Is Pressure Treated Wood Fire Resistant? (No – Here Is Why)
Pressure-treated (PT) lumber resists rot and termites, but it is not fire-resistant – copper-based preservatives do not slow flame spread, so PT wood burns like any untreated softwood. Building codes that require fire-retardant-treated (FRT) wood, tested to a flame spread index of 25 or less under ASTM E84, are never satisfied by ordinary PT lumber. This guide covers the chemical difference, the Class A rating, and where each belongs.
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Pressure-Treated Wood vs. Fire-Retardant Treated (FRT) Wood: The Real Difference
Pressure-treated (PT) wood is lumber infused, under pressure, with copper-based preservatives (ACQ or copper azole) that protect it from rot, fungus, and insects. Fire-retardant-treated (FRT) wood is a separate product from standard pressure-treated wood: kiln-dried lumber or plywood infused, also under pressure, with phosphate-, sulfate-, or borate-based chemicals that slow ignition and flame spread. Both use a pressurized cylinder, but the chemicals – and the job each one does – are completely different.
| Property | Pressure-Treated (PT) Wood | Fire-Retardant Treated (FRT) Wood |
|---|---|---|
| Purpose | Resists rot, fungus, and insects | Slows ignition and flame spread |
| Chemicals used | Copper-based (ACQ, copper azole) | Phosphate, sulfate, or borate salts |
| Fire performance | Burns like untreated softwood | Flame spread index of 25 or less (ASTM E84) |
| Test standard | AWPA treatment standards (decay/insect only) | ASTM E84 / UL 723 / NFPA 255 |
| Typical use | Decks, fences, sill plates, ground contact | Fire-rated walls, roof sheathing, draft-stopping |
| Code recognition for fire ratings | None | Required where IBC Section 2303.2 applies |
Is Pressure-Treated Wood Fire Resistant?
No. Standard pressure-treated wood is not fire-resistant. The copper-based preservatives that protect PT lumber from rot and insects have no meaningful effect on how quickly it ignites or how fast flame spreads across it – a PT deck board burns essentially the same as an untreated board of the same species. The only real-world edge PT wood has is that it tends to be denser, kiln-dried southern yellow pine, which can burn slightly slower than a very dry, low-density softwood – but that is a species and moisture-content effect, not a fire-resistance treatment.
A common misconception is that because PT lumber resists rot so well, it must also resist fire – the two are unrelated wood properties, and marketing around “treated wood” rarely distinguishes them. If a project needs actual fire resistance (a fire-rated wall assembly, a roof within a required fire-separation distance, a multifamily draft stop), ordinary PT lumber from a home center does not satisfy that requirement.
What Is Fire-Retardant Treated (FRT) Wood, and How Is It Rated?
FRT wood starts as kiln-dried lumber or plywood, which is then pressure-impregnated with fire-retardant chemicals – most commonly ammonium phosphate, ammonium sulfate, or borate compounds. These chemicals interfere with the wood’s combustion chemistry: instead of releasing flammable gases quickly, treated wood chars on the surface and releases more water vapor and carbon dioxide relative to combustible gas, which slows flame spread.
📊 Class A rating threshold: a flame spread index of 0-25 and a smoke-developed index of 450 or less under ASTM E84/UL 723/NFPA 255, with no significant progressive combustion for an additional 20 minutes. Source: ASTM E84 test method, referenced by International Building Code Section 2303.2.
This is the information most articles on this topic skip: the Class A rating is not a marketing label, it is a specific, third-party-tested number, and code officials can and do ask for the mill certificate or testing agency label on delivered FRT lumber. A board that “feels chemically treated” or is tinted a certain color is not proof of an ASTM E84 Class A rating.
FRT Wood Can Lose Strength – A Trade-Off Worth Knowing
Fire-retardant chemicals can lower the pH of the wood over time, and in some older FRT formulations (particularly roof sheathing exposed to high attic heat in the 1980s-90s) this caused measurable loss of stiffness and bending strength, occasionally leading to sagging or failure. Modern FRT products manufactured to current AWPA standards are formulated to avoid the worst of this degradation, but it is still a real engineering consideration – FRT lumber used in a structural application should be specified and installed per the manufacturer’s span and exposure limits, not treated as a drop-in swap for untreated framing lumber.
Where Building Codes Actually Require FRT Wood (Not Standard PT)
Standard PT lumber satisfies decay and insect requirements for ground-contact and exterior applications. FRT lumber is a separate code requirement that shows up in different situations, most commonly:
- Fire-rated wall and floor/ceiling assemblies in multifamily and mixed-use construction
- Roof sheathing and rafters within a required fire-separation distance of a property line (common in dense urban infill and wildland-urban interface zones)
- Draft-stopping and fire-blocking in concealed spaces of certain occupancy types
- Wildfire-hazard-zone construction (the same zones that often push homeowners toward composite decking as an alternative), where local amendments to the IBC/IRC (notably in California’s Chapter 7A) require ignition-resistant materials for specific building elements
Always check the specific local amendment in effect – fire-separation distance requirements and wildfire-zone rules vary significantly by jurisdiction, and a building department can require FRT wood even where the base IBC language would not.
Best Fire-Retardant Wood Spray Pick
Master Flame Class A Fire Retardant Spray for Wood, Fabric, & More
A Class A rated, NFPA 701/ASTM E84-tested topical spray for adding fire retardancy to existing untreated or PT wood surfaces – not a substitute for code-required FRT lumber, but useful for sheds, fences, and framing in wildfire-prone areas.
