Ice Damming is when snow buildup on a sloping roof melts and slides down the roof (usually underneath remaining snow) and builds up when it reaches below freezing temperature air - usually at the eaves. Then it will freeze and build up, not allowing further snow to melt and drain properly due to the dam. Then you end up with a "big ole" mess like this:
The constant temperature changes throughout winter combined with either the sun hitting it to cause more melt and/or improper insulation and then re-freezing can cause the ice to creep up under your shingles and cause leaks inside the home. Repeatedly.
Some homes are more prone to Ice Damming, but let's face it, in Winnipeg we're all most likely going to get Ice Damming at one point or another. Homes like 1 1/2 storeys, with their nooks and crevices, as well as homes with little to no overhang for the eaves are more prone to Ice Damming issues. Converted attic spaces can cause problems as the attic should be the same temperature as outside, and if heat is lost either through the roof or through windows (no overhang) right by the eaves, it can cause melt. That's not to say never to buy these homes! I live in a home both with no overhang, and a converted attic space. It's fine. Don't panic. Just be proactive!
Ways to prevent Ice Damming can be really simple! 1- Get yourself a roof rake. They're cheap and basically like a snow shovel for your roof. Whenever a big snowfall happens, shovel off your roof as best you can. 2- Some people also swear by those heat wires by the eaves. You've seen them, zigzagging along the bottom few feet of a roof. 3- If redoing your roof/shingles, there's a membrane that goes under the shingles that stands up to Ice Damming and can make a huge difference. 4- CLEAN YOUR EAVESTROUGHS! Every fall after the leaves have fallen, and every spring is best. If your eaves are clogged, they can't do their job of draining water AWAY from your house! I know it's a pain. Just do it. Or hire someone else to do it.
I hope this helps clear things up! As fall approaches we need to be mindful of getting our homes ready for winter. A little maintenance can go a long way and save a ton of trouble in future!