Venting two bathroom exhausts through one roof hood

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Jeff Handy

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Use a square mounting block made up of pvc trim board (sometimes two pieces glued up double thick) with a hole bored through with a big hole saw.
Aluminum Z flashing can be tucked up and under on top, and along the sides.
Trim the top edge to run an inch wide.
You can flash the bottom same way, but it just ends up laying over the siding, not folding up onto the block.

Then mount the typical plastic vent with flappers, like for a dryer, it covers all the folded over flashing edges.
All the edges will need silicone caulk.

Yes, not as ideal a flashing situation as you would get with the big flange of a roof vent.
But it is a vertical surface, so water will be shedding better anyway.

How many coach lights have we seen hanging off a house, with no mounting block or even a trace of caulking?

You can also get a pre-formed mounting block, usually octagonal, that has side edges which are shaped to follow close to the profile of vinyl or aluminum siding.
Plan your hole so the block sits in just the right place, in the center of the mounting block.

Squirt a thin bead of expanding foam inside along the top and sides, and then place the block onto the siding.
Surround the nearby siding with blue painters’ tape

Clean up the excess once done expanding, add a few screws which the vent will cover, caulk around the vent.
 

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