Step 1 : Measure the inner opening of chimney

Inspect the chimney flue and measure its inside dimensions to determine what size cap to buy. The caps come in a range of sizes from 9-in. square to 13-in.-by-18-in. in black metal or stainless steel. You'll see the package features a minimum and maximum opening size, so choose a size within the range.

Step 2 : Position and fasten the cap

Steady the ladder against the roof and carefully position yourself so you can work on the chimney. Place the cap on top of the chimney and insert the flue tabs of the cap into the chimney.

Use a screwdriver to fasten the screws that came packaged with the cap to hold it in place. Secure all the screws around the outside of the chimney cap base to hold it in place.