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The issue I'm having is that ice builds up on the lower roof, creating a dam. The upper attic is double insulated, but is exposed to sun throughout the day. The lower roof (next to the sidewall) only sees sunlight for an hour or two very late in the day. So, the upper roof experiences some melt due to the sun and drops to the lower roof where it freezes creating a large dam against the sidewall. Eventually the ice backs up into the seams causing water entry into the interior.

Everything is insulated well, so I think I need to install de-icing cable along the upper edge, along the sidewall, and a bit at the bottom. See photo below - does this make sense, or is there a better way to lay it? Or are there other alternatives to avoid this issue?

My other consideration, is doing a piece of metal roofing (flashing) along the side wall, where the meltwater falls on the lower roof.

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    Where on this planet are you? There's probably a better solution to your ice dam woes than brute-forcing it with a heating cable. – ThreePhaseEel Dec 31 '15 at 18:48
  • Go to the websites of the manufacturers of the cables. They have extensive routing, layout and length help. – Speedy Petey Dec 31 '15 at 18:58
  • @ThreePhaseEel, I disagree. Around here depending on your roof orientation and conditions ice dams are inevitable and very troublesome for some homes. – Speedy Petey Dec 31 '15 at 18:59
  • @SpeedyPetey -- even in an unvented cathedralized roof configuration with a vented over-roof? See this article for more details. (Although the OP's problem isn't a roof insulation issue...) – ThreePhaseEel Dec 31 '15 at 19:04
  • Also, at the OP: is there meltwater dripping from the upper roof when this is happening? – ThreePhaseEel Dec 31 '15 at 19:04
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Instead of cable, put a gutter on the upper roof with a down spout directing the runoff past the edge of the lower roof.

  • Not a bad idea. – Jason Jan 3 '16 at 2:50
  • See this post regarding protecting the lower roof from excess wear due to the runoff. It's applicable whether you use a gutter and downspout or not. In fact, you may want to consider some type of drip protection on both sides of the dormer so that water coming off the upper roof does not wear holes in the lower roof. [link] diy.stackexchange.com/questions/71585/… – JKEngineer Jan 3 '16 at 15:30

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