❄️ Ice dam at the edge of the roof: a winter problem not to be neglected

In northern climates like Quebec's, ice dams at the edge of roofs are a common and potentially damaging problem for residential buildings. This phenomenon, often misunderstood by homeowners, can cause water infiltration , damage to roofing materials , and the formation of mold in ceilings and walls.

What is an ice dam?

An ice dam is a buildup of ice that forms at the lower edge of the roof, often above the gutter or eaves. This buildup acts as a plug, preventing meltwater from flowing freely. Backed-up water can then seep under the shingles and penetrate the building envelope.

Training mechanism

The formation of an ice dam is the result of a thermal imbalance between the upper and lower parts of the roof. Here's how the process occurs:

1. Heat from inside the house escapes into the attic , locally heating the underside of the roof.

2. The snow is partly melting on the upper slopes.

3. The water runs down the roof, but when it reaches the colder parts (the cornices), it freezes , forming a roll of ice.

4. This bead blocks the passage of subsequent meltwater, creating a puddle that rises under the shingles .

   
   

Main causes of ice dams

1. Poor insulation

Inadequate insulation in the attic allows interior heat to escape and warm the underside of the roof, promoting localized snow melting.

2. Insufficient airtightness

Hot air infiltration (through recessed lights, access hatches, electrical boxes, ducts) allows steam to accumulate and condense in the attic.

3. Inadequate attic ventilation

Poor ventilation prevents uniform roof cooling and increases differential heating, particularly in the absence of soffit air inlets.

4. Unfavorable architectural design

Low-sloped roofs, “L” or “T” shaped sections, dormers, deep cornices, shaded areas or snow accumulations can promote snow retention and ice formation.

Consequences of ice dams

• Water infiltration inside the residence

• Deterioration of roof covering (shingles, underlayment, plywood)

• Rotting of the structure or insulation

• Mold growth in attics or walls

• Dangerous icicles falling from the roof

• Damage to gutters or deformation of roof edges

Prevention and sustainable solutions

✅ Improve airtightness

• Seal all air leak points leading to the attic.

• Ensure sealing around light fixtures, hatches and conduits.

• Install a continuous and effective air barrier membrane.

✅ Optimize insulation

• Increase the thickness of the insulation blown in or installed in the attic.

• Avoid discontinuities in the insulation around roof trusses, soffits or ducts.

✅ Correct the attic ventilation

• Ensure soffits are ventilated and unobstructed.

• Check that the ventilation ratio meets the Code requirements (1/300 or 1/150 as appropriate).

• Install efficient ridge or roof vents (e.g. VMAX, Maximum).

✅ Install a waterproofing membrane at the edge of the roof

• When re-roofing, a self-adhesive membrane (elastomeric type) should be installed along the entire length of the cornices, extending at least 900 mm (36") above the exterior wall.

✅ Temporary measures (less lasting)

• Clear the snow from the bottom of the slope after each heavy snowfall.

• Install heating cables in a zigzag pattern, although these are energy-intensive and inefficient without correcting the root causes.

  
  

Conclusion

Ice dams are a symptom of a thermal imbalance within the building envelope . Rather than solely addressing the external manifestations, it is imperative to address the root causes: insulation, airtightness, and ventilation . A professional assessment is recommended to identify building-specific deficiencies and develop a sustainable corrective plan.

GEQ – Bâtiment Conseil offers a detailed expert service to diagnose ice dam problems and recommend optimal corrective measures. Preserve the integrity of your roof and the health of your building by consulting our experts.