Wood remains one of the most widely specified structural materials in residential and light commercial construction. It offers a high strength-to-weight ratio, works as a natural insulator, stores carbon for the life of the building, and consumes less energy to produce than steel, concrete, or brick. The weakness that has always limited it is fire, and the building industry has answered with fire retardant treatments that slow flame spread and stop wood from feeding its own combustion. Those treatments have a long record: New York became the first city to accept fire retardant treated wood as an alternative to non-combustible construction in the early 1900s, and the product family has grown steadily since. The same care that goes into maintaining and protecting exterior wood porch floors matters here too, because a sound, dry wood member performs differently in a fire than one that has decayed.
How Fire Retardant Treated Wood Is Made
Fire retardant treated wood, usually called FRTW, comes in two broad families. Pressure impregnation forces chemicals deep into the wood cell structure inside a treatment cylinder, so the protection survives cutting, drilling, and years of exposure. Surface-applied coatings sit on the face of the member and protect only the area they cover. The building code draws a sharp line between the two: pressure impregnated FRTW and surface coatings are not held to the same acceptance criteria.
Pressure Impregnation Versus Surface Coatings
Pressure treatment pushes fire retardant chemicals, including boron compounds and ammonium phosphate salts, into the wood under pressure. When the wood gets hot, these chemicals release water vapor and drive the formation of a char layer that insulates the wood and interrupts the combustion reaction. Surface coatings form a film that performs the same job only where the film is intact, which makes them sensitive to cuts, scratches, and weathering.
What the Treatment Does Inside the Wood
Because the chemical is inside the cell structure, pressure treated FRTW keeps its rating at cut ends and fastener holes. Many formulations also resist decay and insects, which is why treated wood shows up in framing, roof systems, and exterior assemblies. For members that stay visible, choosing the right deck stain and finish is a separate decision, because stains protect against moisture and UV while fire retardants manage flame spread.
The pressure treatment process is industrial and tightly controlled. Lumber is kiln dried, loaded into a treatment cylinder, and subjected to a vacuum that pulls air out of the wood cells before the chemical solution is introduced under pressure. Retention, the amount of chemical left in the wood, is measured in pounds per cubic foot and printed on the treatment certificate, and it is the number the code official checks against the evaluation report.
Flame Spread and Smoke Development Testing
The test that determines whether a product qualifies as FRTW is ASTM E 84, the Standard Test Method for Surface Burning Characteristics of Building Materials, known in the industry as the tunnel test. The test compares a material against two reference points: asbestos cement board, which is assigned a rating of 0, and untreated red oak flooring, which is assigned a rating of 100. Untreated lumber of various species scores between 60 and 230, which is why raw wood fails most code paths that require a low flame spread index.
How the ASTM E 84 Tunnel Test Works
In the tunnel test, a sample of the material is placed over a burner that shoots a 4.5-foot flame along the underside of the specimen. Flame spread and smoke development ratings are established during the first 10 minutes of the test. For wood products seeking a fire retardant rating, the test extends to 30 minutes, and the flame spread must not progress beyond the limit set in the acceptance criteria.
Reading the Numbers
Results come back as a flame spread index and a smoke development index. A flame spread index of 25 or less meets Class A requirements, which is the target for most FRTW products. Smoke development must be 450 or less to satisfy the code, and many fire retardant products finish at 25 or below; smoke is widely considered the leading killer in a fire event, so the smoke number gets as much attention as the flame spread.
| Class | Flame spread index | Smoke development | Typical use |
|---|---|---|---|
| Class A | 0 to 25 | 450 or less | FRTW and most commercial assemblies |
| Class B | 26 to 75 | 450 or less | Corridors in some occupancies |
| Class C | 76 to 200 | 450 or less | General wall and ceiling finishes |
Finish failures are not limited to flame spread. A podcast segment on protecting interior wood finishes from UV rays makes the same point the test lab does: any coating has to be screened for the specific threat it claims to address.
