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Code in many states requires 18 gauge ductwork if it is steel, 22 gauge if it is stainless. Galvanized ductwork can be at a minimum gauge of 18 as long as it can be welded. 16 gauge is sometimes used because it is easier to weld the seams. Welded grease tight seams are required. Code in many states requires 18 gauge ductwork if it is steel, 22 gauge if it is stainless. Galvanized ductwork can be at a minimum gauge of 18 as long as it can be welded. 16 gauge is sometimes used because it is easier to weld the seams. Welded grease tight seams are required; all external seams to be welded liquid tight. Some good quality duct work has integral grease troughs run the length of the hood with removable grease catch cups. A common type of duct work inside the building (not exposed to weather) is 16 gauge hot rolled carbon steel with all seams and joints welded liquid tight. Exterior ductwork (exposed to weather) is 16 gauge galvanized steel fabricated in the same manner. Exhaust duct to hood connections should use a bolt/flange type with high temperature (1500 degree) gasket. The ductwork should be sized to provide a minimum of 1500 fpm and a maximum of 2200 fpm air flow.