The use of interior finishes is mostly regulated by restrictions on their flame-spread rating. However, where finishes are used as protection for foamed plastic insulation, they are required to act as a thermal barrier.

Wood finishes not exceeding 25mm in thickness and having a flame spread rating (FSR) of 150 or less may be used extensively in noncombustible buildings, not considered a high building. They may be used on walls both within and outside suites.

Some restrictions do apply in certain areas of a building. The area permitted to have a FSR of 150 or less is limited as follows:

• in exits - only 10% of total wall area
• in certain lobbies - only 25% of total wall area
• in vertical spaces - only 10% of total wall area 

The use of wood finishes on the ceilings in non-combustible buildings is much more restricted, but not totally excluded. In such cases, the flame-spread rating must be 25 or less. In certain cases, ordinary wood finished can also be used on 10% of the ceiling area of any one-fire compartment, as well as on the ceilings of exits, lobbies and corridors.

Fire retardant treated wood (FRTW) must be used to meet the most restrictive limit of FSR 25. Consequently, it is permitted extensively throughout noncombustible buildings. The only restriction is that it cannot exceed 25mm in thickness when used as a finish, except as wood battens on a ceiling, in which case no maximum thickness applies.

The FSR 75 flame limit for interior wall finishes in certain corridors does not exclude all wood products. Western red cedar, amabilis fir, western hemlock, western white pine and white or sitka spruce all have flame-spread ratings at or lower than 75.

Corridors requiring FSR 75 include:

• public corridors in any occupancy
• corridors used by the public in assembly or care or detention occupancies.
• corridors serving classrooms
• corridors serving sleeping rooms in care and detention occupancies

If these corridors are located in a sprinklered building, wood finishes having FSR 150 or less may be used to cover the entire wall surface.

In high rise buildings regulated by NBCC, Division B, Subsection 3.2.6., wood finishes are permitted within suites or floor areas much as for other buildings of non combustible construction. However, certain additional restrictions apply for:

• exit stairways
• corridors not within suites
• vestibules to exit stairs
• certain lobbies
• elevators cars
• service spaces and service rooms