Chimney Spark Arrestors in California: The Mesh Size and Wire the Code Requires
The first fire of the season is coming, and before it does, it's worth looking at the top of your chimney. A spark arrestor is the metal screen up there that lets smoke out and keeps burning embers in. It also keeps out raccoons, squirrels, rats, and birds, which is why UC's pest management program recommends one too.
In California it isn't optional. The California Residential Code says every chimney attached to a fireplace or appliance that burns solid fuel shall have an approved spark arrester. Here's what "approved" means in numbers, and what changes if you live in a fire hazard zone.
The Opening: 3/8 Inch to 1/2 Inch
The code (CRC Section R1003.9.2) sets a window, not a single size. The screen can't let through anything bigger than a 1/2 inch ball, and it can't block anything smaller than a 3/8 inch ball. In plain terms, the openings have to land between 3/8 and 1/2 inch.
Both ends matter. Bigger than 1/2 inch and embers get out. Smaller than 3/8 inch and the screen starts fighting the chimney. NFPA 211, the national chimney standard, says screening on a cap can't hurt the draft, and the fire safety nonprofit FLASH notes that finer mesh can restrict flow and lead to creosote buildup. That's why the 1/8 inch mesh we sell for ember-resistant vents is the wrong answer on a chimney, and so is 1/4 inch hardware cloth.
The Wire: 19 Gauge Galvanized or 24 Gauge Stainless
The screen has to hold up to heat and corrosion at least as well as 19 gauge galvanized steel or 24 gauge stainless steel. California adds 12 gauge wire to that list. 19 gauge is about 0.041 inch and 24 gauge stainless is about 0.024 inch, so plenty of ordinary mesh clears the statewide minimum. The fire hazard zone rule below is a different story.
The Size of the Cage: 4 Times the Flue
The open area of the screen has to add up to at least four times the open area of the flue it covers. This is the rule that catches homemade caps. A flat screen laid across the top of the flue can't pass, because the wire itself blocks part of every opening, so even a very open screen gives you less than the flue's own area. That's why a spark arrestor is a cage with screened sides, not a lid.
How much screen you need depends on the mesh. Here's the math for 2 mesh, which lands inside the 3/8 to 1/2 inch window in every wire size below.
| Wire | Clear opening | Open area | Screen needed per sq in of flue |
|---|---|---|---|
| 0.041" galvanized (19 ga) | 0.459" | 84% | 4.8 sq in |
| 0.047" stainless | 0.453" | 82% | 4.9 sq in |
| 0.063" stainless | 0.437" | 76% | 5.2 sq in |
| 0.105" stainless (12 ga) | 0.395" | 62% | 6.4 sq in |
So a flue with 100 square inches of opening needs somewhere between about 480 and 640 square inches of screen, depending on the wire. Heavier wire means a bigger cage. Our Mesh Viewer will work out the open area for any other spec.
In a Fire Hazard Zone, the Wire Gets a Lot Heavier
Since January 1, 2026, California's Wildland-Urban Interface Code (Title 24, Part 7) has its own spark arrestor rule in Section 605. It covers chimneys on fireplaces, barbecues, incinerators, and decorative heaters that burn solid or liquid fuel. The screen has to be woven or welded wire of 12 gauge (0.1046 inch) with openings no larger than 1/2 inch, and the 4 times rule still applies.
Two things make this one matter. First, 12 gauge is about two and a half times the diameter of 19 gauge, so light screen that passes the statewide rule doesn't pass this one. Second, this chapter of the code covers existing buildings, not just new ones. A WUI area means a state-designated Fire Hazard Severity Zone or an area your city or county designates. Check your address on the State Fire Marshal's hazard zone map, and confirm with your building department or fire district what they'll accept.
CAL FIRE's home hardening guidance puts it more simply. Cover every chimney and stovepipe outlet with metal mesh in the 3/8 to 1/2 inch range, and close the flue during fire season when you're not using it. For the rest of the house, see our pieces on ember-resistant vents and California's Zone 0 rule.
Masonry or Factory-Built? Check Before You Buy Mesh
The numbers above are written for masonry chimneys. A factory-built chimney, the metal kind that comes as a system with a prefab fireplace or stove, works differently. The code says it has to be listed and installed per the manufacturer's instructions, including how it terminates at the top, and a decorative shroud isn't allowed unless it's listed for that specific system. If the spark arrestor is part of that listed system, NFPA 211 says it has to match the listing too.
