Causes of Ice Dams
Ice Dams can occur on almost any residential roofing system in the Cincinnati area. The most common causes are inadequate insulation and air leakage in an attic space. Ideal conditions for ice dams to occur are often seen on houses with low roof slopes and large overhangs.
Ice dams occur when the outside temperature is approximately 10 degrees below freezing and snow is present on the roof. Homes with poor insulation allow heat to escape from the living quarters into the attic. If the roof system is not properly ventilated, heat will build up in the attic and warm the roof deck. Since the underside of the roof deck is warm, the snow begins to melt. When the melted snow reaches cooler roof areas, usually the gutters or areas near the overhang (eaves) it will refreeze. The thaw-freeze process is not ideal for shingled roofs since the ice can can cause water to pond and get underneath the shingles.
Ice dams will not be prevented by simply installing new shingles. Prevention occurs when the roofing system as a whole is addressed.
- Attic Insulation can be added to minimize the amount of heat that escapes into the attic. Bathroom vents, electrical outlets, plumbing vents, furnace stacks, and access doors can enable significant amounts of heat to escape a house's living quarters. The temperature of the roof sheathing in an ideal attic space should be close to the same temperature as the outside air. (The ambient attic air temperatures will generally be higher than outside air temperatures).
- Mechanical Ventilation cools the roof sheathing and attic space during winter months and can prevents the roof deck from warming. A freeze-thaw process isn't likely to occur on a roof deck that is the same temperature as the outside air. Passive ventilation, or non-mechanical roof vents allow moisture to escape the attic space in the winter but are ineffective in preventing ice dams. They usually cools the roof sheathing no more than a few degrees in the winter.
- Underlayments, including Owens Corning WeatherLock ice and water guard, adhere directly to the roof deck, creating a water tight barrier between the roof deck and the shingles. The ice and water shield is installed around the perimeter where the overhangs exist and at roof penetrations. The use of ice and water guard will protect the roof deck from ice dams but proper attic insulation and/or ventilation will prevent an ice dam from occurring.
The picture above was taken on a 22 degree Cincinnati day. The heat loss from this house occurs almost everywhere above the heated space but not above the overhangs. This phenomenon is similar to snow and ice freezing on bridges before it freezes over solid ground.
This photo was taken on the same 22 degree Cincinnati day. It is a great example of how different roof systems are affected by heat loss. The garage is not heated or insulated, thus the roof deck is the same temperature on both sides. An ice dam will never occur on unheated buildings. The house roof is poorly insulated and interior heat is causing the snow to thaw. An Ice dam may occur on this roof system if the inefficiencies aren't corrected.
This roof system is another demonstration of the phenomenon that occurs in example 2. The roof deck above the porch remains the same temperature as the outside air, 22 degrees. The roof deck above the main house allows the snow to melt because the house is losing heat and is poorly ventilated. It’s important to note that both roof areas are exposed to sunlight and still have different melting patterns.
The photo above shows heat loss around a hot water heater exhaust flue. Heat producing pipes that travel through the attic should be insulated to prevent this from occurring.
Improperly installed insulation can be a problem to a roof system. This house has inadequate insulation near the access to the attic. The (blown-in) insulation contractor installed the correct amount of insulation everywhere in the attic except the area where he was standing.
Installing proper Underlayments, Insulation
is vital in protecting a roof from ice dams. The house shown above in Montgomery, OH 45242 is an example of an efficient roof system that was installed by Deer Park Roofing, Inc.
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