Is composite fencing fire resistant? What to do near the house

Composite fencing is fire resistant only in a limited, flame-spread sense. Wood-plastic composite is combustible: Trex fence is rated Class B for flame spread, and in NIST’s 2022 tests one composite fence burned faster than any other fence and mulch pairing. For the 5 feet next to the house, use metal or masonry instead.

Under California’s Zone 0 rule, composite sits on the same side of the line as wood and vinyl: an attached composite fence needs a noncombustible first 5 feet within 3 to 5 years of the day the rule takes effect, and no new composite fence can go in within 5 feet of the house from that day.

What fire rating does Trex fencing have?

Class B. A Trex fence won’t help spread a fire, but fire will damage it, and because plastic is a major component of composite, it is more likely to melt.

The letter comes from the ASTM E84 tunnel test, which measures how fast flame travels along a surface. Under the International Building Code’s classification (Section 803.1.2), Class A is a flame spread index of 0 to 25, Class B is 26 to 75 and Class C is 76 to 200.

You may see Trex described elsewhere as Class A. The Class A Trex product introduced on 13 January 2026 is Refuge, a cellular PVC decking rated Class A for flame spread under ASTM E84. It is a deck board, not a fence, and it is PVC rather than wood-plastic composite. If you are pricing another composite brand, ask for its E84 test report rather than a class letter on a brochure.

Why isn’t a rated composite fence noncombustible?

Because the Zone 0 rule asks a different question from E84. It defines Noncombustible as “Materials that pass the American Society for Testing and Materials (ASTM) E136 test,” the standard in Section 501.4.1 of the California Wildland-Urban Interface Code (14 CCR 1298.02(f); 1299.02(g)). Its definition of Combustible starts with “Vegetative, wood, or other materials that are likely to ignite and transmit flames in a wildfire.” Composite boards are part wood fiber.

E84 does not answer the E136 question. E84 presumes the material can burn in the first place, and the standard’s own scope calls it “unsuitable for determining the combustibility or noncombustibility of a material.” The noncombustible materials guide explains how the E136 furnace test works.

The WUI Code keeps the two apart. Section 503.2 lists the ways a material can be “ignition-resistant.” Noncombustible material is one path (503.2.1). Composite falls under a separate path, “other ignition-resistant building material” (503.2.4), whose weathering rules name wood-plastic composite (503.2.4.3.1, 503.2.4.3.2). That path needs an extended 30 minute E84 test (or the ASTM E2768 test), a flame spread index “not exceeding 25” (503.2.4.1), a flame front that goes no more than 10 feet 6 inches past the burner (503.2.4.2), and testing after weathering. So the code treats even the best composite as ignition-resistant at most, a broader category than noncombustible. A Class B rating, by definition a flame spread index above 25, would not reach that bar either.

How do composite fences burn?

NIST Technical Note 2228 (2022) includes controlled burns of composite fences. NIST tested two composite privacy fences in three burns at low wind, with a test shed 6 feet away, and calls the results “limited preliminary data.”

• Fence 1, vertical interlocking boards, no mulch. A fire lit at the base “developed into a large fire with flames extending well above the fence and licking the shed from 1.83 m (6 ft) away.” As the composite softened, 6 foot boards fell out of the top and bottom frames to both sides, creating “a fire zone up to 3.6 m (12 ft) wide.”
• Fence 1 with hardwood mulch beneath. Flames rose well above the fence within 6 minutes and the boards fell out again. The flame front took 6 minutes to reach the end of the panel, faster “than for any other fence-mulch combination in this study,” wood fences included.
• Fence 2, horizontal boards in an aluminum frame, with mulch. Very different. The frame held the boards in line as they burned, flames “never reached above the halfway point,” and the fire died down on its own. Its flame front took almost twice as long as fence 1’s to reach the end of the panel.

NIST put “one type of wood-plastic composite fence” in its very high hazard group, alongside parallel wood fences, and noted that a warped frame dropping burning boards on both sides “could block egress.” In small-scale cone tests the composite samples “burned intensely, with peak and total heat release values greatly exceeding” those of western redcedar and vinyl.

NIST’s recommendation covers composite by name: replace combustible landscape features where possible, because “Fire spread is more likely with wood and wood-plastic composite fences than with fences made of vinyl or noncombustible materials such as stone, brick, or steel.” For how this compares with wood, see are wood fences a fire hazard.

Can I keep an existing composite fence?

Under the state rule, an existing composite fence is treated like any other combustible fence (14 CCR 1298.04(b)(8) and (9); 1299.03(d)(8) and (9)):

San Diego’s exception 1 covers “Existing Vinyl and fencing without combustible reinforcement” that runs parallel, sits at least 4 feet from the house, is debris free and has no vegetation between it and the wall. The guidelines do not mention composite, and composite boards are combustible themselves, so do not count on the exception; ask San Diego Fire-Rescue before relying on it. The San Diego fence rules page has the full text.

The Wildfire Prepared Home designation does not permit fire-resistant materials such as vinyl, composite or fire-rated coatings within 5 feet of the home.

What should I do with a composite fence near the house?

1. Measure from the wall. If the fence attaches or ends within 1 foot, the first 5 feet is the section a noncombustible panel or gate replaces. Zone 0 fence requirements covers how the span is measured.
2. Clear the base. NIST’s fastest burn was composite over mulch. Gravel or bare soil under the fence removes the fuel that sped it up.
3. Don’t install new composite inside 5 feet. Steel and masonry are the clear noncombustible choices. Aluminum is the usual third choice; ask for an E136 report on the bare alloy (is aluminum noncombustible covers its one open question). What counts as noncombustible lists the options, and is vinyl fence fire resistant covers the other plastic option.
4. Check composite inserts. A metal frame with composite slats is still a combustible fence inside 5 feet, since the whole span has to be noncombustible. Each infill material needs its own E136 result.
5. Don’t count on a coating. Fire-retardant sprays are not an approved method of protection under the WUI Code; fire retardant for wood fences covers what they do and how fast they wear off.