- Best for: treating existing wood surfaces that were not FRT lumber to begin with
- Why we picked it: gallon size, NFPA 701/ASTM E84 tested, dries clear over PT and untreated wood
- Main drawback: a topical coating, not a substitute for engineered FRT lumber where code requires tested, labeled boards
Compare more fire-retardant wood treatment options
 Option 1 Clore Fabric & Wood Fire Retardant Spray – Class A Flame Resistant Tre
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 Option 3 Flamecheck M-111PA Fire Retardant Paint Additive (8 ounce) Mix 8 ounes
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As an Amazon Associate we earn from qualifying purchases. A topical spray does not replace code-required FRT lumber where the building code specifies it – check with your local building department for structural/code applications.
Never Burn Pressure-Treated Wood – Indoors or Outdoors
This is the most important safety point in this article: never burn pressure-treated wood, in any fireplace, wood stove, campfire, or burn pile. Burning PT lumber releases the copper-based preservative compounds in the smoke and ash. PT wood manufactured before January 1, 2004, may contain chromated copper arsenate (CCA) – the EPA phased out CCA for most residential uses specifically because of arsenic exposure concerns, and burning old CCA lumber concentrates that arsenic in the ash, where it poses an inhalation and contact hazard.
If you are cutting, sanding, or disposing of PT lumber: wear gloves and a dust mask, work outdoors or with dust collection, wash hands and tools before eating, and take offcuts to a facility that accepts treated wood rather than a residential burn pile.
Frequently Asked Questions
Can you have a fire with pressure-treated wood?
You can burn pressure-treated wood only by accident, and you should never do it on purpose. Standard PT lumber is treated with copper-based preservatives (commonly alkaline copper quat, or ACQ), and burning it releases concentrated metal compounds in the smoke and ash. Older PT wood made before 2004 may contain chromated copper arsenate (CCA), which the EPA restricted for residential use specifically because of arsenic exposure risk from burning or handling. Never burn PT lumber in a fireplace, wood stove, campfire, or outdoor burn pile.
What is the actual difference between pressure-treated wood and fire-retardant wood?
Pressure-treated (PT) wood and fire-retardant-treated (FRT) wood are pressurized with completely different chemicals for completely different jobs. PT lumber uses copper-based preservatives (ACQ, copper azole) to stop rot, fungus, and insects – it has no meaningful effect on flammability. FRT lumber uses phosphate, sulfate, or borate-based fire-retardant chemicals to reduce flame spread and smoke development. A board can be one, the other, both (dual-treated, sold by specialty suppliers), or neither – the two treatments are not interchangeable, and standard PT lumber from a home center is not FRT wood.
Does fire-retardant wood actually meet a Class A fire rating?
Yes, when it is tested and labeled correctly. Under ASTM E84 (the same test method as UL 723 and NFPA 255), a material earns a Class A fire rating when its flame spread index falls between 0 and 25 and its smoke-developed index is 450 or less, with no significant progressive combustion during an additional 20-minute burn. The International Building Code (Section 2303.2) requires FRT lumber to meet this standard and to carry a third-party testing agency’s label – an unlabeled board, even if it feels chemically treated, cannot be assumed to meet code.
Is Ipe or another naturally dense hardwood fire resistant?
Dense tropical hardwoods like ipe are more fire-resistant than typical softwood framing lumber (in the same way some engineered panels, like MDF, only gain real fire resistance from an actual chemical treatment, not from density alone) because their tight grain and high density slow ignition and char rate, and ipe carries a Class A/Class 1 rating in some fire-spread tests. That said, dense hardwood is not the same thing as code-recognized FRT lumber, is not chemically treated for fire resistance, and will still burn given enough heat and time. Don’t substitute a naturally dense species for FRT lumber where code specifically requires tested, labeled fire-retardant-treated wood.
Can pressure-treated wood also be fire-retardant treated?
Yes, dual-treated lumber exists, but it is a specialty product, not something stocked at a typical home center. Combining preservative and fire-retardant chemicals in one board is more complex and expensive than either treatment alone, and some fire-retardant chemical formulations are known to react poorly with certain wood preservatives, sometimes accelerating corrosion of metal fasteners or reducing treatment effectiveness. If a project genuinely needs both properties, order dual-treated lumber by spec from a treating plant rather than assuming standard PT deck boards provide any fire protection.
Is it safe to burn pressure-treated wood scraps in a fire pit or outdoor fireplace?
No. The “outdoors” setting does not make burning PT wood safe. The chemicals released in the smoke and ash – primarily copper compounds, and arsenic in pre-2004 CCA lumber – do not simply dissipate harmlessly outside; they can settle on nearby surfaces, contaminate soil, and still be inhaled by anyone near the fire. Dispose of PT wood scraps through a construction debris service or a facility that accepts treated lumber, never in a burn pile.
“A Class A fire rating under ASTM E84 requires a flame spread index of 25 or less and a smoke-developed index no higher than 450, verified by a third-party testing agency – that is the bar the International Building Code sets for fire-retardant-treated wood, and ordinary preservative-treated lumber was never tested against it.”
Pressure-treated and fire-retardant-treated wood solve two different problems with two different chemical treatments – one is not a substitute for the other. If a project needs rot and insect resistance, standard PT lumber is the right call. If it needs a tested, code-recognized fire rating, only labeled FRT lumber (or, for existing surfaces, a Class A topical fire-retardant coating) actually gets you there. And regardless of which type you’re working with, PT lumber should never go into a fireplace, campfire, or burn pile.
