Chim-A-Lator 381212 12" x 12" 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 10" to 12.25" and the length measures from 10" to 12.25".