There are several accredited methods of generating a welded joint for grease exhaust ductwork. The selections will depend upon the code in your jurisdiction but should contain flanged, telescoping, Bell variety or overlapping joints. Promat has made self-supporting systems with no steel liner, such as PROMATECT®-Advert, PROMATECT®-L500 and MASTERBOARD®, which https://greasetightextractductwor13455.webbuzzfeed.com/37490473/the-ultimate-guide-to-grease-tight-extract-ductwork