Residents of an 11 storey building in Old Montreal were forced to leave their homes overnight for what is becoming an increasingly common problem.

Shortly after 10 p.m. Sunday the fire department rushed to 1 McGill St., near De la Commune St., when the roof of the building caught on fire.

Dozens of people rushed out of the building, some with assistance, as emergency crews rushed inside to put out the flames.

Some residents initially stayed in their homes, thinking it was a false alarm, until they realized it was not.

"With the smell of the smoke, and that many fire engines responding, and then we could hear people exiting through the stairwell," said one man. The fire was under control within two hours and everyone was then allowed to return inside.

The cause of the fire was a cigarette stubbed out in a flower pot.

Montreal firefighters launched a campaign earlier this year against the dangerous practice.

Last year cigarettes tossed into flower pots caused roughly 60 fires, and this year several fires have broken out because of careless smokers, including the fire that damaged the building used by Le Devoir newspaper, forcing it to use a backup location.

Firefighters say smokers need to know that flower pots are not filled with dirt: instead they are usually a mix of peat moss and fertilizer where hot embers can smoulder for hours before bursting into flame.

"You think you're putting it out in a bunch of earth, but you're not," said Montreal fire department chief of operations Ian Ritchie. "You're putting it out in a bunch of material or chemicals."

Rocco Isgro, a gardening expert at Pepiniere Pierrefonds said the mixtures are designed to retain water and allow air to circulate, meaning they consist of dry materials that get even drier in the sun.

"What's in potting soil is made to retain water, so there could be coconut husk, peat... It's all there to retain water and nutrients," he said.

You might find this flyer on your doorknob soon. @MTL_SIM education campaign about the dangers of putting out cigarette butts in flower pots has started. The number of fires started this way has doubled in recent years @CTVMontreal pic.twitter.com/aYxmiIXjFy — Kelly Greig (@KellyGreig) June 4, 2018

Ritchie advised that a fire might not break out immediately and that it can take hours of even days for the flames to start after a cigarette has been put out.

"Most of the time, people have left their houses, 'Oh yeah, I smoked at 3:00 in the afternoon,' and the fire starts at 8:00 or 9:00 or in the middle of the night," he said. "It goes to show that the smouldering or the heat that builds up in your planter catches on fire and spreads to your house."