Firefighters from six departments braved hot, humid weather last Thursday to battle a fire at 285 Hayground Road in Water Mill, north of Montauk Highway. The fire was caused by “a major electrical event,” John Rankin, a Southampton Town fire marshal, said this week. It started in the attic and badly damaged the older part of the house, leaving it temporarily uninhabitable. “The fire departments did an excellent job keeping the fire from spreading,” he said. The chief of the Bridgehampton Fire Department, Gary Horsburgh, said the first captain to arrive reported “flames blowing through the roof.” Someone in the house called 911 at about 4:10 p.m., after men working on the house’s chimney spotted smoke coming through the shingles on the opposite side of the roof, Mr. Rankin said. Smoke detectors also sounded. While Mr. Rankin said it appeared the cause was electrical and that it had started in the older part of the house, which has a newer addition. “I couldn’t tell you exactly what the cause is,” he said, noting that insurance investigators were likely to be brought in. The fire was under control in 25 to 30 minutes, according to Jeff White, the second assistant chief of the Bridgehampton department, who was one of the first to arrive. Since flames had gotten behind interior Sheetrock, it was an hour and 15 minutes to an hour and a half before the fire was fully extinguished. The occupants — including the homeowner’s grandchildren, according to Mr. Rankin — were already outside. The south side of the second floor was destroyed, Chief Horsburgh said, and the kitchen below it had heavy water damage. “The north side of the house is in good shape, though. It was a good stop,” he said. Chief Horsburgh said he immediately called for assistance from neighboring departments, partly due to the weather; the temperature was 86 degrees. “Because it was so hot, we have to rotate guys out quickly,” he said. “I didn’t go inside, and I was dying.” Firefighters, wearing 50 pounds of gear or more, can easily overheat, but no problems were reported. Emergency medical technicians with the Southampton Village Volunteer Ambulance joined Bridgehampton E.M.T.s in checking firefighters’ vital signs after they emerged from the building. All firefighters were dismissed by about 6:15 p.m. In all, some 80 to 100 firefighters responded. Bridgehampton had help from the East Hampton, North Sea, Southampton, and Sag Harbor Fire Departments. The latter department brought its rapid intervention team in case anyone needed to be rescued. The Amagansett Fire Department stood by with an engine at Bridgehampton’s headquarters, along with an ambulance from the East Hampton Village Ambulance Association. According to the Southampton Town tax receiver’s office, the property is owned by the 285 Hayground Road limited liability corporation.