Not all building materials offer the same level of fire protection. Understanding Class A, B, and C ratings can help homeowners make smarter decisions about siding, decking, fences, and other exterior improvements/retrofits.
When homeowners hear the term Class A fire rating, they often assume it means a material is fireproof or highly resistant to wildfire. That isn't necessarily true.
Class A, B, and C ratings are commonly used to describe how quickly flames spread across a material's surface under a standardized test. They are useful when comparing products, but they don't tell the entire story when it comes to protecting a home from wildfire.
Understanding what these ratings mean, and what they don't, can help you make better decisions when building, remodeling, or upgrading your property.
What do Class A, B, and C fire ratings mean?
Under ASTM E84, also known as the Steiner Tunnel Test, materials are evaluated for their surface flame-spread characteristics. The results are assigned one of three classifications:
Class A: Flame Spread Index of 0–25. The lowest flame-spread range and generally the preferred classification when selecting materials for fire-conscious construction.
Class B: Flame Spread Index of 26–75. Moderate flame-spread performance.
Class C: Flame Spread Index of 76–200. The highest flame-spread range within these classifications.
A lower number is generally better. However, these ratings describe how flame travels across a tested surface, not whether a product can ignite when exposed to wind driven embers, radiant heat, or direct flame.
The rating also applies to the specific product or assembly tested. Two products made from similar materials may have different ratings depending on their composition, treatment, thickness, and installation.
Common examples of each rating
The following are general examples, not guaranteed ratings for every product. Always verify the manufacturer's test documentation for the exact product you're considering.
Class A examples (WUI Code Required)
Fiber-cement siding and cement board products
Brick, stone, and masonry finishes
Certain fire-retardant-treated wood products
Certain composite decking products tested and listed as Class A
Class B examples (Avoid)
Some wood species and treated wood products
Certain composite or PVC decking products
Some exterior finish products with tested flame-spread performance
Class C examples (Avoid)
Many untreated wood products
Common plywood and oriented strand board (OSB) products
Some wood siding, fencing, and decking products
One important point: a material's Class A rating does not automatically make it noncombustible. A product can achieve a low flame-spread rating and still be capable of igniting under wildfire conditions.
Flame spread versus ignition resistance: What's the difference?
This is where homeowners can easily misunderstand fire ratings.
Flame spread measures how readily fire travels across a material's surface during a standardized test. It helps compare how a material behaves once exposed to flame.
Ignition resistance is a broader question: how well does a material or building assembly resist catching fire when exposed to the conditions associated with wildfire?
Wildfires expose homes to more than surface flames. Wind-driven embers can accumulate in gaps, under decks, against siding, and around fences. Radiant heat and direct flame contact can also ignite materials.
A product that performs well in an ASTM E84 test is not necessarily resistant to ember intrusion or prolonged heat exposure. In California's Wildland-Urban Interface (WUI) building standards, certain ignition-resistant classifications involve additional testing requirements beyond a basic flame-spread rating.
The takeaway: don't use a Class A label as a substitute for understanding a product's actual wildfire performance.
Where should homeowners prioritize better-rated materials?
Not every exterior surface presents the same level of risk. Here are some areas worth evaluating:
Decking: Traditional wood and many composite products are combustible. When replacing a deck, consider noncombustible options such as concrete, stone, pavers, or aluminum. If choosing composite or treated wood, verify the specific product's fire-test documentation. Also pay attention to the underside of the deck, where leaves, needles, and other debris can accumulate.
Siding: Fiber cement, stucco, brick, stone, and metal siding are preferable to combustible wood or vinyl siding from a wildfire-hardening perspective. Pay particular attention to the bottom edge of siding, corners, and areas where decks or landscaping meet the home.NOTE: Intumescent paint is often recommended as a retrofit alternative, but thorough testing by multiple agencies and researchers shows that in less than 1 year the paint is no more effective than traditional paint due to weathering.
Fences and gates: A wood fence connected directly to a home can act as a pathway for fire to reach the structure. Consider replacing the section closest to the home with a noncombustible material, such as metal or masonry, and creating a break between combustible fencing and the house.
Ground-level materials: Wood chips, bark mulch, firewood, and accumulated vegetation can ignite from embers and expose nearby siding or decks to sustained heat and flames. Noncombustible ground coverings such as gravel, concrete, and pavers are better choices immediately around the home.
California's Zone 0 legislation puts this into perspective
California has taken a significant step toward addressing the materials immediately surrounding homes through Assembly Bill 3074, passed in 2020, and subsequent legislation including SB 504 and AB 1455.
These measures direct the state to establish an ember-resistant zone known as Zone 0, covering the first five feet around a home or other qualifying structure in designated wildfire-risk areas.
As of November 10, 2026, the California Board of Forestry has adopted revised emergency regulations, but the latest rule package is still moving through the state approval process. The Board's November 5 notice says it plans to submit the package to the Office of Administrative Law on or after November 13. Homeowners should not mistake the adopted draft for an already-effective statewide rule.
The proposed requirements focus on reducing combustible materials close to structures. They address items such as:
Removing combustible debris, plastic and wood items, and combustible mulch from the immediate five-foot area, subject to specified exceptions.
Keeping leaves, needles, and other combustible debris away from vulnerable areas.
Restricting new fences and gates within Zone 0 to noncombustible construction.
Requiring a five-foot noncombustible section where a fence attaches to, or comes within one foot of, a structure.
Phasing certain requirements for existing properties over time after the regulations take effect.
The details and compliance timelines matter, and homeowners should check the final adopted regulations and local fire authority guidance before making decisions based on legal requirements.
The larger point is that wildfire protection isn't just about the materials used to build a house. It's also about what is directly against it.
Practical retrofit recommendations
You don't necessarily need to replace every exterior material on your property to make meaningful improvements. Start with the areas most likely to expose your home to ignition. It’s important to understand that higher fire resistance materials usually carry a higher price tag, but those products have much less maintenance and longer life spans. The initial investment pays off over time!
Replace combustible ground cover immediately next to the home with gravel, concrete, bare dirt, or pavers.
Consider a noncombustible transition where a wood fence connects to the house. Hog wire, wrought iron, fiber cement, and Class A lumber are common options.
Evaluate aging wood or vinyl siding and consider fiber cement, stucco, masonry, or metal when replacement is appropriate.
When upgrading a deck, compare the actual fire-test ratings of available products and consider noncombustible alternatives.
Remove debris beneath decks and around stairs, porches, and other areas where embers can collect.
Check manufacturer documentation for the exact product and installation assembly rather than relying on a general marketing claim.
The bottom line
Class A is generally the best flame-spread classification, but it is not a guarantee that a product will resist wildfire ignition. Class B and C materials aren't automatically unsuitable, either. Their performance depends on the product, how it's installed, and where it's used.
The smartest approach is to look at the entire home: the siding, deck, fencing, vents, roofline, and the five feet immediately surrounding the structure.
At Fire Hard Property Defense, we help homeowners identify vulnerabilities and prioritize practical improvements using established wildfire-hardening and defensible-space principles. A property doesn't have to be completely rebuilt to become better prepared for wildfire—but the right upgrades, in the right places, can make a meaningful difference.
