Spontaneous combustion of discarded floor-finishing materials sparked a four-alarm fire that raced through two brownstones under renovation in South Boston Monday night, the Boston Fire Department said Tuesday.
“A contractor had stored oily rags in plastic trash bags on the first floor of the building at 797 E. Fourth St. in an unventilated area, which ignited, causing the fire,” said department spokesman Steve MacDonald.
No injuries were reported.
The blaze broke out at 10:25 p.m. Monday on the first floor of the vacant building under renovation at 797 East Fourth St. with flames shooting up through the roof and spreading to an adjacent building under renovation.
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“We are fortunate the firefighters got there quickly. Multiple alarms were sounded to get a lot of manpower in there,” MacDonald said. “It was all open inside the building. The fire had a tremendous headway.”
The blaze caused an estimated $1 million damage to buildings at 795, 797, and 799 East Fourth St. Five residents at an occupied brownstone at 799 East Fourth St. were displaced because of fire damage to their building, MacDonald said.
The fire was brought under control within 45 minutes.
The city Inspectional Services Department will investigate the conditions at the construction site, MacDonald said.
The floor-finishing materials included rags with polyurethane that had been used to renovate the building at 795 East Fourth St., MacDonald said.
MacDonald said the contractor was in the process of completing renovations at that building.
Mike Bello of the Globe Staff contributed to this report. John R. Ellement can be reached at ellement@globe.com. Follow him on Twitter @JREbosglobe.