Rain & Flue Caps   Quick Help   How do I know which kind of Chimney Cap to choose? All chimney caps help keep rain & critters out and sparks in! But, depending on how your chimney terminates at the top or how many flues or openings there are in your chimney; is what will help you choose the correct one What clearance is necessary for proper drafting on a Multi-Flue Cap? A 6" minimum clearance is necessary for proper drafting from the highest protrusion, such as the top of the flue tile, liner or chimney to the lid of the cap. Ex. If your flue tile sticks up 4" from the chimney crown, add 6" for clearance to that and you need a 10" ht cap. How do I measure the chimney for a Multi-Flue Cap? Measure the outside perimeter of your chimney and subtract 3" from each side. This will tell you what screen size you need. Ex. Outside of brick measures 23"x35" and the flue tile sticks up 4". You will need a 17"x29" with a 10" height = MF172910.