Large dimension wood sections have an inherent resistance to fire. Wood burns slowly at approximately .6mm/minute. The char created on the wood surface as it burns helps protect and insulate unburnt wood below the charred layer. The unburnt portion of a thick member retains 85 to 90 percent of its strength.

Hence, a wood member with a large cross-section can burn for a significant amount of time before its size is reduced to the point where it can no longer carry its assigned loads.

Heavy timber construction is defined in the NBCC as: "a type of combustible construction in which a degree of fire safety is attained by placing limitations on the sizes of wood structural members and on thickness and composition of wood floors and roofs and by the avoidance of concealed spaces under floors and roofs."

Both solid-sawn and glue-laminated members qualify under this definition provided they have the minimum sizes given in the table below. Of course, they must be designed to carry the expected loads and actual dimensions must conform to CSA Standard 0141, Softwood Lumber. 

To satisfy heavy timber requirements wood elements must be arranged in solid masses with essentially smooth, flat surfaces to avoid thin sections and sharp projections. This is to reduce to a minimum the surfaces which can be exposed to fire.

For the same reason, when roof arches, trusses, beams or girders are made from several pieces, the connection elements must be a minimum of 64mm thick and be protected by sprinklers. Where not protected by sprinklers, they must be built so that they constitute a solid mass or have the voids blocked off on the underside by a continuous wood cover plate at least 38mm thick.

1. National Building Code of Canada, NRCC No. 38726, National Research Council, Ottawa, ON, 1995.