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How to Choose a Chimney Cap: Flue Type, Mesh, Material and Size
A cap keeps rain, animals and debris out of the flue and screens sparks. The right one depends on what kind of chimney you have, how many flues it has and how the top of the chimney is built.
Start with the type of chimney
The first question is whether the chimney is masonry (brick, block or stone) or a factory-built (prefab) system. See masonry vs factory-built chimney if you are unsure.
- Masonry chimney with clay or stainless liner. Caps are available for single flues and for multi-flue chimneys.
- Factory-built chimney. Home-inspection guidance says a cap must be installed on the top of the flue pipe, and advises against substituting a different cap for a Type B vent if the original is lost or damaged. A professional should identify the correct part for the specific system.
- Chase-covered prefab. The metal chase cover is a separate part; see chase cover materials and corrosion.
Material
The Internet Center for Wildlife Damage Management recommends commercial stainless steel caps and notes that galvanized steel rusts and breaks down. Home-inspection training material says caps are usually galvanized sheet metal or sometimes copper, and that wood must never be used for a crown or cap. Manufacturers also make caps in stainless steel of different thicknesses and finishes; one retailer’s technical page lists 304 stainless steel with 24-gauge body and 18-gauge mesh for a multi-flue cap. Ask for the material and thickness in writing when comparing quotes; see understanding a chimney repair quote.
Mesh and spark screening
A model residential code requires an approved spark arrestor on chimneys serving solid-fuel appliances. As written in that code, the screen must have openings that do not pass spheres larger than 1/2 inch and do not block spheres smaller than 3/8 inch, and the net free area must be at least four times the net free area of the flue outlet. A rain cap on a masonry chimney needs the same four-times net free area under the cap. The cap or screen must be removable so the flue can be cleaned. New Jersey adopts its own version of the model code, so confirm the current requirement with a licensed contractor or the local building department.
The wildlife resource cited above says not to cover a flue with hardware cloth, which rusts, can collect snow and can block gases such as carbon monoxide. CSIA inspection guidance also includes confirming that a rain cap is not obstructed or restrictive. Caps are looked at during inspection and cleaning.
Size and fit
- The same wildlife resource says sizing need not be exact: an 8.5 by 8.5 inch flue can take a 9 by 9 inch cap, and standard sizes are 9×9, 9×13 and 13×13 inches.
- If the flue rises at least 1 inch above the crown, a standard cap can be used. If it does not, use a cap with legs that insert into the flue.
- Measure with the crown in good condition. A cracked crown may need repair first; see crown repair vs replacement.
Single flue vs multi-flue
For two flues less than 6 inches apart, the wildlife resource calls for a multi-flue cap rather than separate caps. One manufacturer’s measuring guidance for a custom multi-flue cap says the dimensions entered are the exact size it will be made to, that the screen height should be at least 6 inches above the tallest flue, and that an outside-mount cap needs extra allowance on each dimension. Check the maker’s own instructions for the model chosen.
| Setup | Cap type to consider | Watch for |
|---|---|---|
| One flue, clay or stainless | Single-flue cap sized to the flue | Flue height above the crown affects the style |
| Two or more flues close together | Multi-flue cap | Measure the whole group; the screen should clear the tallest flue |
| Factory-built chimney | The manufacturer-specified cap | Do not substitute |
What a good installation includes
A cap does not replace a good crown. See crown vs cap vs chase cover. Our caps and chase covers service covers installation and replacement. Our Berkeley Heights project included a cap and chase cover. If animals have been using the flue, see animal noises and nesting first.
Frequently asked questions
Do I need a cap if I never use the fireplace?
A cap helps keep rain and animals out of the flue whether or not it is used. See cold draft from an unused fireplace for related options.
Can I pick any cap from a hardware store?
Fit and material both matter. Galvanized steel is noted to rust and break down, and the size has to suit the flue. A professional can measure and recommend one.
Does a cap stop the chimney from drawing?
A cap that is blocked or too restrictive can interfere with the flue, which is why CSIA inspection guidance includes checking that it is not obstructed or restrictive.
Next step
Request a visit or call (862) 225-6529 and tell us what kind of chimney you have. AM Chimney Sweep is a registered (NJ HIC #13VH14312300) and insured New Jersey home improvement contractor.
Sources
- Internet Center for Wildlife Damage Management, Chimney cap (stainless steel, galvanized rust, hardware cloth, sizing, flue height above crown, multi-flue caps)
- Model residential code text (R1003.9.2 and R1003.9.3), via GoCodeBook (spark arrestor mesh and net free area; this is the California adoption, cited as an example of the model code)
- InterNACHI, How to Inspect Fireplaces, Stoves, and Chimneys (caps on factory-built chimneys, cap materials, no wood, no substitute cap for Type B vent)
- Rockford Chimney Supply, multi-flue cap measuring guidance (example manufacturer measuring and material specifications; cited for general sizing, not as an endorsement)
- CSIA, Level 1 Inspection of a masonry chimney (rain cap should not be obstructed or restrictive)