An ice dam is a ridge, or build-up of ice at the bottom edge of the roof. The problem with ice dams is that they allow additional melt water to pond, or puddle behind them. Shingle roofs are not designed to be waterproof. They are designed to shed water. Once water has accumulated, it generally finds a way into the home through nail penetrations, and behind flashings.
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Ice dams can form in a variety of ways. The most common way is when large amounts of snow accumulate on a roof, and some of the snow begins to melt. The melt water runs down the roof underneath the snow blanket, and when it hits the cold edge of the roof, it refreezes. The ice builds and builds, eventually creating an ice dam that traps the rest of the melt water up on the roof. Other factors leading to ice dams are things like hot spots on the roof. They can occur around vents, skylights, or above an area that lacks sufficient insulation. These hot spots will melt the snow only to have it refreeze as it runs down the roof over a colder area of the roof, usually at the edge.
Preventing and dealing with ice dams
In all Ohio communities it is possible to find homes that do not have ice dams. Ice dams can be prevented by controlling the heat loss from the home.
- Remove snow from the roof. This eliminates one of the ingredients necessary for the formation of an ice dam. A “roof rake” and push broom can be used to remove snow, but may damage the roofing materials.
- In an emergency situation where water is flowing into the house structure, making channels through the ice dam allows the water behind the dam to drain off the roof. Remove ice carefully and apply calcium chlorite . Work upward from the lower edge of the dam. The channel will become ineffective within days and is only a temporary solution to ice dam damage.
- First, make the ceiling air tight so no warm, moist air can flow from the house into the attic space.
- After sealing air leakage paths between the house and attic space, consider increasing the ceiling/roof insulation to cut down on heat loss by conduction.
- Ensure proper ventilation.
Both of these actions will increase the snow load that your roof has to carry because it will no longer melt. Can your roof carry the additional load? If it is built to current codes, there should not be a structural problem. Roofs, like the rest of the home, should have been designed to withstand expected snow loads. In Ohio, plans showing design details to meet expected snow loads are usually required to receive a building permit. The plans for your home may be on file at your local building inspection office. To help you understand the plans, or if you cannot find plans for your home, you may want to contact an architectural engineering firm. A professional engineer should be able to evaluate the structure of your home and answer your questions about the strength of your roof.
- Natural roof ventilation can help maintain uniform roof temperatures, but if the long-term actions described here are done effectively, then only small amounts of roof ventilation are needed to maintain uniform roof surface temperatures. If heat transfer has been reduced substantially, then snow will build up on the roof and cover natural roof ventilation systems, reducing attic ventilation rates. Natural attic ventilation systems are needed to dry the attic space and remove heat buildup during the summer.
Mechanical attic ventilation IS NOT a recommended solution to ice dams in Ohio. It can create other attic moisture problems and may cause undesirable negative pressure in the home.
- Any person on the roof during the winter or performing work on the roof from below is risking injury and risking damage to the roof and house. It is important to contact professionals to carry out this job.
- Whenever a house is tightened up, ventilation systems, exhausting devices, and combustion devices must have enough air to operate safely and effectively!
Interior damage should not be repaired until ceilings and walls are dry. In addition, interior repair should be done together with correcting the heat loss problem that created the ice dam(s) or the damage will occur again.
Preventing ice dams in new homes
The proper new construction practices to prevent ice dams begin with following or exceeding the state code requirements for ceiling/roof insulation levels.
The second absolutely necessary practice is to construct a continuous, 100% effective air barrier through the ceiling. There should not be any air leakage from the house into the attic space!
Recessed lights, skylights, complicated roof designs, and heating ducts in the attic will all increase the risk of ice dam formation.