Ice Dams: How They Push Water Into Ceilings and Walls

Ice dam water damage in Ontario homes happens when heat escaping into the attic melts snow on the upper roof, the meltwater refreezes at the cold eaves, and the ridge of ice traps more water behind it. Shingles are made to shed water running downhill, not to hold a pool, so the trapped water backs up under them and runs into the attic, the ceiling and the top of the outside walls. The lasting fix is in the attic, by stopping the heat loss that starts the melt.

How ice dams form

Three things are needed: snow on the roof, outdoor temperatures below freezing, and a roof surface that is warmer than freezing higher up and colder at the edge. The warm part comes from the house. Heated air leaks into the attic through gaps around pot lights, the attic hatch, plumbing stacks, bathroom fans and the tops of interior walls. Thin or uneven insulation adds to it. The roof deck above warms, the bottom of the snow pack melts, and water runs down under the snow.

The eaves overhang the outside wall, so there is no warm attic under them. The water reaches that cold edge and freezes. Each day adds another layer until there is a thick ridge of ice along the eavestrough and big icicles hanging from it. Winters around London, with regular snow followed by sunny days just below zero and repeated thaws, set this up well.

How the water gets into ceilings and walls

Once the dam is high enough, meltwater ponds behind it. Shingles overlap like scales, and standing water simply rises under the overlaps and finds the nail holes and seams in the roof deck. From there it takes several paths.

• Into the attic insulation near the eaves, where it soaks in and sits.
• Onto the top of the outside wall and down inside the wall cavity, wetting insulation and framing behind the drywall.
• Onto the ceiling drywall, showing as a stain or a drip along the outside edge of a top-floor room.
• Around window and door frames below, where it shows as water at the top of the trim.

Signs of ice dam water damage in Ontario homes

Outside, look for a thick band of ice at the roof edge, icicles forming behind the eavestrough or through the soffit vents, and bare patches high on a roof that is otherwise snow-covered. Inside, look for stains at the junction of the ceiling and an outside wall, paint that blisters at the top of that wall, water at the top of a window frame, and drips that appear on sunny winter afternoons with no rain in days.

What to do while it is happening

1. Catch and contain. Put down buckets and plastic, move furniture, and turn off power to any wet light fixture at the panel.
2. If a ceiling is bulging, pierce the lowest point with a screwdriver over a bucket so it drains instead of collapsing.
3. Remove snow from the lower metre or two of roof with a roof rake while standing on the ground. Less snow means less meltwater feeding the dam.
4. Open a channel through the dam. A fabric tube or old stocking filled with calcium chloride ice melter, laid across the ice so it hangs over the edge, will melt a groove for water to drain. Do not use rock salt, which damages roofing and plants.

Do not climb onto an icy roof, and do not chip at the ice with a hammer or axe. Both are how people get hurt and how shingles get broken. If the dam is large, roofing companies can remove it with low-pressure steam.

Drying out after an ice dam leak

The stain on the ceiling is usually the least of it. Attic insulation at the eaves can hold a lot of water and keep the drywall below wet. Wall cavities that took water from the top stay damp behind the vapour barrier. Both are hidden and both are places where mold starts if they are left.

A moisture meter and thermal camera show how far down the walls and how far across the ceiling the water went. Wet insulation is removed, since soaked insulation packs down and loses its value. Soft drywall comes out, and the framing is dried with air movers and dehumidifiers until it reads under 16% moisture content. Water damage jobs like this typically run $1,950 to $11,000. Many Ontario home policies respond to sudden water entry of this kind, but coverage differs, so call your broker or insurer early and ask.

Preventing the next one

Heat cables and roof raking treat the symptom. The cure is a cold roof. That means sealing the air leaks from the house into the attic first, then bringing the insulation up to a good even depth out to the eaves, and making sure soffit vents are not blocked so outside air can flow under the roof deck. Bathroom fans should vent outdoors through an insulated duct. When the roof is next replaced, ask for ice and water membrane at the eaves and valleys.

This winter, watch where the snow melts first on your roof after a snowfall. Those bare patches show where heat is escaping, and they tell an insulation contractor exactly where to start.

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