Ice dams are usually explained with a picture of icicles hanging off a house's eaves. Flat and low-slope roofs get them too, and the leaks they cause are some of the most confusing calls we get: a roof that never leaks in a summer storm suddenly drips during a sunny February thaw. Here is what is going on.
How ice dams form on a flat roof
Snow sits on the roof. Heat escaping from the building, or a sunny afternoon, melts the bottom of the snowpack, and that meltwater runs toward the drains. If the drain, scupper or gutter is frozen, or the path to it is blocked with ice, the water has nowhere to go. It backs up into a pond under the snow, refreezes overnight, and adds a little more ice to the dam each cycle. Meanwhile the ponded water sits against seams, flashings and laps that were never meant to hold standing water, and eventually finds a way in.
Common contributors:
- Drains near cold edges, such as under an overhang or near a parapet, where the roof stays colder than the field.
- Drain lines through unheated spaces that freeze inside the building.
- Scuppers and gutters that freeze first because they stick out into the cold.
- Heat loss through thin or wet insulation, which melts snow from below on the coldest days.
- Low spots where water already collects in summer.
Signs you have one
- Leaks during a thaw while snow is still on the roof.
- Icicles hanging from scuppers or gutters on a flat roof.
- Ice sheets in the same corner every winter.
- Ceiling stains along outside walls in late winter.
Roof leaking right now?
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How ice dams are removed safely
The goal is to get water moving, not to clear the roof to bare membrane. That means opening frozen drains and scuppers, then cutting controlled channels through the ice to them so the backed-up water can leave. Steam and careful hand work are slower than chopping, but they don't cut the roof, and the most common damage we find in spring is membrane cut during somebody's ice removal. Snow around the drains and the leak area is cleared with plastic tools, leaving a protective layer on the membrane. See ice dam removal.
How to prevent them
- Fall maintenance: clear leaves from drains before freeze-up. A drain half-full of leaves freezes first. See the inspection checklist.
- Insulation: fix heat loss near drains and at the roof edge, and replace wet insulation, which insulates poorly.
- Heat trace: in drains, scuppers, gutters and downspouts that freeze every year.
- Drainage changes: add a drain or overflow scupper, or enlarge a scupper that freezes shut. See drainage and ponding repair.
- Seal the weak details: a seam or flashing that leaks when ice backs up will leak again next winter if it isn't repaired in the season.