Beer Bottle, Sun Caused 2021 Stoneham House Fire: Investigators

STONEHAM, MA — Fire investigators have determined the cause of a house fire on Hersam Street in Stoneham last year was actually caused by sunlight and a beer bottle.

At first, investigators believed the March 6, 2021, house fire was caused by discarded cigarette butts that were found on the second-floor back deck, but it was later discovered that no one had been on the deck in the hours leading up to the fire.

Instead, the fire started when sunlight hit a glass bottle that was left on the back deck with combustible trash items, Stoneham police Sgt. Dave Thistle said. The fire at 24 Hersam St. broke out around 11:15 a.m. and progressed to three alarms. The blaze injured two firefighters and caused more than $700,000 in damage.

Everyone in the three-story building was safely accounted for and the fire was extinguished, but the home was deemed a total loss, Thistle added.

Of the about 16,000 structure fires in Massachusetts every year, only 10 — or less than one one-thousandth of 1 percent — are caused by direct sunlight, State Fire Marshal Peter Ostroskey reported.

"It's uncommon but definitely not unheard of. We've seen several fires involving windows, mirrors, and other glass surfaces," Ostroskey said. "This was the first one we can remember involving glass bottles, but there’s no question these surfaces can concentrate sunlight into a competent heat source under the right circumstances."

Fire investigators say they can't rely on assumptions when conducting investigations. Instead, they follow the evidence, narrowing the investigation by eliminating possible causes until only one remains.

"We had a K-9 come in to make sure there were no accelerants, and we worked to rule out every possible cause we could come up with and narrowed it down to smoking or our rare solar theory," Thistle added.

Investigators concluded that the deck received about five hours of direct sunlight before the fire, which is enough time for the sun's rays to be focused strongly enough through the glass bottles and onto the combustible items nearby.

"We are all familiar with the concentration of sunlight by a convex lens or a concave mirror," said Jeffrey Baumgardner, senior research scientist at the Boston University Center for Space Physics. "Some bottles (with or without some liquid inside) could act as a lens … There are well-documented cases where large buildings with curved faces covered by glass have focused the sun on cars and have melted the plastic in the interiors."