Roof Jack for Exhaust Vent in Existing Foam Roof

Xena99September 6, 2013

We are installing a 6-in.duct to exhaust a range hood. It must go to the roof, as the other option is more complicated (cut into stucco) The roof is an existing foam roof and we are confused about how to secure and seal the duct stack with a roof jack.

Most information about installing exhaust ducts pertains to other roof types and roofs with attic access below. The only information for foam roofs that I found indicates that the roof jack should sit upon the existing roofing. See attached specification from Dura-Foam.

My question is: in the Dura-Foam example how is the roof jack secured to the roof? Simply gravity and mastic?

More importantly, how is the duct stack secured to the roof jack? With duct tape? -- and then mastic??

Just to be clear, we cannot access the underside of the roof; that is, there is no attic access for a person to get in. We think we can extend a drill bit through the ceiling cut the 3 feet to mark the center point for the roof cut, but we did not plan to tear out the kitchen ceiling.

Thank you for your help!

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Sorry I could not load a pdf, so here is the link. You have to open and download the Exhaust Vent specification.

Here is a link that might be useful: Dura Vent Exhaust Vent

    Bookmark   September 6, 2013 at 4:56PM
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The ducting is not suspended from the roof jack and especially not that short a length.
Your approach for locating is correct, but in addition, cut a small hole and adjust from there, allow the ducting to protrude above the foam and use spray foam as a filler.

Use a urethane caulking to attach the roof jack to the foam, then lap the edges of the roof jack with fiberglass tape saturated with the urethane caulking to seal.

    Bookmark   September 6, 2013 at 5:20PM
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Ooohh, very good information! Thank you!

Should we also use fiberglass tape and urethane caulking at the roof jack / duct joint? Or ??

We will anchor the ductwork inside the house using some sort of strapping, but about 4-5 feet below the foam roof, so that is why I am concerned about secure connections.

Many thanks!

    Bookmark   September 6, 2013 at 5:53PM
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All of the duct connections should be reinforced with a quality duct tape, the $12 a rl. not the $2 a rl.
If you are prudent with the size of the roof penetration, just the use of foam as a filler will stabilize the ducting.

    Bookmark   September 6, 2013 at 6:43PM
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I found this Eternabond tape which this site indicates is good for both flashing the roof jack and sealing ductwork. It's expensive, but sounds like just the thing! See the link below

Thank so much snooyb!

Here is a link that might be useful: Eternabond Tape

    Bookmark   September 7, 2013 at 10:59AM
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You are right and for less than $30 in urethane and roofers tape, it's your choice.

    Bookmark   September 7, 2013 at 12:09PM
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