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Ice Damming 101

Many of us in the north have battled the elements every winter and, while it seems like we're doing everything we can to avoid running into issues, often times ice build-up on roof surfaces can lead to major damage before we even know what's happening.

What is Ice Damming?

Video animation credit: Construction Instruction

Ice damming is a term used to describe the buildup of ice on a roof. Generally, this buildup occurs at the eaves (along the gutters), in valleys and near heat sources (heat sources can be exhausts, vents, skylights, etc.). The buildup occurs primarily around where the heated zone (or thermal envelope) of the home ends, and so most leaks resulting from ice damming are found along the outside walls. The ice buildup can take the form of a 'ski jump' which will impede the flow of water and cause it to pond instead of flowing freely off of the roof.

Ice dam formed on mansard-style roof

What is the Cause of Ice Damming and When Does it Occur?

First of all, ice damming is not a result of a poor workmanship, but it is rather a consequence of a lack of ventilation and/or due to building construction design. Adequate ventilation is a product of both air intake and exhaust working together. For example, roof vents work by allowing air to escape the attic space, but in order for this to happen, fresh air must be able to enter the attic space from the soffit area. If the soffits are partially or completely blocked, airflow is restricted.

Ice damming only occurs when snow melts on a roof in sub-zero temperatures.

10 Point Home Inspections
Image Credit: 10 Point Home Inspections

Typically, ice damming will occur in the following scenarios:

  • Throughout the winter below snow-covered heat sources (such as skylights, bathroom/kitchen exhausts, etc.) as some roofs are more prone to large snow drifts accumulating (if this is you, consider reading our other post on this topic regarding skylights).

  • Intermittently, as many buildings are disadvantaged by design. Low-slope roofs, cathedrals and 'storey-and-a-half' homes are the most difficult to ensure continuous air flow, making them subject to excessive ice damming. But it may not happen every year - some buildings need that 'just right' combination of snow accumulation and outside temperature variations to form ice dams.

  • After a heavy dumping of snow that has blocked off exhaust vents, any roof style or design can begin to form ice dams. Heavy 'all-at-once' snow dumps are an inevitable occurrence these days with our weather becoming more and more inconsistent. It's not only the quantity of snow that we're concerned about, but also the frequency. For example, a lot of snow over a long period of time is not usually a problem for most people, but a lot of snow in a short period of time is a whole other kind of thing. Pipes, vents, skylights, etc. will not have the time to naturally melt away accumulations and will instead become completely blocked off, trapping heat under a blanket of melting snow where it will immediately freeze below.

Why is Roof System Ventilation Important?

Here in the north, roof ventilation becomes far more important than in other climates. Not only do we contest with drastic extremes that can result in condensation beneath roofs, but we also have to contend with ice damming as we heat our homes during winter months. Melting of snow during cold days is not only a sign of potentially excessive heat loss from your home, but it also indicates a high likelihood that ice damming is an issue.

While improving the insulating performance of your home is always a worthwhile consideration, it's not the sole solution to ice damming.

Ventilation is the mechanism by which attic spaces and cathedral ceilings dissipate heat away from the roof surface. It also serves to regulate humidity and prevent condensation. The attic space should be approximately the same temperature as outside to mitigate ice damming. This will prevent the melting of snow in subzero temperatures. For most homes, ventilation is achieved by allowing warmer air to escape through roof vents – but this process requires vented soffits as an air intake for cooler air to enter the attic space. Many older homes (70’s and earlier in Greater Sudbury area) are known to have blocked soffits which, in essence, disables the roof vents. Intake and exhaust are both equally important. Blocked soffits must be unblocked whenever possible.

Why is Too Much Snow a Problem?

Besides the obvious problem of exceeding the designed snow load capacity of your roof, excessive snow accumulation is often a serious problem when it comes to ice damming, too. As all homes are continuously heated throughout the winter, a portion of that heat conducts through ceilings and consequently warms the roof surface (by radiation, convection and conduction). Snow works well as an insulator and it insulates the roof surface from the outside freeze, which means it's warmer under the snow than it is on the top. With enough snow accumulation, the roof surface will warm much easier to a point where melting snow on the bottom layer becomes possible. This melt will flow down and away until it passes the heated zone of the home. There, it can freeze along the roof edge creating an ice dam.

