A couple of years back we were dispatched to a structure fire at about 7:30 on a weekday morning. On arrival we found a house under construction with smoke showing. The house was completely framed and finished both inside and out, and in fact the owner was just about to receive his certificate of occupancy. The fire was discovered by a passerby who noticed light to medium gray smoke coming from the structure.

The house was constructed using lightweight materials – wooden I-joists with pressed wood-chip webs, and similar type material for the subfloor. However, we did not find this out until after the fire was extinguished.

The fire began in a plastic bucket of oily rags, which was placed on the bare first floor surface. Spontaneous ignition of the rags caused the fire, which apparently smoldered for quite some time before burning through the 5/8” chipboard floor sheathing and falling onto the basement floor. Once in the basement, the fire continued to burn and involved other materials that also fell onto the basement floor.

As seen in Photo 1, there was no fire showing on our arrival - only light to moderately gray smoke pushing from all cracks and crevices. The house was fully charged with smoke, and the initial hose team elected to make entry through a window on side 1. The first firefighter sounded the floor, then stepped in and immediately fell through the floor in the area seen above and behind the hanging light bulb in the picture. He had apparently sounded the floor above the block foundation wall, which projected about 18 inches into the room. A firm grip on the hose kept him from falling into the basement, and he was rescued by two other firefighters who heard his Mayday. Fortunately he was uninjured.

A second attack team entered the basement from the No. 2 side and carefully made their way to the seat of the fire. The fire was completely extinguished with about 100 gallons of water.

In Photo 2, what you’re looking at is the underside of the first floor as seen from the basement. In the upper center of the picture a saw horse is lying on its side, just about to fall through the roughly 4 x 6 foot hole in the floor. Just below the saw horse, in the background above the block wall, you can see the charred sill plate that bears the first floor outside wall. A deformed natural gas pipe to the left of center in the picture is holding up part of the floor sheathing. The I-joists ran parallel to the pipe and are completely gone in the fire area. The gas pipe probably impeded the firefighter’s fall into the basement.

Photos 2 and 3 really show the extent of the fire damage, an area not more than 15 x 15 feet. About 12 to 15 I-joists were damaged or destroyed, as well as a few sheets of the pressed board sheathing. Beyond the damaged joists, the heat was not enough to melt the 4 inch PVC waste pipe or the light bulbs. Damage to the rest of the house was mainly limited to smoke stains and broken windows; as the house was still under construction, no damage to carpets, furniture or personal belongings occurred.

Photo 3 shows the effects of fire on the I-joists. There isn’t much to burn to begin with, however it appears that heat melted the glue that holds the pressed wood materials together, causing a flaking effect which further weakened the structural member.

Notice the metal studs and the wall surface in the basement in photos 4 and 5: very little heat was present on the wall surface, indicating a relatively low intensity fire in the materials on the floor. Yet the fire continued to burn the underside of the first floor, generating enough heat to melt a light bulb (see photo 4) and drastically weakening the I-joists to the point of failure. It appears to me that the heat of the blazing pile of rags was enough to heat the lightweight materials to the point of vapor production, which sustained a fire within the web space of a few of the I-joists. The result was very little flame spread but complete disintegration of the joists that were involved.

Some people might call this a “good save,” however in reality it was not. There was enough damage to the lightweight first floor framing members that the house had to be completely razed and rebuilt - from the foundation up. From the insurance company’s point of view it was a total loss of about $300,000, so it didn’t really matter whether we made an aggressive interior attack or just stood back and let it burn.

The positive outcome of this fire was that our awareness of lightweight construction dangers has been heightened considerably. We are now marking high hazard structures with reflective red street address signs in place of our standard green ones. Our Chief has been to meetings with our county code enforcement officers to begin a push for a uniform county-wide marking system and database.

Also, by the time we get another fire in a similarly constructed building we will have rehearsed the answer to the questions: Should we, or should we not, make an interior attack on this structure? Is the risk to the interior firefighters worth it? Nope; not on my watch.