A historic Gastown building went up in flames Tuesday morning, causing 20 residents to scramble outside in their pyjamas.

The three-alarm blaze began around 5 a.m. at the four-storey building on Alexander Street at Columbia, which houses a restaurant at street level and apartments on the upper floors.

No one was injured, and a cause has not yet been determined.

Incendio Pizzeria was severely damaged, as were several floors above the restaurant.

Kelly Mullin, battalion chief of the Vancouver Fire Department, said the materials in the century-old building were highly flammable, with the fire spreading from floor to floor via the walls and plaster.

"It apparently started in the lower portion of the building and got into the old plaster walls and it carried on up to the ceiling of each floor and also got into the plumbing chases and got into the ceiling of each floor."

Those made homeless by the blaze have been given temporary emergency housing.