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Damn! Ice dam!

The Old House Web

Note: The following information is used with permission ofServiceMagic, which provides free contractor recommendations from its database of 40,000 home improvement contractors & real estate agents.

Every few winters many cities are plagued by ice dams. They generally appear after a heavy snowfall during an extended period of cold temperatures. Because ice dams occur on roofs buried in snow, few people actually see them, let alone understand their causes.

The only time people think about ice dams is when they occur, which is about the only time that almost nothing can be done! This guide is written to help people better understand the phenomena and take steps to prevent what can be a nightmare of a problem.

Snow covered roofs look picturesque, but can become problematic. -- Photo: OHW, 2004

What are ice dams?

Ice dams are literally dam-like buildups of ice on a roof. They normally occur at the edge of the roof but can occur higher up under certain circumstances. The ice itself is not a problem. What is a problem is that melt water from the snow can form a pool behind the dam. Most roofs are very effective at shedding water. Very few roofs, however, are designed to cope with pools of water.

Water backs up behind the ice, seeping up under the shingles until it finds its way through the roof to the attic and living areas below. While usually temporary in nature, the sometimes massive roof leaks the dams cause can wreak all kinds of havoc with wiring, drywall, paint, carpet, flooring, and all manner of interior furnishings. They can also cause problems for your exterior siding and the wood and insulation behind it.

What causes ice dams?

Ice dams occur most often when we get heavy snow followed by below freezing temperatures, but they can occur with just an inch or two of snow. Some part of your roof warms up enough to cause snow to melt. The melt water flows down to another part of the roof that is cooler, and the water refreezes. The ice forms a small dam that builds up slowly as more and more melt water refreezes. Eventually, water backs up behind the dam and works its way up under the shingles until it begins to leak through the roof into the living space below.

Essentially, there are four causes, three of which we can do something about. They are:

  1. Weather
  2. Household heat escaping to the attic
  3. Uneven roof temperatures
  4. Roofing underlayment that water can penetrate

Our relative ignorance of ice dam causes and remedies makes us more vulnerable in some ways. Every time there are ice dams here, people end up getting injured, damaging their roof, or just wasting their money because they address the problem in inappropriate ways. Ice dam prevention is best achieved by addressing attic insulation and attic ventilation and by using good re-roofing practices. Each of these preventive measures is discussed below, followed by a section answering often-asked questions about ice dams.

Attic insulation

While some warm spots on the roof occur because of sunlight, most are caused by heat escaping from the heated portions of the house. Most homes have attic insulation, but in many cases there isn't a sufficient amount or the insulation has settled, gotten compressed, gotten wet, or has otherwise become less effective. The goal is to have an attic that stays very close to outdoor temperatures. In many homes the problem is breaks or gaps in insulation due to light fixtures, pipes, settling, or foot traffic.

Pull-down attic stairs and other access doors are often quite leaky. Weather-stripping or building a double door (analogous to a storm door on the front of a house) can help seal up these access points.

In many homes, heating ducts run through the attic to reach other parts of the home. If the ducts themselves have a leak or if they are insufficiently insulated, the leaking heat can cause an ice dam.

Some older homes have bathroom or kitchen exhaust fans that blow warm and/or moist air into the attic. These exhausts can be a problem because, besides leading to warm spots, the moisture reduces the effectiveness of the rest of the attic's insulation. All exhaust ducts should be vented to the outside.

Attic ventilation

Even a well-insulated attic is likely to have some heat escaping from the house. It is also likely to be affected unevenly by sunlight. Proper ventilation can smooth out temperature variations and largely eliminate the risk of ice dams. This attic ventilation is usually the easiest preventive step, yet it is the step most often omitted.

Air must be able to circulate freely under the roof. Most attics are designed to take advantage of the fact that heat rises. In a typical home, there is a series of vents under the soffits or eaves. Sometimes people cover these up on the outside, not fully understanding their importance. Sometimes these vents get clogged with dust, vines, leaves, or other debris. If you don't have these vents, have a carpenter or roofer install them. If you do have them, make sure they are clear of debris. These vents are the intake vents in your attic's ventilation system.

To be effective, your attic's ventilation system needs fully functioning intake vents and fully functioning exhaust vents that are as high as possible in the attic. Warm air rises and escapes out of the exhaust vents, drawing fresh, cool air into the intakes. Ideally, this prevents any part of the roof from having the opportunity to become warmed by air escaping from the home below. Good ventilation also helps minimize variations in temperature due to sunlight.

