A five-alarm fire in Montreal’s east-end left a commercial building with several businesses in it heavily damaged Thursday afternoon.

The fire broke out on the roof of the three-storey building on Letourneux St. and Hochelaga St. in Hochelaga-Maisonneuve about 1:30 p.m., according the Montreal fire department.

Workers were installing a new membrane on the roof and that is believed to be the cause of the fire.

The building includes more than two dozen units, including a dentist’s office. The building, which is a former factory with heavy masonry, was badly damaged by water. The roof and the third floor sustained most of the fire and smoke damage.

An early estimate of the damage is at least $500,000.

The fire was brought under control around 3:30 p.m.

One firefighter was treated for exhaustion at the scene.