How to connect vent duct to roof jack?

Chris StrombergerMarch 23, 2007

We are going to have a 10" round duct installed from our kitchen vent hood and it will exhaust thru the roof. I can't find a roof jack off the shelf anywhere that will fit that large a duct, so we might end up getting one made. So, I don't know what size to make it, and I think it depends on how the duct is attached to the roof jack. The roof jacks I've seen are tapered, wider at the bottom than the top. One person suggested that the *top* of it (the narrow end) should be 10" so you can attach the duct there. Someone else said it should be 10" at the bottom and narrow to 8" at the top.

So, having never attached a duct to a roof do they normally attach, and what size does it need to be? It's a smooth metal duct.



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The link below is to a vendor that sells roof caps that accept 10" duct. Pricing should also be much less than having one made! Don't forget to make sure there is a high-quality damper in this duct to minimize leakage into and out of the house; the Fantech models are good models (personal experience). The suggested location for the damper is at the thermal interface of the house - usually where the attic insulation terminates in the attic.

    Bookmark   March 23, 2007 at 2:32PM
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You might try a roofing supply company or a sheet metal fabricator.
I would attach the ductwork to a starting piece. It's a 10 " short piece of duct with bendable fingers on the end. I cut a piece of plywood to fit between the joists and then cut the hole size of the duct. Put the fingers through and bend it back on the plywood. Tack nail it in a few places so it stays where you put it. Then screw the ply to the roof deck. Then just connect the feed duct to it and put a few short screws through it and wrap it with aluminum tape. Pull the insulation over the duct and tape that.
If you can't find a starting duct in the size you need, just cut the end of a duct into bendable segments.
If it's a 10 " duct ,then it's 10 from start to finish.

    Bookmark   March 23, 2007 at 2:34PM
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First, does the hood require such a large duct. Even most residential "commercial style" hoods only require 8". Be sure to follow duct guidelines in the installation manuel or performance may be affected. It is possible to make duct work too large and cause the air velocity to fall beyond an acceptable rate. You should be able to find this size roof jack from a manufacturer at a much better price than having it fabricated. A 10" jack will have a 10" opening at the roof decking, which will allow you to slip the crimped end of a piece of duct into it, support the duct with strapping or angle clips, tape with foil tape, and seal with duct mastic. Or, before the jack is installed, attach a starting collar( available at ac supply) to it, so that the collar will stick below the roof decking and provide an easy connection for the duct. Use foil tape on all joints, not cloth duct tape which will fail in a few years.

    Bookmark   March 31, 2007 at 8:51PM
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