Avoiding an ice dam disaster

Frosty day + toasty cottage = ideal—for a roof leak. How to avoid the trickle down

By Pat LynchPat Lynch


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Slipping out of his snow-covered boots, my dad moved across the cottage floor, arms loaded with clean sheets and bathroom supplies. He was making a beeline for the new bedroom, an addition completed the previous summer that had become a place of relative obsession and, predictably, the first place anyone headed upon opening the building. It was a room where we often stood, collectively marvelling at the wizardry of the woodwork and the genius of our expansion plans. But on this day, it was a place where my dad froze in his tracks, paralyzed by the soggy sock on the end of his left leg and a drip of cold water that trickled down his furrowed brow. There was liquid in paradise and an ice dam in the gutter.

In retrospect, I suppose we should have been comforted by the fact that we weren’t alone in our dam misery last winter; heavy snowfalls, combined with warm spells and cold snaps, created roofing havoc all over cottage country. The water that dripped from the doorway, while liquid gold to local contractors and inspectors, indicated a problem lurking in the attic, a problem connected to that deceptively beautiful icicle hanging just outside the frosted window.

An ice dam is a buildup of ice that forms when heat escapes from the living space, warming the attic enough to melt snow on the roof. As it melts, the snow trickles down the shingles to the overhang, which is colder than the roof and causes the runoff to freeze. The buildup of ice on the overhang creates a “dam” that holds back melting snow as it makes its way down the roof. Left to its own devices, the dam can force melting snow under the shingles where it can leak through the sheathing and onto the attic floor, damaging interior walls and ceilings. In our case, the dam sat in the valley where the new roof met the old, and water had backed up enough to breach the roofing and leak into the addition every time we warmed the cottage upon entry. An unwelcome visitor, the water appeared during the new roof’s first winter and took us all by surprise.

If you make a practice of inspecting your roof after the first frost or light snowfall, you’ll be able to pinpoint problem zones by the areas of melted snow on the shingles. By isolating and addressing potential dam locations before the roof is compromised, you can avoid costly repairs that lie hidden under ice until the next thaw.

Don’t do the quick fix

The common reaction to ice damage comes in the form of a quick fix, which may temporarily alleviate ice formation and leakage but doesn’t prevent dams from recurring in subsequent years. Climbing onto your roof and going at the ice dam with an axe or ice pick is a great way to vent frustrations and your attic in one fell swoop. But shingle damage and the likelihood of taking a tumble should be incentive enough to leave the ladder in the shed. If your cottage is powered by electricity, heating cables strung in a zigzag pattern approximately one metre from the edge of the roof will melt ice along the eaves and move water off the roof before it can freeze and back up in an ice dam. However, they require a significant amount of energy over the course of a winter and are dependent on a constant flow of electricity. And unless you can afford to hire your own snow shovelling/sweeping squad to regularly clean your cottage roof, it’s not likely that you’ll be able to stop drifts from turning into dams through a season of frequent freeze-thaws.

At some point, you’ll have to face up to the long-term solution: The attic must be adequately insulated and ventilated to keep the roof cold enough to hold snow. Bill Crawford, senior technical advisor, research, at the Canada Mortgage and Housing Corporation (CMHC), stresses that “the real way to prevent ice damming is to stop the heat rising from inside the building to the attic. You really want the attic to be as cold as you can have it.”

Start by sealing any potential gaps inside your cottage with caulking or expanding foam, says Crawford. Check around light fixtures, ceiling fans, plumbing pipes, and perimeter walls where warm air can escape. Even tongue-and-groove wood ceilings can leak warm air into your attic space. Next, get up in the attic and check the airflow. Properly insulated, it should be a chilly place during winter, well-ventilated and very dry. The idea, says Orillia-based home inspector John Harris, is to maintain an uninterrupted airflow from the soffit vents to the vents on top of the roof. “Often people think they’ve done a good job of insulating their attics, when what they’ve actually done is prevented airflow by covering the soffit vents with excess insulation.”