While nearly all attics rely on intake vents at the eaves, the exhaust vents take any number of forms. The most common are square, round, or triangular vents the size of a pizza box located near the top of each gable. (The gables are the triangular sections of your home's exterior walls just below the roof line at either end of the house.) These vents also need to be free of debris. Some homes are equipped with a ridge vent, which is a long, often continuous vent that runs the length of the roof's ridge (the roof's peak). Some homes have heat-propelled ventilating turbines. Each turbine is a metal device shaped like a chef's hat with spiraled slots in it and is mounted on the roof near the ridge. These turbines are designed to spin as air rises through them, but can become jammed due to dirt or storm damage. If the turbines don't turn freely, they should be overhauled or replaced.

Many homes with adequate vents have poor air circulation due to obstructed air paths. This typically occurs near the eaves when new insulation is added or old insulation settles in a way that blocks the flow of air from the intake vents to the rest of the attic. There should be at least an inch and a half of space between the insulation and the roof inside the attic at the eaves. There should be no insulation on top of the intake vents themselves.

In summary, the key to good attic ventilation is paired openings (intake low, exhaust high) and an unobstructed air flow that allows the entire attic to get a continuous supply of fresh air. Besides preventing ice dams, good ventilation will help assure the effectiveness of your insulation, extend the life of your roof shingles, and prevent rot, infestation, and other problems associated with excess attic moisture.

Re-roofing strategies

The final line of defense in your home's ice-dam prevention system is called a WSU (Waterproof Shingle Underlayment). This is a special kind of self-sealing asphalt/polymer sheet placed on the roof before shingles are installed. Unlike standard roofing felt or tar paper, this membrane remains waterproof even when nails are hammered through it. Any time you have a new roof put on, ask for this material to be installed at the gutter edge of the roof and along the valleys where two surfaces of the roof meet. Should an ice dam occur in one of these areas, the WSU should prevent the water from getting into the home.


This kind of icicle is okay. The kind that form outside can cause problems. -- Photo: OHW, 2004

Frequently asked questions

Q: What about roof heating cables?
Roof heating cables are a solution that has broad appeal among those without much roofing experience. Roof heating cables are electrical wires run along the edge of the roof and up valleys to provide enough heat to melt ice and snow.

There are a number of problems with the cables. First of all, the wires themselves have relatively thin insulation, which is necessary to allow the wire's heat to escape. Unfortunately, this thin insulation also means that it doesn't take much to penetrate the wire. The extremes in both temperature and moisture on a roof can cause such insulation to break down and create a fire hazard. Secondly, heating cables generally only affect the roof for about four inches in either direction, so they create warm spots bordered by cold spots, which is the ideal formula for ice dams! Ice dams can even occur on the high side of the cable. In general, heating cables are not an effective way to prevent ice dams.

Q: What about shoveling the roof?
Another popular but problematic "solution" employed by folks who don't know better is to keep the roof free of snow by shoveling. The biggest problem is that moving around on a snowy roof is one of the riskiest things a human can do. Every year, lots of well-intended homeowners and service personnel fall while attempting this feat.

Another problem is that shoveling and chipping away at ice on a roof is a great way to destroy the roof. Shingles are much more brittle when cold and can be broken easily. Even walking on shingles when it is very cold can cause cracking and splitting. Finally, because the work is so dangerous and unpleasant, people who attempt it often end up doing only part of the roof. This is often worse than leaving it alone, as a partially shoveled roof leaves warm spots surrounded by cold spots -- the ideal ice dam formula!

Q: Aren't ice dams caused by clogged gutters?
Clogged gutters can certainly aggravate an ice dam problem, but they are almost never the actual cause. Regardless, clogged gutters do cause a wide variety of very expensive problems, and your gutters should be kept cleaned at all times.

Q: What can I do when an ice dam occurs?
Once an ice dam occurs, there is very little that can be done. Desperate attempts at breaking the dam can do more damage than the dam itself. About the only thing you can do is check the insulation and ventilation inside your attic. While a remedy coming this late won't make an existing dam go away, it might prevent it from getting worse. The dam will normally go away on its own in a few days. About all you can do is be ready with buckets, mops, and tarps for the furniture!

Once an ice-dam occurs on your roof, there is very little you can do. You can, however, prevent ice dams by ensuring that your attic is well insulated and properly ventilated.

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