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The above installation to comply with section 505 grease hood duct systems in the standard. mechanical code and to comply with National Fire Protection Associations 96 where applicable. 505.1 Construction Duct systems serving exhaust hoods removing smoke .and grease-laden vapors shall be constructed of and supported by steel (16 ga.) 0.0598 inch (1.52 mm) minimum thickness or stainless steel 0.0478 inch (1.21 mm) minimum thickness or of other material of equivalent strength, fIre and corrosion resistance as approved by the mechanical official. Ducts constructed of materials that are subject to corrosion shall be suitably protected when installed outdoors.. 505.2 Seams And Joints All seams and joints shall be made liquidtight with a continuous external weld. 505.4 Minimum Slope All sections of the duct system shall be constructed and installed without forming dips and traps and shall slope not less than 1 inch per foot (83 mm/m) toward either the hood or an approved residue trap, , SU5.6 Support ] )uct systems shall be properly supported and securely fastened in place at every change in direction. Supports or fasteners shall not penetrate any duet or plcmun, 505.7 Minimum Velocity . ] )uct systems shall be designed and installed in a manner to provide an air velocity within the duct system of not less than 1,500 ft. per minute (7.62 m/s), 505.9 Enclosure V crtica1 and horizontal ducts located within :i building of two stories or more shall be enclosed in a c~)1ltinuous enclosure extending from the ceiling above the hood to or through the roof A minimwn .clearance of 6 inches (152 lum) shall be maintained between the duct and interior surface of the enclosure. The fire resistance rating of the enclosure shall be a minimwn of 1 hour for buildings two through three stories in height and a minimum of2 hours for buildings four stories or more in 'height. the enclosure shall be used exclusively to enclose a single grease exhaust system and shall be used for no other purposes.