In practice, if you have a factory-built chimney, get the replacement cap from the manufacturer instead of building one. A homemade cage on a listed chimney is how a passing install turns into a failing one.
Galvanized or Stainless?
The code treats 19 gauge galvanized and 24 gauge stainless as equal minimums, so either one is legal. We'd still use stainless, and we want to be straight about why. We couldn't find a solid source on how hot the screen at the top of a chimney actually runs, so we won't pretend to know.
Here's what we do know. The Chimney Safety Institute of America says chimney deposits are quite acidic. The galvanizing industry's own guidance puts the limit for continuous heat at 392°F. Galvanized is a zinc coating that wears away over time, and stainless is stainless all the way through. The whole point of a good cap is never having to think about it again. Near the coast, 316 earns its price over 304.
What We'd Use
| Where | What we'd use | Clear opening | Meets |
|---|---|---|---|
| Most masonry chimneys | 2 Mesh 304 stainless, .047" wire | 0.453" | Statewide code |
| Heavier, for a cap you never touch again | 2 Mesh 304 stainless, .063" wire | 0.437" | Statewide code |
| Fire hazard zones | 2 Mesh 304 stainless, .105" wire | 0.395" | Statewide code and the WUI 12 gauge rule |
| Fire hazard zones near the coast | 2 Mesh 316 stainless, .105" wire | 0.395" | Statewide code and the WUI 12 gauge rule |
| Budget option | 2 Mesh 19 ga galvanized hardware cloth | 0.459" | Statewide galvanized minimum |
Before You Put It On
- Make sure nothing's living in there. UC's raccoon guide says to confirm no animals are inside before you cover a chimney. On the West Coast, also watch for Vaux's swifts, which use chimneys as roosts during fall migration, especially in September. They're protected under the federal Migratory Bird Treaty Act and California's Fish and Game Code, and California law also bars needlessly destroying any bird's nest. If birds are using the chimney, wait until they've moved on.
- Inspect it every year. NFPA 211 calls for chimneys to be inspected at least once a year, and for spark arrestors to be checked for soundness and deposits. A screen packed with soot isn't arresting anything except your draft.
- A legal cap doesn't make it a legal fire. The Bay Area Air District bans burning wood or any solid fuel, indoors or out, during a Spare the Air alert. The winter season runs November 1 through the end of February, and the ban applies any time of year an alert is called for particulate pollution. Check before you light.
Frequently Asked Questions
Does California require a spark arrestor on my chimney?
The California Residential Code says every chimney attached to a fireplace or appliance that burns solid fuel shall have one. Whether your city enforces that on an existing chimney nobody's working on is a local question, so ask your building department. In a fire hazard zone, the WUI code applies to existing homes too, and it also covers liquid fuel and barbecues. A gas-only fireplace isn't covered by the residential code requirement.
What size mesh does a spark arrestor need?
Openings between 3/8 and 1/2 inch, which in practice means 2 mesh. Statewide, the wire has to be at least as tough as 19 gauge galvanized or 24 gauge stainless. In a fire hazard zone, the WUI code calls for 12 gauge wire.
Can I use 1/4 inch hardware cloth?
No. A 1/4 inch opening is below the 3/8 inch minimum, and finer mesh can restrict the draft and load up with creosote. Hardware cloth that size is also usually 23 gauge, which is lighter than the code minimum.
Can I use the same mesh as my attic vents?
No. Ember-resistant vent mesh is 1/16 to 1/8 inch, which is far too fine for a chimney. Vents and chimneys have separate rules, and the chimney rule wants the bigger opening.
Can I build my own cap for a prefab metal chimney?
No. A factory-built chimney has to be installed per its listing, and that includes the termination. Get the cap the manufacturer lists for your system.
If you or your chimney contractor is building or rescreening a cap for a masonry chimney, we'll cut 2 mesh stainless to your dimensions, so you're not buying a full roll for one chimney. Send us the size and whether you're in a fire hazard zone, and we'll quote it. Want to see the wire before you order? We'll mail you a free sample. Family-owned since 1938, when we started on Howard Street in San Francisco. Now we're in Hayward, and a real person answers the phone at (510) 887-8787.
This article is general information about wire mesh, not a code compliance determination. Chimney and wildfire requirements vary by jurisdiction and change over time. Verify your requirements with your local building department or fire authority, and have your chimney inspected by a qualified professional.