How to Block off a Chimney Top

What You'll Need
Chimney cap
Protective gloves
Masonry drill
Masonry fasteners
Chalk or marker
Caulking gun

Blocking off your chimney top will prevent undesirable elements from entering your house. Rain and snow will easily come in, causing dampness. Rodents, squirrels and birds may also be attracted by an open chimney. You may soon find yourself with more residents in your house than planned for. Blocking a chimney also has other benefits. It helps to restrict sparks from spreading, acting as a safeguard against accidental fires. A blocked chimney top also improves chimney performance. Below is a guide to help you block off yours.

Step 1: Measure Chimney Flue

Use a ladder to climb onto your roof. Carefully note down the measurements of your chimney flue. This will enable you to order the correct chimney cap from the manufacturer.

Step 2: Fit Cap onto Flue

Once your chimney cap arrives, slip it onto the flue about 3 inches down. Be sure to align the flange of the chimney cap to the chimney crown. If yours is an inside mount chimney cap, this will go inside the flue. The inside mount variety does not need to be secured with much else. Simply push firmly into the flue. Friction will secure the cap in place.

Step 3: Secure the Cap with Screws

Use screw fasteners or adhesive tape to secure the chimney cap in place. This will depend on the type of chimney cap you purchase. In most cases, the manufacturer will supply the screws or adhesive to be used along with the cap. If not, use screw fasteners that measure 1/4 inch diameter and about 1 1/2 inches long. Drill through the flange with a masonry bit at intervals of every foot along the circumference. Be sure to drill right into the chimney crown. Fit in the masonry fasteners as you work each hole. Tighten a bit to keep the cap in place. Once you've fitted in all the fasteners you can proceed to tighten them one by one.

Step 4: Or Secure with Adhesive

Use a chalk or marker to draw a line around the flange. Take your caulking gun and move across the line in a wavy pattern as you lay out the adhesive. A bead of adhesive will suffice. Do not squeeze it out so thick. The width on either side of the line should not exceed 1 inch across the line. Be sure to cover the entire marked flange circumference with adhesive. Press the flange of the chimney cap firmly into the adhesive. Hold in place for a couple of minutes as it sets. This will help secure the chimney cap firmly in place.


  • Always have somebody on the ground whenever you undertake work on the roof. This will prove handy in terms of seeking help should an emergency arise.
  • Wear protective gloves when you block off your chimney top. Chimney caps have sharp edges as they are made of sheet metal. Protective wear will help prevent accidental cuts.