There are many applications for fire-retardant-treated wood (FRTW) in the International Building Code (IBC). The 2015 IBC defines FRTW in Chapter 2 as "Wood products that, when impregnated with chemicals by a pressure process or other means during manufacture, exhibit reduced surface-burning characteristics and resist propagation of fire." Section 2303.2 sets the performance requirements for FRTW, including achieving a listed flame spread index of 25 or less, when tested to ASTM E84 or UL 723; no evidence of progressive combustion when the test is continued for an additional 20-minute period; and, the flame front not progressing more than 10.5 feet from the center of the burners at any time during the test.
Recently, there have been questions as to whether painted, coated, and other surface-treated wood products that may meet the flame spread testing criteria can be approved as FRTW, given the requirement for chemical impregnation of the wood. The short answer is that surface-coated wood products which do not have fire-retardant chemicals impregnated into the wood itself do not meet the strict IBC definition of FRTW, irrespective of meeting the flame spread requirements. However, surface-coated wood products can be approved for use in applications where FRTW is permitted, through the code's alternative materials, design, and methods provisions found in Section 104.11, if appropriate testing and evaluation is conducted that establishes performance equivalency to FRTW.
What are these alternative provisions and on what basis can code officials approve surface-coated wood products? Section 104.11 of the IBC gives the code official authority to approve any alternative determined to be "satisfactory and complies with the intent of the code," and is "not less than equivalent to what is prescribed in the code for quality, strength, effectiveness, fire resistance, durability, and safety." One resource used by code officials to help in making this decision is the International Code Council Evaluation Services (ICC-ES), an independent subsidiary of ICC that provides technical evaluations of building products. ICC-ES relies on published acceptance criteria developed in open hearings and approved by the ICC-ES Evaluation Committee. This criteria provides a technical benchmark by which alternative products seeking to enter the marketplace may be evaluated. ICC-ES Acceptance Criteria is currently available for wood products with surface treatments intended to substitute for FRTW, including ICC-ES AC47, AC124, AC264, and AC479. These approved Acceptance Criteria require evaluation of surface burning characteristics of the product, durability and corrosivity of treatments, and effects of treatments on strength or stiffness of the wood substrate. Durability considerations, for example, help to ensure that the surface burning characteristics of the applied treatment remain effective after exposure to weather such as soak-freeze-thaw cycles or alternating UV/rain exposure. Code officials can use the resulting ICC-ES evaluation reports to approve and accept use of products in accordance with the conditions and requirements of the reports.
For information on changes to Section 2303.2.2 on FRTW appearing in the 2018 IBC, see here.