Causes of Ice Dams

Ice Dams
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 poor ventilation. Ideal conditions for ice dams to occur are often seen in ranch style houses or houses with low roof slopes and large overhangs.
Ice dams occur when the outside temperature is 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, the 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 freeze- thaw process is not ideal for shingled roofs since the ice can travel "upslope" and get underneath the shingles.
Ice Dam Prevention and Protection
Ice dams will not be prevented by simply installing new shingles. Prevention occurs when the roofing system as a whole is addressed.
  1. 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 in an ideal attic space should be close to the same temperature as the outside air.

  2. Ventilation allows heat to escape the attic space during winter months and 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.

  3. 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.
Snow on Overhangs
Example 1
The picture above was taken on a 28 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.
Ice dam
Example 2
This photo was taken on the same 28 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 this garage roof. The house roof is poorly insulated and has exhaust vents only. Heat loss due to inadequate insulation and poor ventilation cause the snow to thaw over the main house roof. An Ice dam may occur on this roof system if the inefficiencies aren't corrected.
Ice dam
Example 3
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, 28 degrees. The roof deck above the main house allows the snow to melt because the house is losing heat and is poorly ventilated.
Ice dam
Example 4
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.
Ice dam
Example 5
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.
Ice dam
Example 6
Installing Underlayments, Insulation, and Ventilation is vital in protecting your 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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