A fire started by a roofer's torch that was thought out caused a four-alarm blaze Tuesday night that gutted one of two buildings of the Harrington Square Apartments, according to Bill Flora, Renton's deputy fire chief.

City investigators determined the cause of the fire was accidental.

Damage to Harrington Square is estimated at $11 million, about half of the $23.5 million value of the total apartment complex. The damage to an adjoining building is estimated at $250,000 and $12,000 to structures downwind from the fire, according to city officials.

About 85 firefighters from agencies in South King County and the Eastside responded to the fire, visible as far away as Tacoma. They battled the fire for about 3 1/2 hours.

No one was hurt fighting the fire and no one was inside the building, which was still under construction.

The general manager of the development company, DRK Development, Inc., of Lakewood, said Thursday the owners plan to rebuild. The complex's other half will open later this summer as planned, according to Joe Kaiser, the general manager.

The developers have worked for seven years to bring the upscale apartment development in the Highlands to reality, including about five years to buy the land and work through the development process.

The second building that was destroyed was about 40 percent complete.

The city supports the project, hoping it will help revitalize that part of the Renton Highlands.

"It's pretty tragic," said Kaiser.

Harrington Square is at 950 Harrington Ave., not far from Highlands Elementary School. One lane of Harrington Avenue in the immediate area until late Monday or Tuesday so that debris can be removed.

The investigation into the fire's cause, completed Thursday, found that the roofers were working Tuesday in the area where the fire started – a front corner of the building, according to Flora.

As sometimes happens in a blow-torch roofing process, some of the roofing material caught fire.

"They thought they had it out," said Flora.

Roofers use a torch to heat up the roofing material so that it bonds to what's underneath. The idea is to create a waterproof seal.

Too much flame can ignite the roofing material, according to Flora. Typically, roofers will stay on site for a time to ensure the fire is out. The fire started at about 8 p.m.

The fire started in the attic space, according to Flora.

The roofers had eight propane tanks; the propane is used to fuel the torch. Investigators found five tanks, all vented of their gas and pressure, which happens in the presence of fire.

The three other tanks weren't found. They were likely empty, but still accounted for the explosions that witnesses heard because they still had some gas residue.

The fire department contacted the federal Bureau of Alcohol, Tobacco and Firearms to cover all bases in its investigation; but federal investigators weren't involved in the probe, according to Flora.

The federal agency, among other things, will investigate possible arson cases. That possibility was ruled out, as were such causes as electrical, fireworks or something natural, Flora said.

Sometimes financial difficulties can lead to arson. This wasn't the case.

"This is probably one of the more respectable contractors that we have worked with," said Flora.

The building was turned back to the owners Thursday. However, the owners and their insurance company will conduct an investigation, as will the insurance company for the roofing company, Patterson Roofing of Fife.

Kaiser, the develoment company's general manager, said Thursday he wouldn't speculate about liability. He said this is the first time his company was worked with Patterson Roofing, calling the company a "great subcontractor."

Never has his company faced such a devastating fire, Kaiser said.

But he is grateful the loss wasn't greater. He credits the Renton fire crews with saving the completed complex of apartments. It suffered little or no damage.

A northerly wind kept the flames away from the other building and fire crews poured a wall of water between the two buildings.

"They did a fantastic job," he said.

Eight to 10 homes on Index Place were evacuated after a report of a roof fire Tuesday night. But through quick defensive protection against falling embers, fire crews were able to prevent more damage to nearby structures.

Next week, the developers will begin cleanup and have engineers determine if there is any structural damage to the building, Kaiser said.

A "wild guess" is that the fire has delayed completion of the second building, which was fully framed, by four or five months, Kaiser said.

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