How to Install a Chimney Cap

Installing a Chimney Cap on a Single-Flue Chimney

1. Measure the Flue Opening

   For square and oval flues, measure the outside length and width of the flue. Measure the outside length and width of square and oval flues, and the outside diameter of a round flue. Then, measure the height, or the amount the flue protrudes from the crown.

   Record these measurements on your phone or a piece of paper and carefully climb down from your roof.

2. Purchase a Chimney Cap

   Take your measurements to a local hardware store, roofing supply store, or an online chimney cap supplier. Purchase a cap within 1/2 to 1 inch of your flue’s length and width or diameter, and that's at least six inches taller than the flue's height. The extra six inches is needed to allow the flue to properly vent. If you can't find the right sized cap, you may have to custom order one.

3. Attach and Secure the Cap

   Climb back onto your roof with the chimney cap. Carefully slide it over the top of the flue, and press down on the cap until it’s fully seated. 

   To secure the cap to a metal flue, use a power drill or impact driver to secure the cap with self-tapping sheet metal screws. These screws should be provided with the cap, and the cap should have pre-drilled holes that specify where the screws go.

   If you have a clay flue, you may need to anchor the cap with masonry screws instead. To do this, purchase masonry screws that are approximately the same size as the provided self-tapping screws, along with a compatible masonry bit that is about 1/8 inch smaller than the the screws' diameter. Attach the masonry bit to a hammer drill, and drill pilot holes through the cap into the flue. Drive the masonry screws into the pilot holes with an impact driver.

   Round flues may come with a clamp instead of screws to secure it to the flue. If that's the case, install a round chimney cap by wrapping the clamp around the circumference of the cap's base, then tightening it down with a screwdriver.

   Seal around the entire perimeter of the cap's base with high temperature sealant.

   Warning

   Clay flues can be brittle and prone to damage if they're drilled or screwed into, so you may want to consult a professional for guidance on installation. In some cases, they may advise that you just use the high temperature sealant instead of screws.

Installing a Chimney Cap on a Double-Flue or Flush Single-Flue Chimney

1. Measure the Flues and Cap

   For double-flue chimneys, measure the combined length and width and the height of the tallest flue. For flush single-flue chimneys, measure the outside length and width or diameter. In both cases, also measure the overall length and width of the crown.

   Record your measurements and climb off the roof.

2. Purchase a Chimney Cap

   Purchase a top-mount cap that’s large enough to cover the flue(s), but small enough to not overhang the ends of the crown. Refer to the manufacturer's instructions to see if the cap is meant to be mounted with masonry screws, adhesive, or both. Purchase the supplies they recommend you use.

3. Prepare the Crown

   Center the cap onto the crown. If the cap is secured with masonry screws, mark out the screw hole locations onto the crown. Drill pilot holes into the marks with a masonry bit attached to a hammer drill. If the cap is secured with adhesive, mark the outside perimeter of the cap that contacts the crown.

   Use a brush to wipe away any dust, dirt, or debris from the area the cap rests on the crown.

   Tip

   You can ensure that your pilot holes are deep enough by marking the drill bit with a piece of tape. Hold the bottom tip of the screw against the tip of the drill bit, and mark the screw head's location on the bit. Drill your pilot holes until the bit reaches the tape.

4. Secure the Cap 

   If construction adhesive is being used, apply a line of the adhesive around the entire perimeter of the cap. Avoid getting the adhesive into any pilot holes, or else you won't be able to remove the screws at a later date. Use an impact driver to drive masonry screws into the pilot holes.