Ice dams are one of the most deceptive forms of winter damage Ontario homeowners face. They form gradually and invisibly, causing leaks and structural damage that often aren't discovered until spring — by which time mold, rot, and stained ceilings are already present. Understanding how they form and how to prevent them saves significant money and headaches.
How Ice Dams Actually Form
Ice dams form through a specific sequence of events that requires understanding to prevent properly. Contrary to what many homeowners believe, ice dams aren't caused by cold temperatures alone — they're caused by a temperature differential between different parts of the roof.
Here's the process:
- Heat escapes through the attic floor. In an under-insulated or poorly ventilated attic, heat from the living space below rises and escapes through the attic into the roof structure, warming the underside of the roof decking.
- The warm roof melts snow from below. Snow sitting on the warmed portions of the roof (above the heated living space) melts from the bottom up, creating meltwater that flows toward the eaves.
- Meltwater reaches the cold eaves and refreezes. The eave overhang extends beyond the heated living space below. Without heat from beneath, the eave remains at outside temperature — below freezing. The meltwater flowing from the warm upper roof reaches the cold eave and refreezes, forming a ridge of ice.
- The ice dam grows and backs up water. As more meltwater flows down and refreezes, the ice ridge grows. Eventually, a dam forms that holds a pool of liquid water on the roof above it. This liquid water, unlike ice, can infiltrate under shingles, find gaps in the roofing membrane, and flow into the attic and wall cavities.
Pro Tip: A key indicator that your roof is susceptible to ice dams: look at neighbouring homes after a heavy snowfall. If your roof shows bare, snow-free patches while your neighbours' roofs are uniformly covered, your attic is warm (from below) and melting snow prematurely. This is the ice dam formation condition in real time.
Damage Ice Dams Cause
The damage from ice dams comes in two forms: immediate and long-term. Both are expensive.
Immediate Damage
- Ceiling stains and leaks: Water backing up under shingles penetrates the roof deck and attic insulation, eventually showing up as water stains or active leaks on interior ceilings — often in the middle of a room, well away from any visible exterior damage.
- Damaged shingles: The ice itself physically damages shingles — the weight and movement of large ice masses can lift, crack, and tear shingles, particularly at the eave area where they're exposed.
- Gutter damage: The weight of ice dams frequently damages gutters — bending them, pulling them away from the fascia, or crushing them entirely. This is why eavestroughs and their hangers often need replacement in spring after a heavy winter.
Long-Term Damage
- Mold growth: Wet insulation and attic framing that doesn't dry quickly enough creates ideal conditions for mold. Attic mold discovered during a home inspection or renovation can cost $3,000–$15,000 or more to remediate properly.
- Rot: Persistent moisture in attic framing from repeated ice dam events can cause wood rot in rafters, sheathing, and fascia boards — structural damage that's expensive to repair and easy to miss until it's advanced.
- Damaged insulation: Wet insulation loses most of its R-value (thermal resistance). Blown-in cellulose or fibreglass batts that have been repeatedly wetted by ice dam infiltration need replacement to restore the attic's thermal performance.
The Long-Term Fix: Attic Insulation
The permanent solution to ice dams is addressing the root cause: heat escaping through the attic into the roof system. This means bringing attic insulation up to recommended levels for Ontario's climate.
Natural Resources Canada recommends R-60 insulation in attics for Ontario's climate zone. R-60 means approximately 60 RSI (or about 38 centimetres of blown cellulose, depending on product). Many older Ontario homes have R-20 to R-30 — well below the recommended level — which is why ice dams are common in the housing stock built in the 1960s through 1990s in Kitchener-Waterloo.
Adding attic insulation to reach R-60 also provides significant energy efficiency benefits — reduced heating bills in winter and cooling bills in summer. Many Ontario homeowners find that the energy savings alone pay back the insulation investment within 5–10 years, making ice dam prevention essentially a bonus benefit.
Attic Ventilation: The Overlooked Factor
Proper attic ventilation works hand-in-hand with insulation to prevent ice dams. Adequate ventilation keeps the attic air close to outside temperature, ensuring that even with some heat loss through the attic floor, the roof deck stays cold enough to prevent snow from melting prematurely.
The standard approach is a soffit-to-ridge ventilation system: intake vents at the soffit (low in the attic) allow cold outdoor air to enter and flow upward, while exhaust vents at or near the ridge allow warm, moist air to escape. When this airflow path is clear and properly sized, the attic temperature stays within a few degrees of outside temperature — eliminating the warm-roof-cold-eave differential that creates ice dams.
