RBG51000 Square Galvanized Roof Vent, Mill, 50 SQIN Net Free Area Per Piece, 16-3/4" x 18-3/4" Base, 9-1/2" Diameter Opening, 8" x 8" Aluminum Screen, Square Dome, 1 Piece Dome Construction For Greater Strength, Louvered On 3 Sides For Maximum Air Flow, Fits All Roof Types, Good For 3/12 To 10/12 Roof Pitch.