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Discussion Starter · #1 ·
Hi, Is there code or rule of thumb for the length of a zip-in screw for attaching the joints of round ductwork? I have a couple of bathroom exhaust fans that will be using galvanized 6" diameter rigid duct for one fan and 4" for the other. The connections of joints will be maximum two layers of sheet metal where the duct, elbow, or termination vent overlap each other.

I have read use #8, 1" long zip screws for 10-12" diameter duct, but these seem excessive for a 6" or 4" duct. (I bought 1" long at the HVAC supply store today, but when attaching some of the 6" diameter duct, the screws seemed unnecessarily long). (This is NOT for dryer exhaust). The screws seem to come in 1/2", 3/4", 1" and longer. When is each size used?

I have looked at the Mechanical Code 603.9, but it refers to SMACNA HVAC Duct Construction Standards for duct joints, seams, and connections. I do not have access to this.

I searched the HVAC forum, but did not see this question. There is a lot of good information though, that I have already put to use! Thank You!
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