Common ventilation problems that contribute to ice dams:
- Soffit vents blocked by insulation pushed against them during insulation installation
- Insufficient total vent area for the attic size
- Ridge vent only, without adequate soffit intake (ventilation requires balanced intake and exhaust)
- Bathroom or kitchen exhaust fans venting into the attic rather than outside (adds moisture and heat directly to attic air)
Heat Cables: The Last Resort
Roof heat cables (also called heat tape or heat trace) are electric resistance cables installed in a zigzag pattern along the eave edge of the roof. When energized, they melt channels through any forming ice dam, allowing meltwater to drain rather than pooling.
Heat cables are a legitimate solution for managing ice dams on specific problem sections of a roof, but they're the last resort, not the first choice, for several reasons:
- Ongoing cost: Heat cables draw significant electricity and run for extended periods during winter. They add measurably to heating costs over the season.
- They don't fix the problem: Heat cables manage the symptom (ice dam formation) but don't address the root cause (heat loss from the attic). The underlying insulation and ventilation deficiency remains.
- Maintenance requirements: Heat cables need annual inspection, and failed sections don't work. They also require proper installation with the correct cable type for exposed-to-elements use.
- They can damage shingles: Poorly installed or incorrect heat cables can cause shingle damage over time.
Heat cables are most appropriate when insulation or ventilation improvements aren't practical (unusual roof geometry, heritage homes with limited attic access) or as a temporary measure while planning more permanent solutions.
What to Do If You Have an Ice Dam Right Now
If you're dealing with an active ice dam during the current winter, here's how to manage it safely without causing additional damage.
The Calcium Chloride Sock Method
The safest DIY approach for an existing ice dam is the calcium chloride sock method. Fill a tube sock or mesh bag with calcium chloride (not rock salt — salt damages shingles and gutters). Lay the sock vertically across the ice dam, extending from the roof edge up through the dam to the snow above. The calcium chloride slowly melts a channel through the dam, allowing trapped water to drain.
Important: use calcium chloride specifically, not sodium chloride (rock salt). Rock salt damages asphalt shingles, kills surrounding vegetation, and corrodes eavestrough metal. Calcium chloride is significantly gentler on these materials.
What NOT to Do to an Active Ice Dam
- Never chip or hack at ice dams with ice picks, shovels, or hammers. The ice dam is sitting on your shingles. Any mechanical removal attempts will inevitably damage shingles, granules, and potentially the underlying roof deck.
- Never pour hot water on an ice dam. The temporary melt is immediately followed by refreezing in subzero temperatures, making the ice layer worse.
- Never use a heat gun or torch on roofing materials. The risk of fire to sheathing and roof structure is real, and heat guns damage shingles.
"We see ice dam damage every spring — ceiling stains, warped trim, mold in attics. Most of it comes from homes with under-insulated attics that formed the same ice dams year after year. Fixing the insulation solves the problem permanently; anything else is just managing a symptom."
— David, D&D Home Services Co-Founder
What Role Do Gutters Play?
Gutters get blamed for ice dams frequently, but the relationship is more nuanced. Gutters don't cause ice dams — the temperature differential in the attic does. However, gutters do get caught in the ice dam formation process, and this has implications for both prevention and damage.
When an ice dam forms at the eave, the gutter (which is beyond the building envelope and therefore very cold) becomes incorporated into the ice mass. The ice exerts significant outward force on the gutter hangers as it expands. This commonly results in gutters pulling away from fascia boards, bent gutter profiles, and hanger failures that require spring replacement.
Gutter guards do not prevent ice dams. They protect gutters from debris, but the ice dam problem occurs on the roof surface itself, not inside the gutter. Similarly, cleaning your gutters — while important for preventing overflow and directing runoff properly — doesn't eliminate ice dams if the attic heat loss is present.
Keeping gutters clean and properly attached before winter does ensure they can drain any meltwater that does reach them without backing up, which reduces but doesn't eliminate ice dam water infiltration risk.
Key Takeaways
- ✓ Ice dams form when attic heat melts roof snow, which refreezes at cold eaves — not from outside cold alone.
- ✓ Long-term damage includes mold, rot, ceiling stains, damaged shingles, and crushed gutters.
- ✓ The permanent fix is R-60 attic insulation and proper soffit-to-ridge ventilation — address the heat source, not the symptom.
- ✓ Heat cables work but are costly to operate and don't solve the underlying insulation problem.
- ✓ For an existing ice dam, use calcium chloride socks — never chip at ice or use salt on shingles.
- ✓ Gutter guards don't prevent ice dams — the problem is the warm attic, not the gutter condition.
Good gutter health is one part of winter preparedness. D&D Home Services provides fall gutter cleaning and gutter guard installation to prepare your eavestroughs for winter. Contact us for a free quote — serving Kitchener, Waterloo, Cambridge, and Guelph.
