An ice dam is a ridge of ice that forms along the edge of your roof, usually right above the gutter line, and blocks melting snow from draining off normally. Once that water has nowhere to go, it backs up under your shingles and can work its way into your attic, your walls, and eventually your ceiling.
How they form
Ice dams start with heat loss. If your attic is warmer than the outside air, it warms the roof deck above it, which melts the bottom layer of snow sitting on the upper part of your roof. That meltwater runs down toward the eaves, but the overhang isn't warmed from below the way the rest of the roof is, so the water refreezes right there. Over time, that refrozen ridge grows and traps more meltwater behind it, which is what eventually pushes water up and under your shingles instead of off the roof.
When to check
Check after any heavy snow accumulation, especially when it's followed by a freeze-thaw pattern: a sunny or above-freezing day that melts snow on the roof, followed by a hard freeze overnight. A few inches of snow that sits for several days with fluctuating temperatures is exactly the setup that grows an ice dam. It's worth a quick look from the ground after every significant snowfall, and a walk through the attic if you've had a stretch of that freeze-thaw weather.
Warning signs
- Icicles along the roofline with a visible ridge of ice behind them, rather than icicles hanging free from a clear gutter
- A noticeable buildup of ice at the eaves that looks thicker than the snow above it
- Water stains or damp spots on ceilings, especially near exterior walls
- Moisture, staining, or ice visible on the underside of the roof deck when you check the attic
What to do if you find one
- Use a roof rake from the ground to clear snow 3 to 4 feet back from the edge of the roof, reducing the amount of snow available to melt and feed the dam
- Fill a nylon stocking with calcium chloride ice melt and lay it vertically across the dam to melt a channel for water to drain through
- Never use rock salt, it can damage roofing materials
- Don't chip or hack at the ice, that risks damaging your shingles
- Stay off a snowy or icy roof. If you can't safely reach the problem area with a roof rake from the ground, call a professional
Preventing them in the first place
The real fix is keeping your attic close to the outdoor temperature, since a cold attic doesn't melt snow from underneath. That means good attic insulation, proper ventilation, and sealing the gaps where warm household air leaks upward, around light fixtures, the attic hatch, and plumbing or duct penetrations. Clean gutters help too, since a gutter already clogged with leaves and debris gives meltwater even less of a place to go.
