A natural-gas explosion blew the roof off a northwest-side home and caused the brick walls to bow out Thursday, but the residents escaped unharmed.

"A couple in their early 70s escaped safely despite extensive damage to the home," Capt. Adam Goldberg, spokesman for the Northwest Fire District, said.

When firefighters arrived just after 12:30 p.m., they found heavy smoke coming from the roof and windows of the home, in the 4400 block of North Obetka Avenue, near West Wetmore and North Flowing Wells roads. The windows had been blown out, the seams of the roof had significant gaps and the ceiling was on fire, Goldberg said.

It took about 15 minutes to control the blaze.

"The brick walls bowed out. It blew the roof off. The roof came down just a smidgen from where it belonged. It was a catastrophic structural failure. The structural ability of the home is no longer safe, and it's uninhabitable," Goldberg said. "At some point in the future, the structure will have to be taken down and rebuilt. The home was a total loss."

Multiple entities are investigating the cause of the explosion including Northwest Fire; Southwest Gas; and insurance companies for the couple renting the home, the out-of-state homeowner and workers at the home a day earlier, Goldberg said.

Contact reporter Kimberly Matas at kmatas@azstarnet.com or at 573-4191.