Chim-A-Lator 381318 13" x 18" Black Galvanized Square Spark Arrestor with 3/4 Mesh is the perfect way to help a chimney function at a optimum level. This Chimney Cap keeps out all of the debris and protects the chimney from any disruptive weather conditions. The outside dimensions have a width from 11" to 13.5" and the length measures from 16" to 18.25".