My cooktop Vent hood has a 6 inch round outlet. I intended to run 6 inch round duct right through the roof, which is really the only option. But I’ve been having trouble finding a flashing collar for 6inch duct. Can I reduce to 4inch or am I asking for trouble. I’m sure I already know the answer to this. I have a very shallow pitched roof with roll roofing on it. Suggestions welcome. Thanks in advance.
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Stick with the 6". There should be no problem finding a 6" roof jack.
If you can't find one, any sheet metal fabricator will be able to make you one. Just bring them the pipe and the correct roof pitch. You will need a storm collar too.
I'm a firm believer in stainless steel instead of galvanized. Specailly after working on the wet, salty Wash. coast