Ice dams: safe removal, the damage they cause and prevention

How an ice dam forms

Heat escaping from the house warms the upper part of the roof and melts the snow lying on it. The meltwater runs down to the eaves, which overhang the outside wall and stay cold, and freezes there. Over days the ice builds into a ridge. Water pooling behind it has nowhere to go but sideways and up, under the shingles.

So an ice dam needs three things: snow on the roof, a warm roof deck above the living space and cold eaves. Take away the heat loss and most ice dams never form.

Signs you have one

• A thick band of ice along the eaves, often with large icicles
• Water stains or drips at the top of exterior walls or on ceilings near them
• Water coming through window or door frames
• Ice or water on the outside of the wall, below the soffit
• Sagging or dripping in the attic insulation near the eaves

Dealing with one safely

From the ground, with a roof rake. Pull the snow off the lowest metre or two of the roof. With less snow feeding it, the dam stops growing. This is the one step most homeowners can safely take.

Steam, by a professional. Low-pressure steam cuts channels through the ice and removes it without harming the shingles.

Inside the house. Catch drips, move belongings and, if a ceiling is bulging with water, keep clear of it.

What not to do

• Do not chip or hack at the ice. Hammers, axes and shovels break shingles and eavestroughs.
• Do not use a pressure washer or a torch.
• Do not put rock salt on the roof. It damages roofing, metal and the plants below.
• Do not climb onto an icy roof.

The damage inside

Water from an ice dam soaks the attic insulation at the eaves, then the top of the exterior wall and the ceiling. Because it is hidden inside the wall and above the ceiling, it often goes unnoticed until stains appear. Wet insulation and drywall that are left in place can grow mold within days once the weather warms.

After a leak, the affected ceiling and wall cavities need to be checked with a moisture meter, wet insulation removed and the structure dried. See water damage restoration.

Insurance

Sudden water entry caused by an ice dam is covered by many home policies, though the cost of removing the ice itself often is not. Check your wording and report the damage promptly.

Prevention

• Air seal the ceiling. Stop warm air leaking into the attic around lights, hatches, stacks and fans.
• Add attic insulation to current levels.
• Ventilate the attic so the roof deck stays cold: clear soffits and adequate roof vents.
• Keep eavestroughs clean before winter.
• Rake the roof edge after heavy snowfalls.
• When re-roofing, make sure an ice and water membrane is installed along the eaves.

Heating cables treat the symptom. Air sealing, insulation and ventilation treat the cause, and they also prevent attic mold.

Which roofs are most at risk

• Older houses with little attic insulation, where a lot of heat reaches the roof deck
• Cathedral ceilings and finished attics, where there is little space for insulation and ventilation
• Complex roofs with valleys and dormers that collect snow
• Low-slope roofs, where water drains slowly
• Roofs with heat sources below, such as pot lights, attic hatches, bathroom fans and ductwork in the attic
• Wide eaves, which stay colder than the rest of the roof

North-facing slopes hold snow longer, but dams often form first on slopes that get sun in the day and freeze at night.

What a professional removal involves

A crew works from the eave upward with low-pressure steam, which melts through the ice without the force that would strip granules from shingles or lift them. Channels are cut first so that trapped water can drain, and then the ice is removed in sections. Snow is cleared from the roof above so that the dam does not re-form immediately.

It is slow, careful work. Be cautious of anyone proposing to use a hammer, an axe or a high-pressure washer.

After the thaw: checking for hidden damage

Even if nothing dripped inside, look for:

• Stains or soft spots along the top of exterior walls and on ceilings near them
• Peeling paint or bubbled drywall
• Damp or compressed insulation at the eaves, seen from the attic
• Water marks on the roof sheathing above the outside walls
• Stained soffits and fascia outside

Wet insulation at the eaves loses its insulating value, which makes the next ice dam more likely. Wet drywall and framing left in place can grow mold once the weather warms.

The permanent fix

An energy auditor or insulation contractor can find the air leaks with a blower door test and seal them, then bring attic insulation up to current levels and make sure soffit and roof vents are clear. When the roof is next replaced, an ice and water membrane along the eaves and in the valleys adds protection.