A massive fire at an Edmonton condo construction project spread to neighbouring duplexes, leveling a city block on Saturday. Some residents believe the fire was fuelled by cheap vinyl siding.

Homeowners said they were stunned to see the spread of destruction, and say the vinyl was highly flammable.

Ken Smy and Beth Simpson said the siding was melting as they were fleeing out the front door.

"It melted it was like taffy falling off the house," Smy said.

Edmonton fire officials say homes are crammed too close together, and that's part of the problem. But outdated building codes are also an issue.

"The building codes are from 1960," said Nikki Booth of Edmonton Fire Rescue. "So they haven't really kept up with changes in building and in technology.

Officials told CTV News that manufacturers recommend exterior grade dry wall be placed underneath the vinyl siding, but that is rarely done if builders aren't required to install it.

If the drywall had been used, the homes likely would have been damaged but not destroyed.

Edmonton fire officials are calling for tougher regulations.

No one was hurt in the massive blaze, but the 149-unit condo building was completely destroyed, except for an elevator shaft.

Nine neighbouring two-family homes were also destroyed, and a fire department official said 38 other two-family homes were damaged in some way. About 100 people have been left homeless.

Fire damage is pegged at $25 million but officials say that number will jump once smoke damage is tallied up. Investigators say it could be a week before they know what caused the fire.

With a report from CTV Edmonton's Deborah Shiry