Iced up downspouts are often cause for concern. You may wonder if the ice is blocking the downspouts due to a blockage of some sort. While this can be a factor in some cases it's more than likely just frozen water. The snow on the roof melts off the roof due to heat loss through the attic. The melted snow then runs down into the downspout which is not heated by the roof or in direct sunlight. The water instantly turns back to ice creating an ice filled downspout.
Adding more downspout or larger downspouts are not effective strategies for solving this problem. They will ice up just the same as the problem downpipe. If you really want to solve this you have to go to the root cause which is generally a heat loss issue in the attic. If fixing the insulation or ventilation in the attic is not feasible due to money or logistic reasons then a heater cable traced down the pipe can be very effective to keep your pipes free of ice.
Here is a quick video of a Toronto home with no ice issues. The pipe stays free of ice and snow.
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