How a dam forms
Heat leaking from the living space into the attic warms the underside of the roof deck. Snow on the warm part of the roof melts, runs down to the eave, which hangs over open air and stays cold, and refreezes there. Do that for a few days and you have a wall of ice with standing water behind it.
Shingles shed water running downhill. They are not built to hold back a pond, so the water works sideways into the laps and through the deck.
What does not fix it
Chipping at the ice with a hammer breaks shingles. Salt tablets leave stains and eat metal flashing. Both are short-term moves at best.
What holds
The real fix is keeping the roof deck cold and giving water a protected path if it does back up.
- Air seal the attic floor so warm indoor air stops leaking up.
- Insulate to the recommended depth, evenly, without blocking the eaves.
- Vent the attic with intake at the soffit and exhaust at the ridge.
- Install ice and water barrier at the eaves whenever the roof is replaced.