Analysis of the accident

The group investigation report revealed that the two workmen had used an excessive amount of construction adhesive. They had immediately enclosed the subfloor framing with two layers of plywood before allowing the construction adhesive to fully dry, off-gas, and ventilate the flammable organic carrier solvents. This effectively trapped the volatile and flammable carrier solvent from the adhesive in the subfloor framing cavities. The use of a pneumatic nailer to adhere the final plywood layer more than likely had ignited a spark into the solvent gases trapped in the framing cavity, which rapidly exploded. Theoretically, a nail either struck another nail or missed the 2×4 sub frame and struck the concrete slab, causing the ignition spark. Either way, the spark detonated the explosive vapors. (Think of this as being similar to the controlled explosion of vaporized gas when it is ignited by a spark plug inside the closed chamber of a car’s engine cylinder.)

The warning label on the adhesive noted several essential conditions for safe use of the product:

• Adhesive contains volatile organic solvents that are flammable and explosive.
• Use only in a well-ventilated area.
• Do not use near an open flame or ignition source.

The investigation revealed that the carpentry crew had no idea of the safety issues involved with using a construction adhesive with a flammable organic carrier solvent in an enclosed cavity in the presence of air (oxygen). The adhesive carrier solvents are some of the components of gasoline and share the same flammability conditions. You wouldn’t enclose gasoline vapors in a floor cavity.
The crew apparently had not read or understood the installation instructions and applications for the construction adhesive because they had been openly smoking in and around the area during the progression of the work.

The odor given off by the adhesive must have been extreme and should have served as a warning to others working in the vicinity. The crew had used nearly 35 tubes of adhesive for work that should have required a third of that. An experienced supervisor might have caught this as the work progressed.

The natural-gas-fired water heater had been on at the time of the accident, so there was an open pilot flame present during the use of the adhesive. Any and all open flames should have been shut off. Tradesmen installing flooring adhesive have known this safety precaution for years.

Filling in the sunken floor with concrete grout may have been a better solution to the requested change. That only becomes apparent through re-evaluation and hindsight.

This was a terrible accident with a combination of unfortunate circumstances contributing to the incident. The blame can go in many directions, but this was an accident. Please review this story with your managers, supervisors, and crews at your next safety meeting to make sure history does not repeat itself.