18 Octagon Gable Vent The octagon gable vent is used to vent the attic. The 2-piece system is fully compatible with all siding profiles, colors, and types. Simply install the base when installing the siding, then snap on the cover later. Rectangular base makes siding cuts easier. Adjustable 5/8 to 1-1/4 for use with all siding styles. Each gable vent includes screen protection to keep pests out. Manufactured with ultraviolet-stabilized color, minimizing the effects of exposure to the elements. Rich textured appearance. Outside dimensions 18 . Ventilated opening dimensions 10 x 12-1/2 . Surface Dimensions 18 , Shape Octagon, Color White, Opening Dimensions 10 x 12-1/2 , Material Vinyl, Net Free Area 50.7 sq in, Pkg Qty 1