39.75 In.H-W 4.5 In.L Aluminum Backdraft Dampr Damper Application Backdraft For Nominal Duct Diameter 40 in Frame Material Galvanized Steel Number of Panels 1 Maximum Air Velocity 2250 fpm Overall Height 39 3/4 in Rough Opening Diameter 40 in Rough Opening Shape Square Fits Fan Diameter 40 in For Duct Shape Square Free Area 10.2 sq ft Overall Width 39 3/4 in Frame Material Thickness 20 ga Overall Length 4 1/2 in Frame Depth 2 in Rough Opening Length 40 in