





I attached 6" insulated flex duct (because the fan I am installing has a 6" duct connector) to the 6" end of the reducer, using a panduit strap and HVAC tape. Then I pulled the insulation up and used another panduit strap to secure it.



And that's how it turned out. Thanks again guys for the ideas.
 
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Last edited by MarlaM; 12-03-20 at 11:56 AM. Reason: added details
 

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