Fire Resistance - Hourly Ratings
Fire retardant treated wood has a surface burning classification and, by itself, does not have a resistance rating in hours any greater than untreated wood. Fire ratings in hours are assigned to door, wall, or deck assemblies, following testing in accordance with ASTM E 119. References such as the Underwriters Laboratories “Fire Resistance Directory” specifically point out that FRT wood may be substituted for untreated wood in any rated assembly. Dricon® FS FRTW can be used as a component of such assemblies in structures where the code does not permit the use of untreated wood. For additional building code guidance for fire retardant treated wood, please reference the 2018 International Building Code Chapter 6 for allowances based on construction classification.
Descriptions of fire resistance rated assemblies incorporating structural lumber are listed in several publications, with the following being those generally referenced in model building codes:
- Fire Resistance Directory, published by Underwriters Laboratories
- Fire Resistance Ratings, published by Engineering and Safety Service of the American Insurance Service Group
- Fire Resistance Design Manual, published by the Gypsum Association
For example, the Gypsum Association’s “Fire Resistance Design Manual” shows a one hour wall or partition assembly (WP 3605) that has wood studs covered by 5/8” Type X gypsum board with specified nailing and positioning of the panels. This assembly could be used for interior, non-bearing partitions, requiring a one hour rating in a noncombustible structure if the studs were Dricon® FS FRTW. In a similar manner, by substituting Dricon® FS FRTW for untreated wood, other one and two hour wall and ceiling assemblies can be used in noncombustible type buildings. The model codes also permit use of ceiling assemblies with the top membrane omitted where only unused attic space is above.
Some model codes and local building officials will accept the Component Additive Method (CAM) for calculating fire resistance in lieu of actual assembly testing. The CAM concept entails adding the resistance rating of individual components to qualify the resistance rating of the assembly. See next page for an example.
The lumber and plywood used in rated assemblies or CAM listings are usually not identified as being untreated or FRT wood, but the model codes generally do require that any wood used in noncombustible types of construction be fire retardant treated.
Also, due to wood’s natural ability to insulate, wood may be more acceptable than unprotected steel in fire resistant assemblies.