As for when to use them they are intended to prevent ice build up when temperatures are causing.
Roof coils to melt the snow.
Zig zag cables for metal roofs even on metal roofs the sun and building heat combine to melt accumulated snow at the roof snow interface.
Install heat cables to melt the snow at the edge of the roof.
Ice is not porous and will trap water.
Secure shingle clips to roof shingles using a flathead screwdriver making sure to align them in a zigzag pattern.
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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.
Clip in the cable along the same zigzag pattern as the shingle clips.
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These are also called deicing cables and they can be found at your local hardware store or online.
Use a ladder to carefully reach the top of your roof and use the clips that come with the cables to attach the cable to the roof.
They will melt snow ice in direct proximity but if you have 3 feet of snow most of it will likely stay there.