Does My Chimney Need a Cap?

What a cap actually does

A chimney cap is a metal hood fastened over the top of a flue. Its solid lid sheds rain and snow, and the mesh around the sides lets smoke and exhaust out while keeping animals and debris from getting in. On a wood-burning flue, that mesh doubles as a spark screen, catching larger embers before they can land on the roof or in the gutters.

Most caps are made from stainless steel, copper or galvanized steel. Galvanized caps are the least durable in salt air and under acidic exhaust and tend to rust through first; stainless and copper hold up much longer on the roof. Whatever the metal, the fasteners and mesh have to match it so they do not corrode where they touch.

A cap also blunts wind blowing across the top of the chimney, which can help with gusty downdrafts, though it does not cure every draft problem.

• Keeps rain off the damper, the smoke chamber and the flue tile
• Blocks birds, squirrels, raccoons and bats
• Stops leaves and twigs from clogging the flue
• Catches larger sparks before they reach the roof
• Takes some of the bite out of wind gusting down the flue

What an open flue costs over time

An uncapped flue is a pipe pointed at the sky. Every storm sends water straight down the inside, and in a freeze-thaw climate that water does slow, steady damage. The damper rusts and seizes. Mortar joints in the smoke chamber wash out. Clay tile soaks up water, freezes and flakes. In a wood-burning flue, wet creosote gives off a strong smell in humid weather.

Animals are the other cost. Birds build nests on the smoke shelf, squirrels drop in and cannot climb out, and raccoons sometimes raise young on the damper. A nest in a fireplace flue is a fire hazard; a nest in a heating flue can block exhaust and push it back into the house.

How to tell if your chimney is capped

From the sidewalk or the backyard, look at the very top of the chimney, with binoculars on a tall house. A cap looks like a small metal box or roof sitting over each flue tile, or one wide metal cover across the whole top. Bare clay tiles sticking up with open tops mean those flues are uncapped. A rusted, tilted or half-missing cap counts as uncapped too.

From inside, open the damper on a bright day and look up with a flashlight. A rusty damper, water in the firebox after rain, or leaves and twigs on the smoke shelf are good hints that the top is open.

On a heating flue, the cellar gives clues as well. Open the cellar cleanout with the boiler off: leaves, feathers, twigs or a puddle of water there suggest the top is open or the cap has failed. Close the door firmly afterward so the flue draws properly.

Which flues need one most

Every open flue is exposed, but some take more damage than others. In rough order of urgency:

• A heating flue (boiler, furnace or water heater), where a nest or debris can block exhaust
• A wood fireplace in regular use, because of sparks and creosote that holds moisture
• A fireplace that is never used, which collects nests and rain without anyone noticing
• Any flue in a house surrounded by trees, which drop leaves and invite squirrels
• A flue whose damper has already started to rust or stick

Matching the cap to what the flue serves

Caps are not one-size-fits-all. The fuel and the appliance change what the cap has to do, and gas appliance instructions sometimes name the exact termination to use.

• Wood fireplace or stove: sheds rain, blocks animals and arrests sparks; check the mesh for creosote
• Oil boiler or furnace: sheds rain and blocks animals without a fine mesh that oil soot can clog
• Gas boiler or water heater: keeps the draft clear and follows the appliance's venting instructions
• Several flues in one stack: a cap on each flue, or one full-width cover that suits every flue under it
• A flue no longer used: sealed at the top instead of capped, so water and animals stay out for good

Brownstones, rowhouses and multi-flue stacks

Chimneys on older attached houses, from Brooklyn brownstones to Philadelphia rows, often carry three, four or more flues in one stack: fireplaces on each floor, a boiler flue and sometimes a water heater. It is common to find the boiler flue capped and the fireplace flues open, or one cap installed years ago and the rest forgotten.

Each flue needs its own protection, or a single cover across the entire top that suits all of them. On a party-wall chimney that serves two houses, the flues may belong to two owners, so it helps to know which flue is yours before ordering anything.

When a cap is not the whole answer

A cap keeps water from falling down the flue. It does not stop water coming through a cracked crown, failed flashing or porous brick, so a leak that continues after a cap goes on usually has another source. Caps also need occasional attention: on a wood flue the mesh can collect creosote and choke the draft, and on an oil flue soot can do the same.

If a fireplace smokes after a cap is installed, the cap may be too low, too restrictive or clogged. That is worth checking rather than living with.

Signs an existing cap has failed

A cap that was installed years ago can be present and still not doing its job. Look at it from the ground after a storm and once each spring. Rust streaks running down the brick below it, a lid that sits crooked, mesh that is torn, bent outward or missing a panel, and a cap leaning off the flue are all signs it needs attention.

Inside, the clues are the same ones an uncapped flue gives: scratching or chirping in the chimney, twigs on the smoke shelf, water in the firebox, or a damper that has started to rust. On a heating flue, a boiler that begins to soot up or a CO alarm alert after a windy week is a reason to have the top checked for a blocked or collapsed cap. Animals that get past a damaged screen often nest right under it, where they are hardest to see.

Getting a cap fitted properly

Caps are measured to the flue tile or the chimney's top, then clamped or fastened so wind cannot lift them. A loose or wrong-size cap is the main reason caps end up in the backyard after a nor'easter. Fitting one is roof work, so leave it to a crew with ladders and fall protection.

While they are up there, the crown, flashing and top courses of brick should get a look too, since the cap sits on all of them. You get a straight answer on what fits each flue, and an estimate in writing before anything is installed.