The most common building material in residential structures and sometimes used in commercial structures, is, as you could guess, wood. Wood is an extremely good building material for smaller structures with overall lighter loads, it is easy to work with, relatively inexpensive, and has remarkably good structural properties, especially engineered lumber. Unfortunately, wood is the most combustible building material. Wood can ignite at around 450°F, with char layers developing around 550°F. Overall, these are relatively low temperatures considering the extreme temperatures that can be produced by a typical house fire. The good news for wood members is once the wood member has developed a char layer, the char layer acts as an insulator and protects the inner layers of the wood requiring greater temperatures to degrade the inner core.
The Char Layer
The char layer obviously has no structural capacity; however, the degraded layers can be removed through scraping or media blasting, and the remaining wood still contains residual structural capacity, although less than its original capacity prior to the fire. A structural engineer can take the remaining wood section and determine if the member contains sufficient remaining strength to be used in service again.
These types of calculations are best used for framing members having a larger initial cross-sectional area and are not recommended for smaller, light-framing members such as 2X’s. 2X members that have sustained significant charring should be removed completely. Typically, these members are designed up to their load capacity and do not contain enough residual strength to handle a reduction in cross-sectional area and overall reduction in strength. However, a char layer of 1/16-inch or up 10% of the cross-section area could be justified depending on the loading on the member.
Wood trusses should be treated the same way as 2X members; however, one additional condition needs to be addressed. The connection of the truss member is usually made with a steel gusset plate. If there is damage or discoloration of the plate or the wood members being connected under the plate are charred, the connection is considered to have failed and it is recommended that the members and plate be repaired or replaced.