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Building Science by Norman Aceron Garcia

Ice dams

by Norman Aceron Garcia

ice damAn ice dam is a hill-like ice formation on the edge of a roof that blocks melting snow from draining off the roof. As water collects behind the dam, it can seep through the roof and cause deterioration to roof underlayment, ceilings, walls, insulation and other areas. Ice dams are formed by the interactions between: 1. snow covering the roof; 2. warm portions of the upper roof (warmer than 0° C) and; 3. heat lost through the roof when the attic gets cold. Melted snow from the warmer portions will refreeze when it pours down to the colder area, thus forming ice dams.

Even though the major cause of snow melting on the roof is heat loss from the building’s interior, solar radiation also provides plenty of heat to melt snow. In Winnipeg, sufficient sunlight can be transmitted through 15 cm of snow cover on a clear sunny day to cause melting even when the outside temperature is minus 10° C and attic temperature is minus 5° C. Contrary to popular belief, gutters do not cause ice dam formations. However, gutters assist in ice accumulation in susceptible areas where parts of the house can strip away due to the weight of the ice and come plunging to the ground.

Ice dams can lead to various problems when water leaks from the roof into the building envelope. It can significantly reduce the effectiveness of building insulation. Damp insulation doesn’t work well and will not decompress even when it dries. With an ineffective insulation, more heat will flow to the roof to melt more snow, causing more ice dams to form. Water that penetrates in the wall cavities can cause rotting of the structural wood members and blistering and peeling of exterior paint. Furthermore, trapped water can lead to respiratory illnesses due to the growth of mould.

There are many ways to prevent the formation of ice dams:

  • Seal all air leaks in the attic floor, particularly those surrounding attic hatches, ceiling light fixtures, and duct and plumbing penetrations.
  • Increase the thickness of the insulation of pipes, ducts and chimneys that passes through the attic.
  • Increase the height of vents that terminate just above the roof.
  • Provide a minimum 75 mm of air space between the top of the insulation and roof sheathing in sloped ceilings.
  • Remove snow from the roof using a special roof rake from the ground. Be careful not to dislodge dangerous icicles or damage roofing materials. Do not use shovels, hammers, ice picks, or lawn rakes.
  • Make channels in the ice by hosing it with warm water. However, this procedure deliberately adds water to the roof and must be done only in emergencies when there is too much water is flowing through the roof, and when the outside temperature is warm enough that the hose water can drain before it freezes.
  • Install electric heat cables. However, this system rarely works, requires a professional installer, consumes electricity, and can make asphalt shingles brittle.

Summing up, ice dams are caused by insufficient attic insulation, but homeowners can take certain preventative measures to minimize its formation and deleterious effects.

Norman Aceron Garcia is a registered Professional Engineer and a Certified Property Inspector of Mr. Peg Property Inspections Inc. Please visit for more information on home inspection, building science and home maintenance tips.

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