Fire Phraseology FAQs: FRT versus Fire Rating

A quick explanation of some commonly used fire safety terms and their implications for building construction.

Fireproof? Probably Not.

Because most inhabited buildings have flammable contents and furnishings, there’s no such thing as a truly “fireproof” building. If these contents catch fire, they can quickly generate enough smoke and heat to cause extensive damage. 

While “fireproof” is probably not attainable, fire resistant construction is well within reach. Extensive testing of building components and systems has resulted in UL-certified fire-rated assemblies that retain the structural ability to contain a fire long enough for inhabitants to escape.

Fire Resistance Rated (Fire Rated) Assemblies

A fire resistance rating, often shortened to “fire rating,” measures the length of time that a structure can continue to perform its function when exposed to fire. The rating is determined through testing methods prescribed in the building code and is expressed as an “X”-hour fire resistance rated assembly.

For example, a UL listed one-hour fire rated floor assembly, properly built to specification, will support its design load without collapsing or transmitting flame and high temperatures for at least one hour after a fire commences.

Some building products, such as glulam, may also carry a fire rating. But do not confuse fire rating with fire retardant treatment (FRT); the two are in no way equivalent.

Fire Retardant Treated (FRT) Wood

A variety of wood products are available with fire retardant treatments (FRT), a pressure impregnation process that forces a flame-inhibiting substance into the wood fiber. This treatment inhibits flame spread across the surface of the wood, which may qualify for a reduced fire hazard classification—and yet, this does not necessarily impact fire rating.

False Equivalency: FRT ≠ Fire Rating

Fire retardant treatment is mostly helpful for containing smaller, cooler fires, as it retards surface flame spread. But, at a high enough temperature, FRT doesn’t make much of a difference. Flame spread inhibition becomes irrelevant if the fire is very hot or very large, and FRT treatment does little to contain damages under these conditions.

Fire-Rated Glulam

 

A structural member’s fire resistance is measured by the time it can support its design load during a fire. An exposed beam or column sized for a minimum one-hour fire resistance will support a design load for at least one hour during standard fire test conditions, which simulate an actual fire. To qualify for a one-hour fire rating, a glulam will be manufactured differently, with the core lamination adjacent to the tension lamination replaced by a second tension lamination. As with all other structural framing, specifications of one-hour fire resistance rated glulam should be checked by a professional designer to assure compliance with local codes.

FRT Glulam

While APA doesn’t currently recommend FRT glulams, we understand there are some treaters producing this material. As treatment may cause checking or other impacts on the strength and stiffness of the glulam, designers should request a code report for approval. More information on FRT glulam can be found in APA’s newly issued publication, “Fire-Retardant-Treated Structural Glued Laminated Timber.”

Get this resource and others and learn more about FRT wood products and fire rated assemblies at https://www.apawood.org/fire-phraseology-faqs.