As mentioned in an earlier post about preparing your home to survive an Edmonton winter, I'm going to talk a bit about ice damming; what is an ice dam, why is it dangerous to your home and what you can do to prevent them.
To understand why ice dams are a problem that need to be prevented, you first have to understand what exactly it is. An ice dam is a layer of ice that forms along the bottom row of your roof's shingles. If there is warm air in your attic, it will heat the shingles and cause the snow to melt. The water then runs down your shingles underneath the snow that's built up on the roof. When that water reaches the colder edge of the roof, the water freezes before it runs into the gutter (or if the gutter is blocked by leaves and debris, the ice will build up in the gutter and onto the shingles). The ice that is created forms a dam at the edge of the roof and you now have a dam problem.
This ice dam becomes a problem because as the snow continues to melt, it's not able to drain properly and backs up further and further and gets under your shingles. Since water expands when it freezes, it will start to lift your shingles allowing water to get underneath. Most houses will have another layer of protection installed under the shingles, a roofing paper, however this is not always the case, especially if the roof was a Do-It-Yourself job from a previous home owner (we re-shingled the roof of our first house in 2001 and found that it didn't have any roofing paper under the shingles). Even if you do have roofing paper, it is designed to shed water, not protect against ice. Plus, a lifted shingle is still a problem that can lead to bigger problems.
Lifted shingles are not only more likely to be blown away in a strong wind, but they also will allow rain to be blown up under them. Nothing will deteriorate the plywood sheathing under shingles faster than water that is allowed to accumulate and soak in. Water will also leak through the plywood joints, run into your attic and begin to deteriorate the roof structure. If your attic is warm, you now have a nice little environment where mold could possibly grow unnoticed by the occupants. The exterior protection of your home has now been compromised and you have more than just ice on your shingles to worry about.
The best way to prevent ice dams is to ensure that you have proper ventilation in your attic. Your goal with ventilation is to have enough air flow to keep the attic temperature consistent with the exterior temperature. This will keep the snow from melting as mentioned above.
Attic ventilation is provided through various means: soffit vents, ridge vents, passive vents, gable vents and turbines are the most common ones in the Edmonton area. Some homes will also have electronic controlled fans to keep the air moving. Ensuring that these vents don't get covered by snow is important to maintaining their functionality. Snow accumulation on the roof will sometimes cover the vents which limits air movement. You also want to make sure that your attic insulation is not covering your soffit vents (more on this topic will be covered in a future post).
The other thing you want to do is prevent warm air from your house leaking into your attic. Not only does this warm your attic, but it also increases your heating costs as you're losing that warmth through the attic. The most common area where air leaks into the attic is through the attic access hatch. This can easily be sealed by putting weather stripping around the edge to ensure a nice seal between the hatch door and the frame.
Making sure the air in your attic is flowing smoothly will go a long way to preventing ice dams from occurring on your roof, which will prolong the life of your roof and home.
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