The strange case of the fiery deck

Trent Ernst, Editor


It wasn’t something that fire chief Matt Treit has seen every day.

Indeed, the call, about a fire that started outside on a deck is one of the odder ones of his career.

According to Treit, the fire department was called out to investigate a fire that started on someone’s deck with no apparent source of ignition. “I think it was spontaneous combustion,” says Treit.

His theory is fairly radical, but it’s the best guess he has for why the fire started on a hot afternoon last week. “The fire started in the only spot on the deck that had any rot. There was no source of ignition that we could determine. Everyone was inside the house, and there wasn’t any glass sitting out on the deck that could have focused the sunlight. According to them, it just ignited.”

The deck in question belongs to Stephanie Jones, though she was not at home at the time the fire started. “My girlfriend was here with all the kids. I was out running some errands. She was inside putting the baby down for a nap. All the kids were inside the house, cooling off, and one of them said ‘auntie, the deck’s on fire.’”

Auntie’s first reaction was disbelief. “She said, ‘yeah, right,’” relates Jones. “But she looked out and it was on fire. Flames, not just smoking.”

Jones’ girlfriend grabbed a bucket and doused it with water. Jones called the fire department just to have a look and determine the cause.

And that’s where the trouble starts, because there is no apparent cause for the fire. No fire pits. No nearby barbecues. No glass bottles that might have caused the sunlight’s rays to be focused onto the deck. Just a singed section of the deck’s railing, the most charred sections found inside the wood and not on the surface.

Treit says that he’s never heard of treated lumber spontaneously catching on fire. “Haystacks can spontaneously ignite, as can big piles of sawdust.”

As wood rots, bacteria within the rotting wood causes the temperature inside the wood to rise. However, “there must be a sufficient depth of fuel to provide adequate insulation,” says a paper written for the Forest Fire Research Institute by J. Armstrong. “The actual depth depends on the thermal conductivity of the substance being considered.”

This is where the theory is at its weakest. The piece of wood that caught on fire, while indeed starting to rot, was a only two inches thick, like most dimensional lumber. Spontaneous combustion typically occurs within large piles of sawdust in mills, for instance, or in peat bogs, not in two by fours or two by sixes.

Treit says he ran his theory by some other fire professionals, and they believe that it is more likely that another, unknown ignition source was present, but Treit says the fire department couldn’t find anything. “If I had to put money on it, I’d say spontaneous combustion. I wouldn’t stake my life on it, though.”