No. FP-2013-22 May 28, 2013
Learning Objective: The student shall be able to describe the elements that identify heavy timber construction (Type IV).
Building codes classify different methods of construction into five “types” for the purposes of establishing requirements for size, height, fire resistance, life safety, occupancy and fire protection. (See Coffee Break Training FP-2009-45 for a summary.)
One of these categories is known as heavy timber construction (Type IV) due to its reliance on heavy solid or laminated wooden components in the structural framing. Typically, heavy timber construction buildings have noncombustible (usually masonry) exterior walls, but the model building codes allow fire-retardant-treated wood framing in exterior wall assemblies with a two-hour fire-resistance rating or less. Where a horizontal separation of 20 feet (6096 millimeters) or more is provided, wood columns and arches conforming to heavy timber sizes are permitted to be used on the weather side of exterior walls.
Concealed spaces (such as drop ceilings, roof/ceiling assemblies, floor/ceiling assemblies or other enclosed spaces where fire could hide) are not permitted.
For fire-resistant properties, heavy timber construction is desirable because the inherent thickness of the wooden elements resists fire impingement. Furthermore, as these thick wooden elements are exposed to flames, a surface layer of char builds up that acts like a layer of insulation and slows the burning rate.
The following table describes the minimum sizes of various construction elements that enable a structure to qualify as heavy timber