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Reading Your Own Roof

Lesson 12. Ice Dams, Start to Finish

An ice dam is caused by heat escaping into your attic, which is why the same few houses on a street get one every winter and their neighbors never do.

At a Glance

  • Cold weather does not cause ice dams. A warm attic does
  • Never a hammer, never salt, never a pressure washer
  • A roof rake from the ground is safe and genuinely helps
  • The permanent fix is air sealing, insulation and ventilation

The Sequence

  1. Heat leaks from the house into the attic
  2. The attic warms the roof deck from underneath
  3. Snow on the warm part of the roof melts, even at twenty degrees outside
  4. The water runs down to the eave, which overhangs open air and stays cold
  5. It refreezes there and a ridge of ice builds
  6. More meltwater arrives and pools behind the ice
  7. Standing water works backward under the shingles and into the house

Shingles shed water running downhill. Nothing about them holds back a pond, and a pond is what the dam creates.

Why Your Neighbor Does Not Get One

Same street, same weather, same snow. The difference is heat loss. Recessed lights cut into an insulated ceiling, an unsealed attic hatch, a bathroom fan venting into the attic, gaps around a chimney chase, or simply not enough insulation.

A house that gets an ice dam every year is telling you it is losing heat every year. The ice is a receipt for a heating bill you are already paying.

Getting One Off Without Wrecking the Roof

  • Steam is the only fully safe method and it is what a professional uses
  • Never a hammer, an axe or a chisel. Those break the shingles under the ice
  • Never a pressure washer. It strips granules and forces water under the shingles
  • Never rock salt. It stains the roof and corrodes metal and plants below
  • Calcium chloride in a sock laid across the dam will melt a channel. It is a stopgap
Worth knowing. A roof rake used from the ground, clearing the first three or four feet above the eave after each real snowfall, prevents most dams before they form. Do it from the ground only.

Ending It Permanently

Three steps, in this order. Doing them out of order wastes money.

  1. Air seal. Find and close every gap where warm air enters the attic
  2. Insulate. Bring the attic up to a modern depth, evenly, without blocking the eaves
  3. Ventilate. Make sure soffit and ridge venting is open and continuous

Done properly, the roof deck sits close to outside temperature and the snow simply stays there, the way it does on your neighbor's house. Your heating bill drops on the way past.

Next. Lesson 13. How Roofing Is Priced
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