Fire retardant for wood fence: does it work by the house?

Fire retardant for a wood fence cannot make it noncombustible, so neither a spray nor factory-treated lumber fixes the 5 feet where a wood fence meets the house. For that section, the answer is metal or masonry. Factory pressure-treated, exterior-rated fire-retardant-treated wood (FRTW) is a recognized ignition-resistant material; the building codes do not accept sprays and paints as protection.

Under California’s Zone 0 rule, a wood section at the house has to be replaced within 3 to 5 years of the day the rule takes effect, whether or not it was treated. The FRTW versus spray distinction matters most in San Diego, whose local guidelines allow listed FRTW for one kind of new fence.

What is the difference between fire-retardant-treated wood and a spray?

The California Building Code draws the line. Section 2303.2 defines fire-retardant-treated wood (FRTW) as wood “impregnated with chemicals by a pressure process or other means during manufacture” that has a listed flame spread index of 25 or less in the ASTM E84 test, with the test run an extra 20 minutes and the flame front staying within 10.5 feet of the burners.

• Pressure treatment must happen “in closed vessels under pressures not less than 50 pounds per square inch gauge” (2303.2.2).
• Other factory methods must be “an integral part of the manufacturing process” (2303.2.3), and the same section says “The use of paints, coating, stains or other surface treatments is not an approved method of protection.”
• Outdoors, FRTW has to be identified as “Exterior” (2303.2.7), and its label must say “No increase in the listed classification when subjected to the Standard Rain Test” (ASTM D2898) (2303.2.5).

The WUI Code repeats both points. FRTW counts as ignition-resistant only if “identified for exterior use” and meeting 2303.2 (503.2.2), and “Use of paints, coatings, stains or other surface treatments are not an approved method of protection as required in this chapter” (503.2.5).

Does fire-retardant wood count as noncombustible for Zone 0?

No. The Zone 0 rule defines Noncombustible as “Materials that pass the American Society for Testing and Materials (ASTM) E136 test” (14 CCR 1298.02(f); 1299.02(g)) and Combustible as “Vegetative, wood, or other materials that are likely to ignite and transmit flames in a wildfire.” FRTW qualifies through an E84 flame spread result, which is a different test. It is still wood.

The WUI Code keeps the categories apart too: noncombustible material is one route to ignition-resistant (503.2.1) and FRTW is another (503.2.2). The noncombustible materials guide explains the E136 test.

So treated wood does not satisfy the 5 foot span where a fence meets the house. A treated wood fence built within 5 feet after the effective date also falls under the ban on “new Combustible fences or gates” (14 CCR 1298.04(b)(9)(B); 1299.03(d)(9)), because the Combustible definition names wood and FRTW does not pass E136. The Board’s own guidance says to “Replace the first 5 feet of attached fences with metal or non-combustible materials.” Zone 0 fence requirements covers the span and the timelines.

Where is fire-retardant-treated wood allowed?

In San Diego, for one layout. The city’s Zone 0 Guidelines, section 4.6, exception 2: “New parallel wood fencing is permitted if it is Fire Retardant Treated Wood (FRTW) and complies with all requirements for vinyl fencing, and it is on the California Office of the State Fire Marshal’s Wildland Urban Interface (WUI) Products Handbook and approved for exterior use.”

The vinyl requirements it borrows are that the fence runs parallel to the walls, sits no less than 4 feet from the structure, is kept free of debris, isn’t an unreasonable risk from its condition, height or fuel load, and has no vegetation between it and the house. It never covers the attached 5 foot span, and the city warns that “Your insurance provider and/or the IBHS may not allow vinyl or FRTW within 5 feet of a structure.” Details are on San Diego fence rules page.

Outside San Diego’s exception, the Wildfire Prepared Home designation and San Rafael’s fence reimbursement program both exclude treated wood: the designation does not permit fire-resistant materials such as vinyl, composite or fire-rated coatings within 5 feet, and San Rafael does not reimburse treated wood.

Do fire retardant sprays work on a wood fence?

On new, clean wood, some reduce flammability. Outdoors, the effect fades fast.

NIST Technical Note 2094 (May 2020) tested ten commercial coatings sold for exterior wood, applied to western red cedar to the makers’ instructions. Four film-forming coatings performed well when new. After accelerated weathering equal to “an estimated outdoor time of 14 days to 28 days,” all of them “became ineffective.” NIST tried eight coating and top coat pairings; only two were compatible and kept their fire performance, and the better one was estimated to last “up to seven months.” Those outdoor times are NIST’s estimates for a humid subtropical climate, not California field data. NIST’s comment: “it is unlikely that homeowners would reapply coatings every seven months.” It recommends checking a maker’s fire test data after weathering. NIST did not name the products it tested.

Two sprays marketed for outdoor wood show what the makers claim:

Neither sheet reports fire testing after weathering, which is the data NIST says to look for. Flame Stop II also asks for wood at 5 to 15% moisture, a compatibility test, and notes that runoff on vertical surfaces may call for several coats.

A worked example: the 5 feet of a 6 foot privacy fence next to a house, both faces, is 60 square feet. At Flame Stop II’s 125 square feet per gallon that is about half a gallon per coat. It is a small job, but the section is still combustible under the rule, so the coating does not settle the span question.

What should I do with a wood fence near my house?

1. Replace the 5 feet at the house with metal or masonry. A noncombustible section is the only change that meets the rule’s span. What to do with an existing wood fence walks through the options.
2. Clear the base. In NIST’s 2022 fence burns, wood fences with mulch at the base “were more hazardous than either the fence or the mulch bed separately,” and lifting a fence 6 inches above shredded hardwood mulch “decoupled the burning behavior of the fence from the mulch between posts.”
3. Deal with back-to-back fences. NIST saw a large fire even with two wood privacy fences 3 feet apart. Removing one, or making one noncombustible, does more than any coating.
4. If you build a new parallel wood fence in San Diego, at least 4 feet from the house, buy listed, exterior-type FRTW from the State Fire Marshal’s handbook, check the label, and confirm the layout with San Diego Fire-Rescue first.
5. Treat sprays as an extra, beyond 5 feet, and budget for reapplying on the maker’s schedule or sooner.

For a fence that is already composite rather than wood, see is composite fencing fire resistant.