The same rating system applies to wall and ceiling finishes, not just structural wood. Paneling, plywood, and acoustic tiles all carry flame spread classifications, and the interior finish requirements in the code reference the same ASTM E 84 numbers. That is why a fire retardant plywood panel and a fire retardant stud share the same vocabulary even though they play different roles in the assembly.
Fire Resistance Ratings and Building Code Requirements
FRTW is not non-combustible, and the code does not treat it that way. It is accepted as an alternative where the code allows protected wood construction, and that acceptance is documented through evaluation reports issued under acceptance criteria published by the International Code Council. The relevant document is AC66, the Acceptance Criteria for Fire Retardant Treated Wood, and every pressure treated FRTW product must demonstrate compliance with it.
AC66 and the Path to Code Acceptance
The evaluation process checks more than flame spread. AC66 covers the effect of treatment on structural properties, the risk of hygroscopicity, which means the treated wood picking up moisture from humid air, corrosion of fasteners in contact with the treated material, and, for exterior products, weathering and the ability to dry out after getting wet. Interior and exterior FRTW are evaluated differently because they face different moisture loads.
Code references to FRTW appear throughout the International Building Code. Roof assemblies, attic floors, exterior walls on certain occupancies, and stage platforms all have sections where fire retardant treated wood is accepted in place of non-combustible construction. The approval path runs through an ICC evaluation report, so the spec should name the product and its report number, not just say fire retardant treated wood on the drawings.
Wood has enemies beyond fire. Protecting wood from borers requires identifying the insect and applying the right treatment, and it illustrates why penetration matters: treatments that reach deep into the wood cell protect against more than flame.
Applications in Commercial and Residential Construction
Pressure impregnated FRTW is common in schools, multi-family buildings, hotels, airports, shopping centers, sports stadiums, and convention centers. The typical applications are roof systems and attic spaces where sprinkler coverage is limited, plenums, exterior decks and balconies in some jurisdictions, stage platforms, and wood frame walls in buildings that need an extra margin of fire performance.
Typical Applications by Building Type
- Schools and daycares: roof framing, attic separation, corridor bulkheads
- Multi-family housing: balconies, stair enclosures, attic floors
- Hotels: egress corridors, roof assemblies above sleeping rooms
- Airports and stadiums: long-span roof structures, concourse framing
- Retail: mezzanines and storage separations
Material selection is one layer of a fire strategy. Protecting a building facility from fire covers prevention, insurance, and recovery, and treated wood is most effective when it works with detection, suppression, and good housekeeping.
FRTW is not for every situation. Products rated for interior use can pick up moisture in continuously damp spaces, and no treatment eliminates the need for a fire barrier where the code demands one. Ground contact, standing water, and environments above 140 degrees Fahrenheit are all outside the normal service range for most formulations.
Working With FRTW on the Job Site
FRTW behaves differently from untreated lumber on site. It is heavier, it can be more abrasive on tooling, and some formulations are hygroscopic, so the material should be stored under cover and allowed to dry before it is enclosed. Cut ends on interior-rated products keep their protection because the chemical is in the cells; exterior-rated products may require a field-applied treatment on cut ends.
Cutting, Fastening, and Field Modifications
- Cut with carbide-tipped blades and collect the dust; wear a respirator during cutting and sanding.
- Treat exposed cut ends with the manufacturer’s field solution where required for exterior exposure.
- Use hot-dip galvanized or stainless fasteners, because some fire retardant formulations accelerate corrosion.
- Store material under cover and keep it dry before and during installation.
- Allow the wood to acclimate before enclosing it in walls or roof assemblies.
In timber and log homes the same moisture discipline applies. Protecting log home wood from insects, moisture, and mold is a separate campaign, but it protects the same asset a fire treatment does.
The defense of a wood house starts outside. Protecting a wood home from the elements with landscaping, drainage, and overhang strategies keeps water away from the structure, which preserves both the wood and whatever treatment is in it.