Chimney Cap Types Compared

The three families

Single-flue caps fit over one flue tile. A trade article in Masonry Magazine, published by the Mason Contractors Association of America, describes them as the choice when a clay flue tile projects from the crown. They clamp or bolt to the tile.

Full-width caps, also called multi-flue or crown-mount caps, are a larger box with a lid that covers the entire top of the chimney. The same article describes crown-mounted caps for covering several flues with a single cap, or when no flue tile extends above the crown. Because the lid extends over the crown, it also shelters part of it from direct rain.

Top-sealing damper caps mount on the top of a flue and include a spring-loaded lid with a gasket, operated by a cable that runs down to the firebox. Fine Homebuilding describes how these close down on top of the flue liner, keeping rain and animals out and conditioned air in. When open, many work as a screened cap; when closed, they seal the flue. Our guide to throat dampers and top-sealing dampers covers the damper side.

Step 1: How many flues, and how close together?

Start at the top of the chimney.

• One flue, with a sound tile projecting above the crown: a single-flue cap is the simplest fit. Go to step 2.
• Two or more flues spaced well apart: individual caps can work. Masonry Magazine notes that single-flue caps need at least 7 inches of space between them because the lids overhang. If the tiles are too close for that, go to the next line.
• Flues close together, a tile that does not project, or a cracked tile that will not hold a clamp: a full-width cap fastened to the crown or the chimney is usually the practical choice, after any needed tile or crown repair.

The multi-flue chimney guide explains why each flue must still vent freely under a shared lid.

Step 2: What burns below each flue?

A cap sits in the exhaust stream, so the appliance matters.

• Wood-burning fireplace or stove: the screen catches larger embers and keeps animals out, but it also collects soot and creosote. A screen that clogs restricts draft, which is why the cap is part of every sweep.
• Oil furnace or boiler: oil soot and acidic exhaust are hard on screens and some metals. A cap for an oil flue has to stay open enough to vent and resist corrosion.
• Gas appliance: some gas appliance and liner manufacturers specify the termination cap. Using a fireplace-style screen cap on a gas vent may not match those instructions.
• A flue serving nothing: it still needs to be closed off from rain and animals, and the choice depends on whether it will ever be used again.

If the flues under one full-width cap serve different fuels, the cap has to suit all of them.

Step 3: Do you also want the flue sealed?

If the fireplace has a rusted throat damper, a damper that leaks cold air, or you want a tighter seal when the fireplace sits idle, a top-sealing damper cap does two jobs at once. CSIA suggests a top-mounted damper on the least-used fireplace as one way to stop smoke being drawn down that flue from another fireplace.

Things that limit this choice:

• It needs a sound flue top to mount on and seal against.
• The cable eventually wears. Fine Homebuilding notes that the stainless steel cables have to be replaced at some point, after a couple of years or decades depending on use.
• It must be fully open whenever there is a fire or hot ashes.
• It belongs on a fireplace flue. Furnace, boiler and water heater flues must stay open at all times.

We install top-sealing dampers as well as repairing and replacing throat dampers; see chimney damper repair and replacement.

Step 4: Screen size and what it trades

The mesh is a compromise. Smaller openings stop smaller sparks but clog sooner with soot, creosote and, in winter, ice. Larger openings stay clearer but stop less.

• Masonry Magazine reports that a 5/8-inch screen opening is required in much of California and parts of Nevada for spark protection, while 3/4-inch mesh is used in most other areas because it is less likely to clog with soot or ice.
• The 2003 International Residential Code text reproduced by Buckley Rumford Fireplaces set spark arrestor openings that do not let spheres larger than 1/2 inch pass and do not block spheres smaller than 3/8 inch. Your town's current code edition governs, and it may differ.
• The Internet Center for Wildlife Damage Management advises against covering a flue with hardware cloth, which rusts and can catch snow and block the flue, trapping gases such as carbon monoxide inside.

Whatever the mesh, a screen in a wood-burning flue needs checking during each sweep and cleaning when it loads up.

Step 5: Choosing the metal

The common choices are stainless steel, galvanized steel (usually painted), and copper, with some caps made in stainless steel with a copper-colored finish, as Masonry Magazine describes. Rough guidance:

• Stainless steel is the usual default for durability in a wet, acidic flue environment.
• Painted galvanized steel costs less, but once the coating wears, rust follows, and rust streaks on the crown and brick below a cap are a common sign of it.
• Copper is chosen mostly for appearance on visible chimneys and weathers to a brown and then green patina.

For flues that vent oil or gas appliances, the cap and liner manufacturers' instructions should guide the metal choice.

Factory-built chimneys and chase tops

A factory-built fireplace or stove chimney is tested and listed as a system, and the termination cap is one of its parts. DuraVent's installation instructions for its DuraTech chimney, for example, describe the cap attaching to the top chimney section and a spark arrestor assembly that needs periodic cleaning. Replacement caps for these systems should come from the same manufacturer and match the listing.

When that chimney runs up through a framed chase, the top of the chase is closed by a chase cover, a metal pan that the listed cap passes through. Our chase covers are aluminum and made to measure for each chase. See chase covers and crown vs. cap.

Questions homeowners ask

Sources

1. Masonry Magazine (MCAA): Installing the Correct Chimney Cap
2. Fine Homebuilding: Installing a Flue-Top Damper (PDF)
3. 2003 International Residential Code, Chapter 10, Chimneys and Fireplaces (text reproduced by Buckley Rumford Fireplaces)
4. Internet Center for Wildlife Damage Management: Chimney Cap
5. CSIA: Homeowner Resources and FAQ
6. DuraVent: DuraTech LDC large diameter chimney installation instructions (PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.