Worse, a heavy, all-at-once dumping of snow from a winter storm - as mentioned earlier - can quickly cover roof exhaust vents and other heat sources like skylights. Normally, heat sources are able to passively clear themselves, melting away snow from normal accumulations and maintaining an open exhaust for air flow. With a heavy dumping of snow, it's a different story: exhaust vents become blocked off, cutting off all air flow and resulting in rapidly increasing attic space temperatures. As the attic temperature increases, so does the roof surface temperature, until snow beings to melt and the ice dam begins to form.

How Does the Leak Actually Occur?

Asphalt shingle roofs are the most common style of lapped roofing in the North. Any 'lapped' roof system is designed to shed water in one direction only. An ice dam will prevent water from flowing freely off the roof, causing ponding on sloped roofs which forces water under shingle laps during freeze expansion.

As described earlier, the ice buildup takes the form of a 'ski jump.' During some of the relatively warmer winter days, this mass of ice may partially thaw on the underside (nearest the heated zone), but remain frozen solid at the roof edge. This allows water to continue to find its way underneath the ice dam with no way to escape. Our experience over the years has been that, with each nightfall or drop in temperature outside, the water trapped under the ice dam freezes and expands, forcing its way up under laps and through just about anything (including an 'ice and water shield' membrane as shingles nailed through it compromise its effectiveness) where it then forms icicles beneath the roof surface. In general, water freezes and expands with such great force that it can push through most residential roofing materials that rely on lapping to shed water.

On a mild day, the ice that has formed in the attic or beneath the roof surface will begin to melt, dripping onto ceilings along outside walls where it may then infiltrate the home. Keep in mind that a leak can occur long before you notice it if it remains frozen above your ceiling. This is why some of the worst ice dams that occur during very cold months are so devastating - a substantial amount of ice can build up in the attic space, while remaining unnoticed in the living space below. Once it becomes more mild outside and the ice begins to thaw, a substantial leak occurs - usually along exterior walls.

What Can be Done to Prevent Ice Damming?

You may be asking "...what about ice and water shield membranes?"

Our common response to this question:

An 'ice and water shield' membrane is better to have than not - but it is not an impenetrable barrier! In fact, asphalt shingles must be nailed in place, which means any ice and water shield membrane underneath will have an average 4 nail penetrations every square foot (likely more). We'd prefer to call it an 'asphalt membrane' and leave it at that. These membranes are an additional layer of protection but ice dam related leaks occur all the time, even with a membrane present beneath the shingles.

In short, an ice and water shield membrane is not your insurance to a known ice damming problem. It is a good added layer of protection, that is all.

Here's what you can do (in order of priority):

  • To start, the space above your ceiling needs good ventilation. For simple designs, this can be achieved by adding more attic exhaust vents while ensuring the soffit is open and acting as an air intake without being blocked by insulation. We recommend high-profile exhaust vents whenever possible. They stay clear of snow in most situations. Check out our article on ventilation: Ensuring Proper Roof Ventilation

  • Ensure ceilings are adequately insulated and vapour barriers are continuous.

  • Eliminate or reduce heat sources in attics where you can, such as bathroom exhausts, holes in vapour barrier, mechanical equipment, potlights, etc.

  • Keep skylights and other heat sources clear of snow, especially after a heavy dumping of snow. Alternatively, an insulated enclosure may be installed over skylights during winter months (see our post on that topic here).

  • Periodic removal of snow and removal of ice dams as they occur is one way to get through a winter in a jam.

  • If your eavestrough/gutters are taking a beating because of the ice, consider: The Toughest Gutter in the North

  • Higher quality 'SBS' asphalt membranes provide better protection. Goldshield (IKO) or Lastobond Shield (Soprema) are both good quality products we use for added peace of mind. 'SBS' modified asphalt membranes are far more flexible and provide a superior bond/seal.

Some homes, by design, are more troublesome until properly address. If you have a 'storey-and-a-half' home, a low-slope cathedral ceiling home or an older home with multiple additions, there is hope - get in touch with our team for a free con.

Sudbury and area, contact us for an assessment and service:

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Ice Damming
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