By: Patrick Carl, P.E.

I-joists are engineered wood products that have become widely used in residential construction.  Wood I-joists consist of top and bottom flange members glued to a thin web member, forming an “I” shape (Figure 1).  Wood I-joists provide several benefits to homebuilders and designers compared with traditional sawn lumber joists: they are lighter, more consistent dimensionally, and easier to work with than sawn lumber.  I-joists can be used over longer spans than sawn lumber, providing more design options and larger open spaces for the home designer.

Figure 1: I-joists supporting the floor of a residence

With these advantages comes one major disadvantage: I-joists are extremely susceptible to fire damage.  The web section is thin compared to sawn lumber, and structural integrity of the web section is quickly compromised when exposed to fire.  A fire can damage I-joists in three ways: the fire can consume portions of the I-joists, heat from the fire can cause failure of the adhesive used in manufacturing the I-joists, and long-term exposure to moisture after the fire can result in moisture damage to the I-joists.

Fire Damage:

Fire damage to the I-joists is identified by charred wood or wood that has been consumed by fire.  Charring of the wood indicates that the cross-sectional area of the I-joist has been reduced.  Since cross-sectional area of the I-joist is related to the strength of the I-joist, charring indicates loss of strength.  As a general rule, I-joists that have been charred should be sistered or replaced.  Close study of the webs should be made because they are thinner than the top and bottom flanges, and are likely to char before the flanges.  The web in the I-joist shown in Figure 2 was consumed by the fire.  The flanges in the vicinity of the consumed web incurred only moderate charring.  This is an example of the susceptibility of the webs to fire damage.  I-joists should be considered fire damaged anywhere from 2 to 4 feet beyond the point where charring is no longer identified.  This provides a reasonable factor of safety against charring that is hidden from view or overlooked during the study.

Figure 2: Fire-damaged I-joist

Heat Damage:

Heat damage to the adhesives used to connect the flanges to the webs can cause loss of strength in the web-to-flange connections.  The adhesive used in manufacture of I-joists varies among manufacturers, and a single manufacturer can change the adhesive formula at any time.  Therefore, it is important to gather the manufacturer’s information directly from the I-joists in order to contact the manufacturer concerning the specific I-joists studied.  The manufacturer’s information is typically stamped on one of the flanges as seen on the top flange of the right I-joist in Figure 1 above.

ASTM D7247[1] is the standardized test method used to measure the affects of heat on adhesives used in laminated wood products.  When contacting the manufacturer, information should be specifically requested on adhesive testing using ASTM D7247.  Test results will indicate the oven temperature used for testing, and this temperature should be used to evaluate heat damage to the I-joists.

It is helpful to know that char oxidation of wood becomes significant around 200° C (400° F).  Since charring indicates fire damage, study of the adhesives is unnecessary for an adhesive meeting ASTM D7247 at a temperature greater than 200° C.  If ASTM D7247 testing of an adhesive does not exceed 200° C, then heat damage to the adhesive should be suspected at any I-joists in the vicinity of charred wood.  Heat-damaged I-joists can be repaired in place, and do not generally require replacement.  Repair of heat-damaged I-joists is not costly, and a conservative estimate on the extent of heat damage is warranted.

Moisture Damage:

Moisture damage to I-joists results from prolonged and continuous exposure to water or high humidity.  Moisture damage should be suspected and studied closely where water stains or mold are present.  Moisture damage associated with a fire is typically found in areas that are exposed to repeated wetting, such as areas in the vicinity of fire-damaged roofs and at walls that allow water intrusion.

Moisture damage is also typically found in areas that are not ventilated.  This can occur between floors where water becomes trapped between the floor sheathing and drywall ceiling, in crawlspaces and basements where water ponds, and in buildings that generally lack ventilation after the fire. 

Moisture damage is not typically caused by water used to extinguish fires.  Although a large volume of water is used to extinguish fires, this water is applied over a short period of time and cannot produce long-term moisture damage, unless the water becomes trapped in the building.

Typically, moisture damage in I-joists first occurs at the web.  The web can be checked for water damage by using a screwdriver or similar tool.  If the web delaminates under the pressure of a screwdriver, or has undergone dimensional change as indicated by areas that are swollen, then the web is considered water damaged.  Mold on I-joists is an indication of long-term exposure to moisture, but does not necessarily indicate damage.  Rot requires a prolonged period of time to cause damage, and unless the structure has been in a state of disrepair for a year or more after the fire, then the rot probably pre-dates the fire.

Repair of fire-, heat-, or moisture-damaged I-joists should be designed by a registered professional engineer.  Several repair options are available to an engineer with I-joist experience.  I-joists exhibiting moisture damage and those with heat damage to the adhesive can be repaired by installing APA-rated sheathing on both sides of flanges, effectively creating a box beam.  I-joists that exhibit charring of the webs but not in the flanges can be repaired using this same method.  I-joists exhibiting charring of the flanges or extensive fire damage to the webs require replacement or sistering.

As with all fire-damaged structures, study of the structure should be accomplished with extreme care.  A general study of the overall structure should be made to identify hazardous areas before detailed study of the structure begins.