These are also called deicing cables and they can be found at your local hardware store or online.
Roof coils to melt the snow.
To be fair heat cables are pretty inexpensive so it s worth a try especially if you can go up on the roof yourself and attach them.
Most coils operate by just plugging them in but some will be connected to a switch or thermostat.
Secure shingle clips to roof shingles using a flathead screwdriver making sure to align them in a zigzag pattern.
Clip in the cable along the same zigzag pattern as the shingle clips.
They will melt snow ice in direct proximity but if you have 3 feet of snow most of it will likely stay there.
Ice is not porous and will trap water.
Install heat cables to melt the snow at the edge of the roof.
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As for when to use them they are intended to prevent ice build up when temperatures are causing.
Snow is porous and allows water to flow.