1. How to Install a Chimney Cap Step by Step 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 21 in by 21 in in stainless steel or copper You ll see 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 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