Do you need a chimney cap?

Cap, crown and flashing: what's what

These three get mixed up all the time, and they do different jobs. When a sweep tells you one of them needs attention, it helps to know which is which.

A cap protects the inside of the flue. It doesn't stop water getting in through a cracked crown, porous brick or failed flashing. For those, see signs your chimney is leaking.

What a chimney cap does

Keeps rain out of the flue

An uncapped flue is an open hole in your roof. Rain that falls down it rusts the damper and firebox parts, soaks the smoke chamber, and wears away the mortar between flue tiles. In winter, that water freezes and thaws inside the chimney. A cap is the simplest way to stop it.

Keeps animals out

Birds, squirrels and raccoons all see an open chimney as a good place to nest. Nests block the flue, which can push smoke and, for gas and oil appliances, carbon monoxide back into the house. Animals can also get stuck, die in the flue or find their way into the house. Mesh sides keep them out.

Stops sparks and embers

The mesh on a cap works as a spark arrestor. It catches larger embers that would otherwise land on your roof, in the gutters or on dry leaves nearby. A cap won't prevent a chimney fire, but it reduces the chance of embers starting a fire outside the chimney.

Keeps debris out

Leaves, twigs and windblown trash collect in an uncapped flue, especially under tall trees. That debris can block the flue or catch fire.

Can help with downdrafts

On some chimneys, wind blowing across the top pushes air and smoke back down the flue. A cap can help, and there are specialty caps designed for chimneys with persistent wind problems. It isn't a cure for every draft problem, though, so it's worth diagnosing first.

A note on chimney swifts

Not every bird in a chimney can be removed on the spot. Chimney swifts are a migratory species protected under federal law, so an active nest generally has to be left alone until the birds leave for the season. After they've gone, the chimney can be cleaned and capped so they nest somewhere else next year. Other animals can usually be removed humanely right away. See our animal and nest removal service.

Types of caps and materials

Getting the right fit

A cap has to match the chimney and the appliance it serves. A few things that matter:

• Size. It must fit the flue properly and leave enough open area for the chimney to draft.
• Mesh. The mesh needs to be small enough to stop embers and animals but open enough not to clog quickly. Wood-burning chimneys with heavy use can clog the mesh with creosote, so it should be checked at every sweep.
• The appliance. Some gas and oil appliances, and factory-built fireplaces, call for a specific type of cap or termination. Using the wrong one can affect venting.
• Secure fastening. Pittsburgh's wind and storms will find a loose cap.

Signs your cap needs replacing

• Rust, or rust stains running down the crown or brick
• Torn, bent or missing mesh
• A cap that's tilted, loose or missing after a storm
• Mesh clogged with creosote, which can restrict draft
• Animal noises or nesting material in the firebox, even with a cap on

What to expect at an install

Fitting a cap is usually a short job, but it's also a good chance to look at the top of the chimney, which most people never see. At a typical visit we:

1. Measure each flue that needs covering, and note the type of appliance it serves.
2. Check the crown, brick and flashing. A new cap on a badly cracked crown protects the flue but leaves the masonry exposed, so you'll want to know about it.
3. Look down the flue for nests, debris or damage before it's covered.
4. Fasten the cap securely, so wind can't lift it.
5. Send you photos of the finished cap and anything else we noticed.

If we find a problem with the crown or flashing while we're up there, you'll get a free written estimate. You decide whether and when to go ahead.

The bottom line

Common questions