What does a chimney cap do?

A chimney cap is a covering fitted over the top of the flue. It keeps rain, snow, birds, other animals and debris out of the chimney, and many designs also stop sparks from escaping or help the chimney draw in wind. Fairfax County Fire and Rescue advises that you install a chimney cap to keep debris and animals out of the chimney^[3].

What a cap does

Keeps water out. A masonry chimney is an open pipe pointed at the sky. Woodstock Soapstone, a stove maker, says a masonry chimney should have a cap to keep out moisture, birds or other pests^[6]. Water that gets into the flue soaks the masonry, rusts dampers and damages liners, and can drip into the firebox.

Keeps animals and debris out. Birds and small animals look for a sheltered place to nest, and an open flue is one. Even a chimney that is rarely used can fill with nests, and a blocked flue can stop it working. On a factory-built metal chimney, Woodstock says a round top cap is added to the last section of chimney pipe to keep out rain and snow as well as birds or other small animals that might be tempted to nest in the chimney^[7]. Its liner kits work the same way: the cap on its liner kit keeps rain, snow and small animals out of the chimney system^[6].

Slows sparks. Many caps have a mesh screen around the sides. Prince William County Fire and Rescue says to install spark screens and caps on chimneys^[4], because embers carried up a flue can land on a dry roof or in leaves.

Helps in wind. Some caps are designed for windy sites. Woodstock says that a stove that puffs smoke back on windy days may call for a special cap that creates more draft when the wind blows, and that a wind cap can eliminate downdrafts and backpuffing caused by wind^[5]. A cap is one possible fix for smoke blowing back into the room, not a cure for every draft problem. If your fireplace smokes, read why a fireplace smokes into the room before assuming the cap is the cause.

Why the cap has to be the right one

A cap can do harm if it is wrong for the chimney. A mesh that is too fine, or a cap that sits too close to the flue opening, can clog with soot and restrict the draft. Whoever fits it should check that the cap is not obstructed or restrictive.

On a factory-built fireplace or metal chimney, the cap is part of the tested system. DuraVent, a chimney manufacturer, says the supplied cap should never be replaced or left off for aesthetic or other reasons^[2]. If you think a wind problem calls for a different design, DuraVent says swapping in a specialized cap should only be done with the approval of the fireplace manufacturer^[2].

Size matters too. A cap made for a masonry flue does not fit a metal pipe, and one that is too small chokes the flue. A cap taken off the shelf is not always right for a particular chimney.

Keep an eye on it

You can see the cap from the ground with binoculars. Look for a missing cap, a bent or rusted one, a clogged screen, and tree limbs close to the top. Fairfax County says to keep the top of chimneys clear of tree limbs or debris^[3].

A damaged cap can also be a clue to something worse. If the cap looks scorched or warped, do not use the fireplace or stove until a sweep has inspected the whole system.

When to call a licensed pro

For fitting or replacing a cap. The cap sits at the top of the chimney, so the work means going on the roof, and a cap that does not fit, or is fastened badly, can blow off in a storm or restrict the flue. Choosing the right cap for your flue size, fuel and chimney type is also a job for someone who has looked at the chimney.

DPOR says Virginia requires that if the work is valued at $1,000 or more, a valid Virginia Contractor's License is required for the license category in which the contractor is to work^[1]. A cap alone may come in under that, but a cap job that turns into crown, flashing or masonry repair may not. Check the licence on DPOR before you pay anything.

A chimney sweep can check the cap during a routine inspection. See what a chimney sweep does and, for what sits below the cap, what a chimney liner is. Browse chimney and fireplace services or all chimney answers.

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