What is a chimney cap, and should you cap your chimney?
Short answer. A chimney cap is a metal cover, usually with a mesh screen, that sits on top of the flue to keep out rain, animals and leaves and, on wood-burning fireplaces, to catch sparks. Most chimneys should have one, since it is inexpensive protection against water damage and nesting animals; the main exception is a decommissioned flue that a professional has sealed at the top.
Cap, crown or chase cover?
- Chimney cap: the hood and screen over the flue opening.
- Crown: the concrete or mortar slab sealing the top of a masonry chimney around the flue tile. It should slope and overhang so water drips clear of the brick.
- Chase cover: the metal top on a framed chimney chase; the cap mounts on its collar.
What a cap does
- Keeps rain out of the flue, where it mixes with creosote, rusts the damper and soaks the masonry.
- Keeps out birds, squirrels and raccoons. Chimney swifts, a protected migratory bird, nest in open flues in summer, so caps are installed after they leave.
- Catches sparks from wood fires before they land on the roof.
- Can reduce downdrafts on windy days.
Choosing one
- Stainless steel or copper lasts longest; galvanized caps rust within a few years.
- Single-flue caps clamp to the flue tile; multi-flue caps cover the whole crown.
- Gas appliance flues need caps that suit the appliance's venting.
- Measure the flue carefully, or have a sweep fit it.
What a cap will not fix
A cap does nothing for flashing leaks at the roofline or a cracked crown. If water is coming in around the chimney rather than down the flue, check those too. See chimney leaks and crickets and roof flashing explained.