Wildfire seasons keep getting longer, and building codes in fire-prone areas keep getting stricter. For exterior walls, the siding is the largest exposed surface, so it is the first line of defense against flame and embers. Builders in high-risk zones now have more options than ever, including wood products that carry genuine fire ratings instead of just a natural look.
One recent introduction shows the direction of the market: an exterior Class A fire retardant treated redwood siding in 1×6 and 1×8 beveled profiles, treated with a clear fire retardant that preserves the wood’s appearance while meeting the highest flame spread classification. It is aimed squarely at wildfire-prone areas and launched first in the Central Region covering Colorado, Kansas, Nebraska, New Mexico, South Dakota, Texas, and Wyoming.
Before specifying any cladding, it helps to understand what fire retardant treatment actually does, how redwood compares with other siding materials, and what installation and maintenance look like in practice. The fundamentals of installing vinyl siding right, weather barriers, flashing, fasteners, and spacing, decide how long any cladding lasts.
What Fire Retardant Treated Siding Is
Fire retardant treated lumber, FRT for short, is wood that has been pressure-impregnated with chemicals that slow ignition and reduce flame spread. It is not fireproof, and it is not the same as fire-treated or charred wood. The treatment changes how the wood behaves in a fire: it burns slower, produces less flame, and does not add as much fuel to a structure fire. The clear formulations used for siding leave the grain visible, so the board keeps the look of natural wood.
How the Treatment Is Applied
The process starts with kiln-dried, planed lumber. Boards are loaded into a large cylinder, air is evacuated to pull moisture out of the wood cells, and the fire retardant solution is pumped in under pressure so it penetrates deep into the material. After treatment, the wood is redried to a stable moisture content and often planed again. Because the chemical is inside the wood rather than on the surface, it does not wash off or wear away the way a surface coating would.
What Class A Means
Class A, sometimes called Class I, is the highest flame spread classification for building materials under the ASTM E84 tunnel test. Class A materials have a flame spread index of 25 or less and a smoke developed index of 450 or less. The chart below shows the three classes.
| Class | Flame spread index | Smoke developed index | Typical materials |
|---|---|---|---|
| Class A (I) | 0 to 25 | 0 to 450 | Fire retardant treated wood, fiber cement, gypsum |
| Class B (II) | 26 to 75 | 0 to 450 | Some treated woods, certain composites |
| Class C (III) | 76 to 200 | 0 to 450 | Untreated wood siding, many plywoods |
A Class A rating on siding does not make a house fireproof, but it slows the spread of flame along the wall and buys time for occupants and firefighters. The treatment also does not end maintenance. Weathered boards still need attention, and the steps used to clean and stain weathered redwood siding keep treated wood looking and performing well.
Why Redwood for Exterior Siding
Redwood has been a premium siding choice on the West Coast for more than a century, and the reasons are physical. The heartwood contains natural extractives that resist decay and insects, the wood is dimensionally stable, and it holds paint and stain better than most softwoods. It is also light, roughly 25 pounds per cubic foot when dry, which makes it easy to handle on a ladder and kind to wall framing.
The BEE profile, a beveled pattern sold in 1×6 and 1×8 widths, is the most common redwood siding style. The bevel sheds water, the wide face reads as traditional clapboard, and the boards bend easily around corners. Other patterns include channel rustic, board and batten, shiplap, and tongue and groove.
Heartwood vs Sapwood
Not all redwood is equal. Heartwood, the darker center of the log, carries the natural decay resistance that makes redwood famous. Sapwood, the lighter outer band, has little natural resistance and should be treated or used only where it will be protected. Buyers should confirm they are getting heartwood or treated material for ground contact and high-exposure locations.
How It Handles Weather
Redwood shrinks less than most softwoods, which means fewer popped nails and fewer gaps at the joints. It also accepts a wide range of finishes, from transparent water repellents that let the grain show to solid stains and paints that hide it. The practical questions builders ask, from sealing can lights, porch conversions, and redwood siding to flashing details, show up constantly in trade discussions, and the answers usually come down to keeping water out of the wall cavity.
