SMACNA HVAC Duct Construction Standard, Metal & Flex The new construction standards contain re-rated rectangular, round, oval and flexible duct constructions for positive or negative pressures up to 10 inches water gage (2500 Pa). Widths go to 120″ in gages 26 to 16. Diameters go to 84″ in gages 28 to 16. New performance requirements facilitate equivalent ratings for non-illustrated construction. The data have been reorganized for easy use and reference; includes metrics. Other revisions occur in duct liner, hangers, fittings, vanes, dampers, tie rods and tests. Lead shielding has been added. Seal classes, casings, louvers, rooftop vents, sheet gage tolerances and accessories are retained.