Maintenance and Repairs

October 8, 2026 Published by Toronto and Area Chapter - By Matthew Foster

Ice Dammed If You Don’t

From the Volume 29, Fall 2026 issue of the CCI GHC Condo News Magazine

This past winter was a notably cold one across the GTA. While cold weather brings fun winter activities that we Canadians enjoy, prolonged periods of snow and freezing temperatures can expose buildings to a lurking terror: ice dam’nation.

Ice damming is a phenomenon that you’ve likely seen, but perhaps not understood how it develops. Large icicles hanging from the eavestroughs, ice along the edge of a roof, or snow melting unevenly across a roof can all be warning signs.

What is Ice Damming?

Ice damming typically forms on sloped roofs when snow on the roof melts. The melted snow sheds to the colder edge of the roof, where it refreezes. If the weather remains consistently cold and snowy, this process will continue. The migration of water as it melts and refreezes near the edge of the roof creates an ice buildup or ‘dam’. Once the dam has built-up high enough, it will cause the melted water to pool on top of the shingle roof. Shingle roofs are not designed to hold standing water, as they rely on water being shed from the roof. When water pools on a sloped roof, this can cause a leak in the house.

Figure 1: Ice damming at a house in Hamilton, Ontario, February 2026

Figure 2: Generated image depicting how Ice Damming occurs from steps 1 to 6.

Why is snow on a roof melting in cold weather?

If the weather is cold, it may seem strange that snow on the roof is melting. In general, snow that melts on a roof is above an attic where heat is inadvertently being transferred to the roof from the interior living spaces below. Once the liquid water hits the colder edge of the roof beyond the footprint of the attic, it freezes.

Another cause of snowmelt and ice damming can be from solar gains combined with complex roof geometries and/or exposures. Roofs featuring deep valleys and orientations that only partially face the sun can create shaded regions. Sunlight heats snow at exposed roof elevations, causing it to melt and shed into valleys or shaded areas, where it refreezes once it’s no longer heated by the sunlight. During winter, the lower path of the sun arc through the sky increases the extent of shaded areas on most roofs, exacerbating this problem. Trying to permanently resolve this type of ice damming is usually more difficult, as it could require significant reconstruction of the roof framing. Generally, this type of ice damming is sometimes “managed” by using heating cables in the shaded areas of the roof, which has its own limitations.

The edges of roofs are more susceptible to ice damming issues. To protect these areas, the Ontario Building Code (OBC) requires a waterproofing membrane under the shingles, along the eaves, which is commonly referred to as “Ice and Water Shield”.

While this membrane can help reduce the risk of water entering the building, it does not address the demon that needs to be vanquished to resolve your ice dam’nation problems.

The Three Main Factors Behind Ice Damming

In our Canadian climate, attics typically include insulation, air/vapour control layers and ventilation, which all play a role in the performance of an attic space.

1. Insulation

Low insulation levels in the attic can allow increased heat transfer from the interior of the unit into the attic space via conduction, which can result in more heat transfer to the roof and ultimately more melted snow.

Figure 3: An attic space with missing insulation.

2. Air and Vapour Control

While there are several types of air and vapour control layers, most attics include a polyethylene sheet under the insulation that is meant to act as the air and vapour barrier. This barrier is meant to prevent warm interior air from leaking into the attic space.

For a polyethylene sheet to perform as an air barrier, all the ceiling penetrations need to be properly sealed. This includes electrical boxes, plumbing stacks, exhaust fans, light fixtures, attic hatches, and other penetrations. Even a small hole in the air barrier can allow substantial air leakage under the right conditions.

Unfortunately, it can be difficult to review the air barrier and the associated detailing, as it is often hidden under the insulation. You may not know you have an issue until ice damming symptoms manifest.

Figure 4: Unsealed air barrier around electrical box.

Figure 5: Unsealed air barrier around fan box

The same polyethylene sheet that acts as an air barrier is often meant to act as a vapour barrier. It is meant to prevent water vapour (i.e. water that is dissolved in the interior air) from diffusing through the building materials. Excessive moisture in attic spaces can cause condensation and associated water damage, which is a separate but often concurrent issue to ice damming.

3. Ventilation

Attics are meant to be ventilated to remove stagnant air and supply fresh exterior air. This helps prevent the accumulation of moisture and heat within the attic.

It may seem counterintuitive, but standing in a well-performing attic should feel like you are standing outside.

To promote air circulation, attics are required by the OBC to have vents at the top and bottom of the attic space. Ventilation at the bottom is typically provided by perforated soffits. At the top, ventilation is generally provided by ridge vents or vents near the roof peaks.

The OBC requires that the area of the vents at the top and the bottom of the attic space be relatively balanced, which is more critical than one might think. If you’ve ever stuck a straw in a glass of water, covered the top with your finger, and pulled it out, the water trapped in the straw is the same concept of how air can get trapped in an attic without balanced ventilation.

Figure 6: Blocked soffit vents

Figure 7: Blocked rooftop vent

Signals of Ice Damming to Watch For

An ice dam does not automatically mean that a building has a major defect as harsh weather can cause ice damming even in well performing attics, but repeated or severe ice damming is worth investigating.

Some warning signs include:

Large or recurring icicles along the roof edge • Thick ice buildup in or above eavestroughs or lower roof edges • Snow melting noticeably faster over certain areas of the building • Water staining on ceilings or exterior walls during winter • Frost or condensation inside the attic • Wet or compressed attic insulation

It can also be helpful to compare similar units within the same townhouse or Condominium complex. If snow always seems to melt on one roof faster than others, with a similar orientation and exposure, then this is a signal that there are attic issues to be reviewed.

Ice Damming Can Be Difficult to Diagnose

The factors behind ice damming can be affected by the smallest of installation details, beyond what we can dive into for this article. For example, small details that can affect attic performance and lead to ice damming include:

Missing or poorly distributed insulation • Thermal bridging • Blocked soffit or peak vents • Improperly installed attic baffles • Gaps around electrical or mechanical penetrations • Poorly sealed or insulated attic hatches • Bathroom or kitchen exhaust ducts that are disconnected, leaking, or improperly terminated • An inadequate type, number, or distribution of attic vents • Gaps or defects in the air and vapour barrier • Understanding where solar gains and roof geometries may be an added factor

Resolving one factor on its own is often not enough to resolve the full issue. The devil truly is in the details, which requires a professional who understands the full dynamics of an attic space, otherwise you could be ice-dammed if you don’t.

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Matthew Foster, P.Eng., BSS, is a Managing Engineer at Brown & Beattie Ltd. with over 13 years of experience. He leads the GTA West Building Restoration Team, focusing on building restoration projects for all condominium building types. He is a member of the Golden Horseshoe CCI Education Committee and prides himself with sharing knowledge with everyone in the Condominium space.