How to install a blower on a flat roof

ecrannyMay 18, 2011

I posted this on the home repair forum - thought I should try here as well...

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

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Not an expert by any stretch - do own a home w/2 flat roof similar as you explain. I have done my own repairs, painted the alligatoriting surface w/rubberized paint etc.

I do know that penetrating a flat roof with what you explain is inviting a whole host of potential problems. And will look really odd. If the roof is intact, not leaking leave it alone. I also have to wonder if what you describe is within code.
We have flat roof above part of kitchen where range is & vented through exterior wall.

Find an alternative option -

    Bookmark   May 19, 2011 at 9:10AM
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Is yours a typical flat roof where there is little or no space between the interior ceiling sheathing and the roof? If so, then the typical reason for using a roof-mounted blower for the range vent (eliminating noise from the kitchen) may not apply to you so much. Most of the time a remote blower like that is located an attic distance away from the kitchen. In yours, you would be gaining less than a foot of "remoteness". So a good quality range hood with an internal blower may be the answer for you.

If you are tied to the external blower, consider checking around the area with roofing companies familiar with your type of roofing, or vendors who install skylights. I would think that the process of installing and sealing a skylight would be very similar to what you're envisioning. I'm not sure I'd DIY this.

    Bookmark   May 19, 2011 at 10:02AM
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see if the ventilator company offers a roof curb. This would be a metal framed box that is designed to seat the unit on it, and seal watertight to the roof.

    Bookmark   May 20, 2011 at 7:02PM
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If you google the roofing system you will find the necessary details but avoid thin paint-like coatings. The silver paint is there to reflect sunlight.

Anything that touches a bitumen (modified asphalt) membrane must be bitumen based; no synthetic rubber or PVC roofing material should be in contact with it.

You should hire a roofer experienced with this kind of membrane roofing. Water damage from roof leaks can easily exceed the expense of the roofer.

    Bookmark   May 20, 2011 at 7:17PM
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