55.75 In.H-W 4.5 In.L Aluminum Backdraft Dampr, Damper Application Backdraft, For Nominal Duct Diameter 56 in, Frame Material Galvanized Steel, Number of Panels 2, Maximum Air Velocity 2000 fpm, Overall Height 55 3/4 in, Rough Opening Diameter 56 in, Rough Opening Shape Square, Fits Fan Diameter 54 in, For Duct Shape Square, Free Area 21.5 sq ft, Overall Width 55 3/4 in, Frame Material Thickness 20 ga, Overall Length 4 1/2 in, Frame Depth 2 in, Rough Opening Length 56 in