FRTW specified in the NBCC must have a flame-spread rating of not more than 25. It therefore qualifies as an interior finish for any application since the most restrictive flame-spread rating is 25. FRTW must be identified by a label (as shown below) from an independent testing laboratory which indicates that the necessary tests were made and production controls maintained.
For many wood species, and particularly plywood and lumber in sizes common to frame construction, treatment results in chemical retentions high enough to obtain a flame-spread rating of 25 or less. It should be noted that the chemicals will not usually penetrate the entire wood member, refusal will usually occur when the chemicals have penetrated approximately 13mm from the outer surface.
The actual flame-spread rating of treated lumber or plywood depends on the fire-retardant chemicals used and the amount of chemicals retained in the wood.
Commonly used chemicals are proprietary mixtures which are free of halogens, sulphates, ammonium phosphate and formaldehyde. These provide superior performance characteristics over previous formulations and lower corrosivity to metal fasteners. These water-soluble chemicals are effective in reducing flame spread, and through careful proportioning succeed in reducing smoke development and afterglow.