Chimney caps are an essential part of any properly functioning chimney system. A cap covers the top of your chimney liner, keeping out rainwater and debris while a wire mesh wraps around the sides to block animals and birds from nesting inside. That same mesh also acts as a spark arrestor, catching large embers before they can escape and potentially ignite dry grass or leaves on the ground below.
Getting the right chimney cap and installing it properly is critical for maintaining safe, efficient chimney operation. Some chimneys have persistent draft problems, and specific cap designs can help improve airflow — but for most homes, a standard chimney cap will do the job perfectly, as long as it's the right size for your flue or liner.
Choose the Right Cap for Your Liner Type
Before buying a chimney cap, consider your type of chimney liner. Terracotta or clay chimney liners can accept caps that mount either inside the liner or outside of it. Outside-mounting caps only work if the clay liner extends a few inches above the chimney crown — otherwise, there's nothing to attach the cap to. For flexible metal chimney liners, caps are always mounted on the inside.
Getting the Measurements Right
For outside-mount caps, measure the outside diameter of the liner. For inside-mount caps, you'll need the inside diameter, since this type of cap slides snugly into the liner. You probably won't find a chimney cap that exactly matches your measurements — that's normal. Most cap packages list a range of sizes; just pick the one that includes your dimensions.
Installing a Standard Chimney Cap
Once you've chosen your cap and completed any assembly, it's time to install it. You'll need a screwdriver that matches the included screws, a sturdy ladder, and a good comfort level with heights. Slide the cap into or onto the chimney liner and use the provided screws to secure it in place, following the manufacturer's instructions.
Attaching most chimney caps is straightforward — some mount on the outside of the liner, others on the inside. For inside-mount caps, make sure the screwdriver you bring up the ladder will fit comfortably inside the liner while you're tightening screws. Fewer trips up and down the ladder means a safer, easier project.
Crown-Mounted Chimney Caps
Beyond standard caps, there are literally hundreds of different types of chimney caps in every size and shape imaginable. Some mount directly to the chimney crown instead of the liner. These crown-mounted caps are typically much larger than the flue itself, and they hold up better in high winds because they're anchored directly to the masonry. They're especially useful for chimneys with multiple flues, since a single large cap can cover all of them at once.
Measuring for a Crown-Mounted Cap
For crown-mounted caps, you'll need to measure the chimney crown itself. Choose a cap that's a few inches smaller on every side so there's enough room to drive the screws in properly. Also make sure there's a few inches of clearance between the sides of the chimney liners and the sides of the cap. Pay attention to the space between the top of the flue and the underside of the cap — if it's too tight, airflow will be restricted and draft will suffer.
Drilling Pilot Holes for a Clean Install
Some crown-mounted caps come with a masonry drill bit for pilot holes — if yours doesn't, you'll need to pick one up that's the right size for the included screws. Drilling pilot holes is important for two reasons:
- It makes driving screws into the concrete crown much easier.
- It prevents cracking the crown, which is far more expensive to replace than a masonry bit.
Center the cap on the crown and mark where the holes should go. Remove the cap, drill the holes, then reposition the cap and insert the screws. Use a good drill for this — don't try to force screws in with a hand screwdriver, especially while balancing on a ladder.
When in Doubt, Call a Professional
If you're not sure which chimney cap is right for your home, don't hesitate to contact a local chimney professional. The right cap can make a real difference in your chimney's efficiency — and that's something every homeowner can appreciate.