Fire-Retardant-Treated Wood
CSI Division 06, Wood, Plastics & Composites
Lumber or plywood pressure-treated with fire-retardant chemicals to reduce its flame spread rating, required by code in specific applications, such as certain roof assemblies in combustible construction types near property lines, distinct from wood that's simply painted with an intumescent coating after the fact.
Why it matters
Some older fire-retardant treatments are known to chemically degrade the wood itself over time, especially in hot, poorly ventilated attic spaces, weakening roof sheathing enough to cause real structural concerns, which is why treated roof sheathing in older buildings sometimes needs inspection or replacement even without any fire ever having occurred.
On a real project
A builder specifies fire-retardant-treated plywood roof sheathing for a section of a multifamily building located close to the property line, meeting the code's flame spread requirement for that specific fire-separation condition.
Who this matters most to
A Carpenter installs fire-retardant-treated wood the same as untreated lumber, but has to confirm each piece is actually stamped as treated where code requires it. A Structural Engineer accounts for the reduced allowable design values some treatments impose on the wood's structural capacity.
Where this goes wrong
A framing crew runs out of fire-retardant-treated plywood mid-installation and substitutes standard sheathing for the remaining few sheets in a code-required area, intending to swap it later. The substitution is forgotten, and drywall closes over the roof sheathing days afterward. It's the general contractor's own closeout documentation review, reconciling delivered stamped-panel counts against installed square footage, that turns up the shortfall, forcing the untreated panels to be located and replaced before final inspection.