If you have a single flue chimney cap, the type that fits over the flue, just slip it onto the flue. It should cover the top 1 or 3 inches of the flue. Make sure there is at least 5 inches between the top of the chimney flue and the lid of the chimney cap to allow for good air flow. Now firmly tighten the screws on all sides of the chimney cap.
Slip the chimney cap over the flue, covering at least one inch of the flue, and firmly tighten the screws (see the red arrows, above)
on all sides of the chimney cap.
If you don't yet have your chimney cap for a single rectangular, square or round
flue, take the FireplaceMall Chimney Cap EASY MEASURE Guide up on the roof with you to help you correctly measure for the type of chimney cap you need.