Ice Damming

Post date: Feb 11, 2013 1:05:55 AM

Ice dam
Ice damming


Ice dams are the accumulation of ice at the eaves and in the valleys of roofs. This can be the result of several factors:

  • not enough attic insulation;

  • not enough attic ventilation, or

  • the combination of both

These conditions can cause roof snow to melt and slide down to the eaves, where the roof surface is cooler and the melting snow freezes to the roof. The melting ice and snow causes water to pool behind this dam and icicles to form at the eaves.


Sometimes the water backs up and seeps between the shingle layers, wetting the roof sheathing, and often leaking into the walls and ceilings of the interior spaces below.


1. Make sure that all attic/ceiling penetrations are well sealed and insulated to keep warm, moist air (air leakage) out of the attic. Areas that can be checked:

  • attic hatch (needs to be both insulated and weatherstripped. Foam board to insulate the hatch, and foam weatherstripping around the hatch.)

  • electrical/light fixtures

  • bathroom exhaust fans

  • plumbing stacks

  • chimneys

  • duct work (bathroom exhaust)

Inspected by 42 - Home Inspections provides a consulting service using an infrared camera to pin point specific locations in the attic where warm air is leaking into the attic space.

2. Make sure there is free and unobstructed air movement beneath the roof surface to provide adequate attic ventilation (approx. ratio of 1 to 300 or 1 sq.ft./300 sq.ft. of ceiling area).

3. Make sure that soffit vents are not blocked by insulation. Use appropriate baffles/barriers between the rafters/trusses to avoid loose fill/blown-in insulation from settling in the eaves. A minimum insulation value of R-40 to R-50 is recommended, and it is generally better to insulate to a higher R-value than the minimum recommended level.

Tip: You can determine the thickness of your attic's insulation; for example, to achieve R-40, you will need approximately 13.5 inches of fibreglass blanket insulation. For R-50, you would need about 16.75 in. Check the insulation values for various types of insulation in the following article:

4. Make sure that eaves troughing,downspouts and downspout extensions are able to discharging water. Eaves troughing that is not properly sloped or not cleared of leaves and debris, can also cause ice dams.

5. All homes are different, you can always contact a roofing professional for expert for advice if you have a ice dam problem that persists.

See Also:

Asphalt Roofing Technical Bulletin #10 -

Keeping the Heat In -

For a very detailed and professional Ottawa home inspection, ice dam consultation or infrarad scan, call (613-799-3698) or email Barry at Inspection by 42 Home Inspections ( We inspect homes in Carp, Bridlewood, Bells Corners, Manotic, Greely, Constance Bay, Kanata, Ottawa, Barrhaven, Orleans, Stittsville and surrounding areas.