Families just weeks away from moving in to their dream homes will now have to start all over again after a large fire ripped through a cluster of houses under construction in Richmond Hill Monday.

The fire broke out at about 4 p.m. and left a group of homes on Canyon Hill Avenue badly burned.

Fire crews say the houses will have to be knocked down and rebuilt from scratch.

Deputy Fire Chief Bryan Burbidge said the blaze spread quickly because there was no drywall inside the houses to act as a fire break.

"Every piece of wood is exposed, the fire can take hold very quickly and it spreads very quickly throughout the building," he said.

Several neighbours believe the fire may have started in one of the houses where a welder was working.

Fire investigators say that will be part of the investigation.