Fits outside flue tile dimensions of 18 1/2" x 20" to 9 1/4" x 20 1/4" Constructed of 18 ga 304 Stainless Steel with a 3/4" expanded stainless steel mesh. These caps mount to the outside of the flue tile providing greater free air space for better draft. Use the HY-C leg kits (separate item) to attach cap to the inside of the flue if your flue tile doesn’t extend above the crown.