To help improve airflow you can install ventilation baffles – polystyrene vents that fit between rafters and assist the flow of fresh air from soffit to peak. Baffles ensure that the insulation on your attic floor does not block the crucial ventilation path along the underside of the roof’s sheathing. However, in cottages with low-sloping roofs, ventilation baffles will often leave only a 10 cm gap between roof and floor for insulation. In order to reduce heat loss above the exterior wall, you need to fill that gap using an insulation product with a high R-value. Blown foam works well as it seals and insulates at the same time. Polystyrene insulation board can also be wedged horizontally between joists near the roof edge, with good insulation placed between it and the attic floor. Leave a 2.5 cm space, at minimum, between the sheathing and the insulation board and you’ve created your own baffle while heavily insulating against heat loss above the exterior wall.

It’s also important to ensure that the upper attic vents are not blocked by snow. Because ridge vents tend to get stepped on and crushed during roof work, and must be carefully installed and maintained to work effectively, John Harris prefers to see turbine vents (whirligigs) on cottage roofs. These are less prone to becoming blocked by snow and require little or no maintenance over winter months.

Ventilating a cathedral ceiling can be slightly more difficult, since there isn’t a typical attic space to work with. A tightly sealed, properly insulated cathedral ceiling will not need ventilation, but if you’re not sure that you sealed it perfectly from inside the cottage, you might consider adding ventilation baffles under the sheathing from soffit to peak. However, since you can usually access cathedral ceiling cavities only from the outside, by lifting the sheathing during the re-roofing process, the ventilation process is labour-intensive and costly.

An alternative for those with cathedral ceilings, as well as regular attics, is to make the roof impermeable to water by covering it with a self-sealing membrane underneath the shingles. Commonly known as an ice-and-water shield, or simply ice shield, this membrane is a rubberized asphalt adhesive that bonds tightly to the roof deck. Rolls come in 90 cm widths and varying lengths, ranging in price from $70–$110 a roll. While an ice shield doesn’t prevent future damming problems and will still leave your roof vulnerable to shingle and gutter damage, it will stop leakage into the attic. The Ontario Building Code requires new buildings to have, at minimum, 900 mm of membrane extending up the roof’s slope from the eave. The farther it’s extended, the better the protection. Reg Moore, the Township of McKellar’s chief building official, visited our leaky addition on a number of occasions. He says that after the proliferation of ice dams in cottage country last winter, many cottagers are taking roofing protection to heart and installing shields.

“A lot of people have been doing the whole roof in ice-and-water shield when they’re re-shingling. And after what I saw last winter, I’d say it’s a good idea. Bite the bullet and ice shield the whole thing. It adds a lot of cost, no doubt, but it’s worth it in the end, especially in roofs with a lot of hips, valleys, and dormers.”

Dealing with the leaks

When it came time to deal with the leaks caused by our ice dam, contractor Bob Swann opted for an even heavier-duty, slightly overboard solution. “Because of the low pitch and a flat area on the roof,” says Swann, “we used torch-down membrane, which gives you a durable, long-lasting double ply.” Torch-down systems, which cost about twice as much as an ice shield, often come in multiple layers, the first of which is a two-sided base sheet made of a modified bitumen (tar product) coated with a thin polyethylene film that fuses to the roof and the next layer when heated. They are typically used on flat, unshingled roofs and actually stand on their own as a roofing material, but because our problem involved water settling in a small, unsloped valley, Swann opted for the more labour-intensive roofing solution. “It’s similar to an ice shield,” Swann continues, “but you heat it with a propane torch, which melts it to the roof, then you melt a cap sheet over top of that. Both layers are bonded together and to the roof.” With a patch in place, Swann also slightly altered the pitch of the valley where the ice dam had formed to encourage water to move off the roof rather than settle.

This winter, as the snow begins to fly and the icicles hang low, there will undoubtedly be some anxious moments as we traipse across the cottage floor, searching for the telltale puddle and chilly beads of dripping water. But with good ventilation, fresh membrane in the valley, and a little bit of luck, those dam leaks should be little more than a soggy memory of a winter gone by.

This article was originally published on December 19, 2002

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