Chimney caps are protective coverings that prevent water and wildlife from getting into the fireplace. Without a chimney cap, water can enter the fireplace's flue (the clay or metal tube that vents smoke and combustion byproducts outside), which can damage the fireplace and surrounding roofing material. In addition, nests formed by birds and rodents around or inside the flue can pose a serious fire hazard.
A chimney may have one flue (single-flue chimneys) or two flues (double-flue chimneys). Both can either protrude from the top of the crown (the section of concrete surrounding the flue) or be flush with it. Additionally, chimney flues can be square, round, or oval-shaped. All of these factors will influence what type of chimney cap you should install.
Single-flue chimneys that protrude from the crown typically use caps that screw or clamp directly onto the flue. Single-flue chimneys that are flush to the crown often require a top-mount chimney cap that covers the entire crown and can be anchored to the crown with masonry screws, construction adhesive, or both. Double-flue chimneys also require a top-mount chimney cap.
Installing a chimney cap involves working on your roof, which can be dangerous. Take the following safety measures when working on your roof:
- Use an extension ladder with a ladder stabilizer (ladder jack), and extend the top rungs three feet above the gutters. Avoid using an A-frame ladder, since they aren’t as stable as extension ladders.
- Work with a partner that can hold your ladder, hand you tools, and call for help in the event of an accident or emergency.
- Wear a safety harness to safeguard against falling.
Consider hiring a profession to install your chimney cap if your roof is too steep to walk on or it doesn’t provide enough room to maneuver around the chimney or if you don't have the proper safety equipment or are uncomfortable working on the roof.
Equipment / Tools
- Caulking gun
- Extension ladder with ladder stabilizer (optional)
- Power drill or impact driver
- Hammer drill (optional)
- Safety harness (optional)
- Marking pen or pencil
- High temperature sealant or construction adhesive
- Masonry screws and drill bit (optional)
Installing a Chimney Cap on a Single-Flue Chimney
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.
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.
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, wrap the clamp around the circumference of the cap's base, and tighten it down with a screwdriver.
Seal around the entire perimeter of the cap's base with high temperature sealant.
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
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.
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.
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.
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.