Fireside Knockdown style Multi Flue Hinged Chimney Caps,Screen Height 10",Dimension 18x24 All caps are manufactured with 304 stainless steel and come complete with easy to install instructions and LIFETIME WARRANTY. The sturdiest most durable cap on the market. When measuring, please subtract 3" from the length measurement and 3 " from the width measurement.This will take into account the fold down hinged mounting strip width around the chimney cap.