Comparing Siding Materials for Fire-Prone Areas
Siding choice in a wildfire zone is a trade-off among fire performance, appearance, maintenance, and cost. Fire retardant treated redwood is one option, but it competes with fiber cement, metal, and untreated woods. The table below summarizes the practical differences.
| Material | Fire rating | Maintenance | Appearance | Relative cost |
|---|---|---|---|---|
| FRT redwood | Class A when treated | Moderate; refinish on schedule | Natural wood grain | Premium |
| Untreated redwood | Class C typical | Moderate | Natural wood grain | Moderate to premium |
| Vinyl | Melts in fire; not rated for walls | Low | Molded profiles | Low |
| Fiber cement | Class A | Low; repaint every 10 to 15 years | Wood-like texture | Moderate |
| Cedar shake and shingle | Class C typical | High | Distinctive texture | Moderate to premium |
| Steel and aluminum | Noncombustible | Low; denting and paint touch-up | Panel or lap profiles | Moderate to premium |
The comparison changes in a wildfire zone. Vinyl siding can melt and expose the wall, and untreated wood feeds a fire, which is why ignition-resistant construction rules in many wildland-urban interface areas restrict which materials can go on the exterior. Treated wood, fiber cement, and metal satisfy those rules while keeping different looks.
Reading the Labels
Specifiers should look for the flame spread and smoke developed ratings on the product’s listing, plus the mark of an accredited agency such as UL or ICC-ES. The treatment chemical matters too: some formulations are hygroscopic and can draw moisture in humid climates, so the manufacturer’s moisture and humidity limits should match the project site.
Whatever the material, weathering does not stop at the factory. Boards exposed to sun and rain will gray, and the routine for cleaning and staining weathered redwood siding restores the color and extends the life of the finish.
Installing Fire Retardant Treated Redwood
Installation follows the same sequence as any wood siding, with a few details specific to treated boards. The general rules for exterior siding installation for long-lasting results, weather barriers, flashing, and ventilation, apply to redwood exactly as they do to other cladding.
Step-by-Step Installation
- Store the siding flat, off the ground, and protected from weather, and let it acclimate to the site before installation.
- Install the weather barrier and all flashing before the first board, including sill flashing and window and door flashings.
- Snap level lines and set the first course, starting at the bottom and working up.
- Fasten each board through the top edge so the next course covers the fasteners, or follow the profile’s prescribed nailing pattern.
- Leave 1/8 inch gaps at butt joints and corners to allow movement, and pre-drill near board ends to prevent splitting.
- Apply the manufacturer’s end-cut treatment to any field cuts if the treatment system requires it.
- Finish promptly with the specified stain, paint, or water repellent.
Fastener Choice
Fasteners for redwood should be stainless steel or hot-dipped galvanized. Electro-galvanized nails can react with the natural extractives in redwood and leave dark streaks, and corrosion accelerates in coastal or treated-wood applications. Ring-shank or spiral nails add holding power in high-wind zones.
Cutting treated wood produces dust that should not be inhaled, so wear a dust mask and cut outdoors when possible. Keep the cut ends clean and apply any required supplemental treatment before the board goes on the wall.
Finishing, Maintenance, and Working Solo
A finish on redwood is a maintenance contract, not a one-time event. Transparent water repellents may need renewal every one to three years, penetrating stains every three to five, and paints longer, depending on exposure. The treatment does not change that schedule, although it adds a layer of fire protection behind the finish.
A Seasonal Maintenance Checklist
- Spring: inspect for popped nails, loose boards, and damaged finish after winter.
- Summer: wash with a mild detergent and soft brush; avoid high-pressure washing that drives water behind the boards.
- Fall: touch up stains and seal any gaps before wet weather.
- Every few years: strip and refinish when the coating shows wear.
Long siding runs are heavy, and a one-person crew can waste hours juggling boards on a ladder. Many solo builders speed the work by building and using homemade hangers designed for one-person siding jobs, which hold the board in place while the top edge gets nailed.
Choosing Siding for Wildfire-Prone Areas
Homes in the wildland-urban interface face two threats: radiant heat from a front of flames and windblown embers that can land on the roof and against the walls. Codes in these zones increasingly require ignition-resistant materials for exterior walls, and treated wood siding is one way to keep a natural look while meeting the requirement.
The Whole Assembly, Not Just the Cladding
The decision is about the whole assembly, not just the siding. Eaves, vents, decks, and the gap between siding and grade all matter, and the siding’s fire rating only helps if the wall behind it is detailed correctly. Availability is improving as suppliers bring treated wood to new regions, with the Central Region rollout covering seven states from Colorado to Texas.
Texture and pattern still drive many choices. Shake and shingle walls remain one of the most requested looks in wood siding, and the installation techniques used for cedar shake and shingle siding transfer directly to other species, treated or not.
For a builder in a fire-prone county, the practical answer is to compare the whole package: flame spread rating, maintenance schedule, installed cost, and the look clients want. Fire retardant treated redwood sits in the middle of that trade-off, offering the appearance of natural wood with a Class A rating that untreated cladding cannot match.