MONTREAL -- Icicles hanging from rooftops may be a mainstay of Canada's postcard winter season, but don’t be fooled by their charm, they're dangerous, and could be an indicator of problems in your house.

“Icicles are daggers, and they drop,” said home renovations expert Jon Eakes. “And if they drop on your head, when you walk out your front door, this is not a good thing.”

Murder-mystery fanatics have long hailed the icicle as the perfect murder weapon (spoiler: it’s not), for its potential to melt after use. But the realities of rooftop icicles are even more chilling.

Eakes told CTV News icicles and ice dams (formations of ice that build up at the roof’s edge), are indications of poor ventilation or heat loss inside the home. If those problems go unchecked, leaks can develop, and ice can even build up indoors.

So how do icicles form in the first place? Eakes said that if heat from the home reaches the roof, it can melt the snow on top of the house.

When the snow melts, water will cascade down to a point where it freezes again, whether that’s just before the roofs edge as an ice dam, or just below it, in the form of a pointy icicle.

Ice dams can allow water to seep through tiny holes in the roof, causing problems inside. And of course, icicles can fall, hurting people and animals.

There are several problems that can create heat loss or poor ventilation in the house. Eakes said an easy, and sometimes surprising way to find the problems in your home is to take a look at your roof after a light snowfall.

“You can go out and you can count all the hot lights you have in the upstairs ceilings, because there are melted spots … and in really bad cases, they melt even the heavy snow on the roof.”

Lights are one issue, misplaced insulation is another. But there’s one issue in particular that is special to Quebec:

“…The god-awful habit in Quebec of running heating ducts through the attic,” said Eakes, who said people who do that “should be hung up by the icicles and left to dry.”

Eakes recounted one particular episode with a client where he found three large icicles forming inside someone’s home.

How did they get there? Eakes said it became clear when the clients showed him “three beautiful brand-new pot lights put in over a sauna” the floor below.

Air from the sauna had risen through the lights in the ceiling. The moisture in the air had collected on the chilly underside of fiberglass in the attic, forming three-foot-long icicles.

“This was an Agatha Christie mystery about to happen. How did those people die in the sauna? There was no weapon, but when their body has been pierced by something.”

So, what should you do about the shimmering, glass-like shards of ice hanging from your roof? Take a picture, and then break them, said Eakes.

“You want to take photographs because … a renovator can use [them] to go right to the problem,” he said.

Irregular formations of icicles can point a contractor straight to a specific problem and fix it. Even, picturesque icicles can point to bigger problems.

“If it's icicles all along nice and even like a Christmas decoration, the whole attic is in trouble.”

Do you have icicles on your roof? Send a picture to CTV News Montreal on Twitter, Facebook or Instagram for a chance to be featured on our site.