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How do I install an exhaust fan on a flat roof

Posted by ecranny (My Page) on
Wed, May 18, 11 at 16:32

I hope this is the right forum for this question...

I am thinking about installing a large blower unit for a cooking range ventilator on the flat roof above my kitchen. The roof looks like a torch-down modified bitumen with metallic paint over it. The blower unit is 30" by 26" and 12" high. I was thinking I could build a 2x4 frame (screwed and cemented onto the roof sheathing) encased in a plywood box and mount the blower on top of that. I am not sure how to go about weather-proofing the whole thing, and I don't have any experience with roofing, so I am looking for advice on what materials/techniques to use to seal it all up. I am really looking for a way to do it with minimal impact on the existing roof - all the holes for screws and the ducting would be inside the plywood box - but I will need to seal the box where the blower sits on top, and also where the box meets the roof. The box itself will need to be waterproof - is there some kind of paint I could use for that?

I live in Washington State, and it rains pretty heavily for 8 months of the year, so it is important that I prevent all leaks.

Thanks in advance for any advice you can give me


Follow-Up Postings:

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RE: How do I install an exhaust fan on a flat roof

You mean a range hood. 30" by 26" and 12" high is the size of the hood inside. Read the instructions, it probably only needs a 4" vent.

Do not go cutting holes in your roof if you confuse interior size with vent size.


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RE: How do I install an exhaust fan on a flat roof

This is just one example of many metal caps. A wider flange would be better for your application. Building out of plywood is asking for problems. The diameter hole is determined by the size of the vent pipe plus the required by code clearance between the sheathing/framing/combustibles and the vent pipe although I dont see range hood venting pipes getting all tha hot. Ask your building dept. for info on those clearances.

Here is a link that might be useful: vent cap


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RE: How do I install an exhaust fan on a flat roof

Thanks for the response, but I am afraid I didn't make myself clear. The blower unit is an exterior fan, 1500 cfm, and it is the fan unit that is 30" by 26". I want to mount the fan on the roof. It does have a 4" flange all around it. I want to build a box on the roof that the fan will sit on, hence the plywood box. I need to find out how to weather-proof the box, and seal it where it sits on the roof, and where the fan unit sits on top of it.

gammyt - I do not mean a range hood. The range hood will be inside the house, with a 10" duct going through the ceiling and into the fan on the roof. I will be cutting a 10" hole in the roof for the duct.

sierraeast - its not a cap, its a large rectangular blower. Luckily my building dept doesn't care about range hood venting systems (except they do care about downdraught systems)

Hope this make things a bit more clear


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RE: How do I install an exhaust fan on a flat roof

Im wondering if you would be better off having an hvac/sheet metal shop look at the possibility of making up a box out of galvalume rather than the plywood route. Another avenue would be to line the ply box with metal cladding. Any type of paint or sealer/coating on plywood would be high maintenance and need attention often being with the high rain averages in your area. I would look into a reputable metals shop and get their take. An hvac shop would more than likely have answers as they no doubt have run into these types of situations if they are reputable and experienced and have been in business for awhile.


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RE: How do I install an exhaust fan on a flat roof

Flat roofs get a curb built up and protected with roofing material and flashing.

It is not especially hard, but requires a good knowledge of roofing material and a lot of attention to detail.

The overall goal is to NOT rely on calking of ANY kind to keep out water.
That is the job of flashing and counter-flashing.

Bitumen should be wrapped over the curb and the corners sealed correctly (if the cuts are down carefully and correctly the existing material can often be used to cover the curb with the only seams at the corners).

Counter flashing then covers the joint between the fan enclosure and the bitumen so that water is shed without having a chance to enter.

Things get even more critical if the flat roof is designed to hold water for later release (not common in residential use).


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RE: How do I install an exhaust fan on a flat roof

I am looking for the same design detail. I need to intall a 1200 cfm exhaust fan on a flat concrete roof over a range rood with a 10 in round exhaust duct. Obvioulsy the detail needs to make the fan secure, and protected from weather.

Anybody have knowlege to help solve this problem.


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RE: How do I install an exhaust fan on a flat roof

"Obvioulsy the detail needs to make the fan secure, and protected from weather. "

The fan itself should not need any protection if it is designed for exterior use.
Every restaurant has them for the kitchen ventilation.

The detail is on making sure the box they are typically mounted on is correctly installed to prevent leaks.

A lot depend son what type of flat roof you have.
Built up can be a real PITA to flash and seal, while EPDM is not nearly as hard.

In either case it is not a DIY thing, especially in a commercial setting.


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