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cbaileyau 08-23-2010 02:34 PM

Installing Vent in Flat Roof
So we just got a new microwave - the kind with the built-in convection oven. It's great, but the vent currently just recirculates in the kitchen, making it hot. I wanted to upgrade it to vent outside. Its not located on an exterior wall so its gotta go up through the roof. The problem I am having is our house has a flat roof with a fibered aluminum coating on it. What is the best procedure to install a vent in this type of roof?

Do I just cut a hole, glue down the vent flange, then cover the flange and some of the vent in more fibered aluminum coating to seal it all? Can I just patch fibered aluminum coating? what is the best way to prep the existing coating for the patch.

Thanks for the help.

Grumpy 08-23-2010 05:16 PM

You can do that if you want it to leak. I am assuming your roof is an asphalt based, perhaps modified bitumen or maybe smooth BUR. My suggestion would be to hire a roofer to do this because if you do it yourself, you are going to have to play with fire. Well, no, you won't "have" to play with fire but by using a torch and some torch applied modified bitumen you can achieve the best bond to that silver coat.

Here is the scope of work if I were proposing to do the job for a customer.

You cut the hole and run your vent through the roof.
I prime the area around the vent approximately 6-8" onto the field of the roof. This is necessary due to the existing silver coat.
I would then allow the primer to cure and flame prime the same area and allow that to cool and prime approximately 1' up on the metal vent pipe.
Cut my strips of modified bitumen, the method I use would depend on the shape of the pipe, I'd then adhere my modified bitumen strips to the roof and pipe with open flame, making sure to have a fire extinguisher within arms reach.
I'd secure at the top of the pipe mechanically either using termination bar if it's a square pipe or a hose clamp if it's a round pipe.
Finally I'd silver coat the exposed asphalt modifiued bitumen.

You could use adhesives and various other membranes, and really depending on the cindition of your roof maybe you do just slap some tar on it if you are considering a new roof in the next year or less.

OldNBroken 08-23-2010 08:00 PM

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Grumps, or install a roof flashing to run the pipe through with a storm collar. Then just fasten the base and 3-course or, better yet, torchdown to just the base of the flashing.

federer 08-23-2010 11:28 PM

you guys are good at this sh1t!!!

OldNBroken 08-24-2010 12:10 AM

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cbailey, it sounds like you intend to just put a normal vent cover on your roof. Don't do this. You need to run a duct a minimum of 12" above the roofdeck, then just cap it with a china cap. I did find one picture but you can't see the bottom of the roof flashing under the growing medium. Hopefully the rest will give you an idea of what you want to do.

BTW, this is also a bathroom vent coming through a flat roof.

federer 08-24-2010 01:27 AM

why at least 12"

seeyou 08-24-2010 04:41 AM


Originally Posted by federer (Post 490302)
why at least 12"


seeyou 08-24-2010 04:42 AM

Does the vent need to be dampered? Is there a damper on the fan unit?

cbaileyau 08-24-2010 08:29 AM

Grumpy, so what your saying is just put some duck tape around it and spray it real good with WD-40 and I should be good for 30 years. Thanks for the detailed response. Definitely sounds like this is something that should be left to the pros.

OldnBroken, we are lucky to see an inch of snow a winter so, height wouldn't be much of an issue.

Maybe the best option would be to hang a hard right in the attic and cover the 8ft to go out through the wall. It's a little longer run, and I would have to punch through the 12" 80 yr old block wall, but There wouldn't be a risk of setting the house on fire and dealing with leaks in the future.

Dsmalen 09-01-2010 10:53 PM

longer Run
Ctting through an aluminum coated roof is dangerous even for old roofers, Grumpy is right, you would be best to do that and replace the roof at the same time if its not to large an area. It maybe old anyway if its coated already.
Your duct size is also important coming through the roof as you dont want to reduce the opening from the microwave , so your hole in the roof shold be probably 8" diam. or what ever the manudfacturer calls for.
Going through the attic if its not to far sounds like your best bet, I wonder if those in-line exhaust fans could be used as a booster for a long run if needed, maybe check with an electrician on that. I know they use them for long runs in bathrooms and they are venting wet steam.
Good luck

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