1. The Science of Ice Dams

Ice dams are not caused by the weather alone; they are a symptom of a building envelope failure. Understanding the mechanics is essential for proper remediation.

How They Form

An ice dam occurs when there is a temperature differential between the upper portion of the roof and the eaves.

1. Heat Loss: Warm air leaks from the living space into the attic due to poor insulation or air sealing.
2. Melt: This warm air heats the underside of the roof deck, melting the snow on the upper roof.
3. Flow: The meltwater runs down the roof under the insulating blanket of snow.
4. Refreeze: When the water reaches the eaves (which overhang the house and are not heated by the attic), it freezes.
5. Accumulation: As this cycle repeats, a ridge of ice forms, trapping subsequent meltwater. This water pools and eventually seeps under the shingles/flashing and into the home.

2. Evaluating Snow Load Risk

While most residential roofs are designed to withstand significant weight (typically 20–40 lbs per square foot in snow-prone regions), specific conditions can exceed these limits.

Understanding Snow Weight

The danger depends on the density of the snow, not just the depth:

• Fresh, Fluffy Snow: Weighs approximately 3–5 lbs per square foot.
• Packed/Wet Snow: Weighs approximately 12–20 lbs per square foot.
• Ice: Weighs approximately 57 lbs per square foot.

3. Emergency Mitigation Strategies

If water intrusion is occurring, immediate action is required to minimize damage to the interior structure. Safety is paramount; homeowners should never climb onto an icy roof.

Interior Relief (For Active Leaks)

If water is pooling in the ceiling (visible as a sagging paint blister or bulge):

Procedure: Place a catch bucket beneath the area. Using a screwdriver or awl, puncture the center of the bulge. This allows the water to drain in a controlled manner, relieving weight and preventing the drywall ceiling from collapsing.

Exterior Removal (The “Melt Channel” Method)

Do not use rock salt (sodium chloride) on roofs, as it is corrosive to metal fasteners and harmful to vegetation. Do not use impact tools (axes/hammers), as they will shatter frozen shingles.

The Calcium Chloride Technique:

1. Fill a leg of nylon pantyhose with Calcium Chloride ice melt.
2. Safely position the stocking vertically across the ice dam so it overhangs the gutter.
3. The chemical reaction will generate heat and melt a channel through the ice, allowing trapped water to drain.

4. Long-Term Prevention: The “Cold Roof” System

Permanent prevention of ice dams requires addressing the root cause: attic temperature. The goal is to maintain a “Cold Roof,” where the roof deck temperature matches the outdoor temperature.

Step 1: Air Sealing (Critical)

Insulation alone is insufficient if warm air can bypass it. Common leakage points include:

• Around chimney chases and plumbing vent stacks.
• Recessed lighting fixtures (can lights).
• Attic hatches and drop-down stairs.

Step 2: Ventilation

A proper ventilation system uses the “Stack Effect” to wash the underside of the roof sheathing with cold outside air.

• Intake: Soffit vents allow cold air to enter at the eaves.
• Exhaust: Ridge vents allow warmer air to escape at the peak.
• Baffles: Ensure insulation does not block the airflow from the soffit vents.