Fire Damaged Trusses
I have been hired by an insurance company to determine the extent of damage to roof trusses exposed to fire. How much fire damage compromises the structural integrity of the truss?
There is no definitive method of determining the degradation of plates and lumber after trusses have survived a fire. We contacted some engineers in the industry to get an informal description of their methods for dealing with this situation. Some will specify repair or removal of any charred material. Some will specify repair or removal of lumber that has lost over 10 percent of its cross section due to charring. Some will allow up to 1/16 in. char depth on the assumption that it will not reduce the strength markedly. Lumber that is discolored by smoke damage but not charred is usually considered acceptable after it has been cleaned. If there is damage to the plate area, the plate is discolored or there is charring under the plate, the plate should be considered ineffective.
Truss chords and webs can be repaired using properly sized and attached lumber scabs over the damaged areas. Joints are often repaired using plywood or OSB gussets that are properly sized and attached to transfer 100 percent of the forces in that joint. In some cases, the entire truss is replaced. All of the engineers we spoke to stressed the fact that these were not one-size-fits-all solutions and that one should consider the specific circumstances before choosing a repair strategy.
Each situation requires a separate professional engineering assessment. The most conservative solution is to replace all charred or smoke damaged trusses.
For additional information on fire issues related to trusses, see the fire section of SBCA’s website.