Ice build up on your roof

This winter has been one of the worst in recent memory for leaks caused by ice dams.  The majority of   roofs in Ontario are covered in a deep layer of snow, some with ice and icicles hanging off the eaves. The snow alone, however, is not the problem. Normally you need three conditions for ice dams to form: snow, heat to melt the snow and cold to refreeze the melted snow into solid ice.  

Most days this winter have been cold enough that snow on roofs has not melted much, if at all.  However deeper snow on the roof acts as an insulator, trapping more indoor heat beneath the roof deck and warming the roof sheathing.  And every time we get a warm spell, we have even better conditions for ice damming.  When air temperatures warm up closer to the freezing point, the surface of the roof warms up as well.  The heat that builds up within the attic as a result of radiant heat from the sun or heat loss from the home rises to the higher points of the roof, causing the snow cover to melt in those areas.  This melting snow runs down the roof surface below the snow, where it contacts the colder areas of the roof along the eave or overhangs, where it re-freezes.  The continuous freezing of melting snow forms a ridge of ice or "ice dam".  As more water from continuing snow melt arrives at the ice dam, it is blocked and begins backing up under, and through, the roofing materials.


Here's an example of how bad ice dams and icicles can get during a winter with heavy snowfall.  The weight of all that ice in the gutters can tear them right off.  Some winters ice dams have caused millions of dollars of damage to homes across Canada and the US.  Much of the damage is apparent: gutters torn off, dislodged roof shingles, water stained ceilings and walls and peeling paint are all easily recognized, and usually repaired when weather or budgets permit.  But other damage is not as obvious and often goes unchecked.  Roof leaks wet the insulation, which then doesn't work as well, accellerating the problem.  


Water often leaks down within the wall cavities, resulting in rot and mold growth.  Structural framing members can decay, fasteners corrode.  Mold and mildew can form on interior surfaces, and interior and exterior painted surfaces may blister and peel.  If you have icicles hanging down through your soffit vents, you know you have a serious problem.

The two things that clients ask me are how to get rid of the ice dams that are already there, and how to prevent them in the future.  I have heard of many methods for removing ice dams yourself, but hesitate to recommend any of them.  If you can reach your roof from the ground (as in the case of some bungalows or cottages), you can use a “snow rake” to pull heavy snow off the roof.  Unless this is done after every heavy snowfall, however, the melting process will begin, and ice dams will eventually form.  Attempting snow removal is also dangerous for inexperienced homeowners, and can easily damage the surface of the roof.  It’s safer to hire a contractor, although the steam or high pressure water used can also damage to roof surface.  Heating cables have been employed by some homeowners for many years, with mixed results.  The heat pattern is irregular, and ice dams often re-form above the line of cables.  They are also expensive to operate, and the heat can make the shingles brittle.  Some people swear by pouring vertical bands of salt or “ice melter” on top of the dams, to melt channels through the ice to allow the free passage of snow melt.  I’ve even heard of filling pantyhose with salt, and laying vertically across the ice dams.  I don’t know if this chemical approach will have any long term effect on the shingles, or vegetation below.  Keeping downspouts clear of ice is also important, allowing gutters to drain easily as the snow melts on warmer day.


This is a diagram of the cause of ice damming in action.  Water that's trapped above the ice has nowhere to go but down through the roof, into the house.

The best way to solving the problem is prevention.  This means taking steps to keep the entire surface of the roof cold.  Look at the roofs on unheated structures such as garages and sheds; ice dams don’t form on these structures because there is no source of heat to cause uneven melting and freezing.

    • Stop warm air from leaking into the attic from the house.  As the primary cause of ice damming, this should be addressed first. This can’t be stressed enough.  Seal all holes in the floor of the attic for wiring and plumbing runs, and gaps at intersections of interior walls and ceiling.  Seal all leaks around ceiling fixtures, bathroom exhaust fans, attic hatches, etc.  Almost any house over 20 years old will have “chases” or passages for chimneys, gas appliance vents, and vent systems, which allow large volumes of warm air to escape.  Often “bulkheads” above kitchen cabinets, or lowered ceilings in showers are open to the attic.  If you want to seal these air leaks yourself, consult with a home inspector about using Infrared Thermography to highlight them, as they are mostly hidden under the insulation and can be very difficult to find.  Simply adding more insulation won’t help, as most commonly used materials do not do much to stop the airflow (that’s why fiberglass is used in furnace filters: it offers little resistance to airflow).

    • Ensure adequate levels of insulation.  A layer of insulation, applied evenly with no gaps, equivalent to R-32 is the bare minimum required.  R-40 or more is better.  It is common to find only a couple of inches of insulation, if any, above the top of exterior walls.  There is little room for insulation below the roof framing here, and this area is hard for installers to reach.  Adding insulation to fill these areas will reduce heat loss here, and ice damming.  The best approach is to use an insulation which can be packed tightly in place, like cellulose, or use a material with a higher R-value per inch – specifically, closed-cell spray foam insulation.  It’s not necessary to have the entire attic lid insulated with spray foam, but having it installed at the eaves is a great way to help compensate for the lack of space here.  The only downside to having this type of insulation installed is that it’s more expensive than other types of insulation, and the work should be performed by an experienced spray-foam insulation installer.  It’s not a DIY project.

  • Provide sufficient passive ventilation to keep roof sheathing cool.  A combination of soffit and peak or ridge vents of adequate size will exhaust any heat which does accumulate in the attic.  Be sure soffit vents are not blocked by insulation, and that the roof sheathing below peak vents has been cut open properly.  The old formula to determine adequate ventilation was for 1 square foot of vent area for every 300 square feet of attic floor, to be divided equally between soffit and peak or ridge venting.  This is a minimum requirement, more is obviously better.  Gable and turbine vents used on older houses are simply not adequate, and should be augmented with soffit and peak or ridge vents.

If you see any signs of potential damage from ice dams, consider consulting a home inspector who performs infrared scans, an energy auditor, or an insulation contractor for solutions.  It’s a good bet that money spent preventing ice dams will be considerably less than the cost of repairing roof or interior damage caused by water leaks.


Here are two photos of the "chase" around an exhaust vent for the furnace, open all the way from the basement (left) to the attic (right), creating a constant supply of heat into the attic.


The photo on the left shows the infrared image of heat loss around pot lights, through the insulation in the attic.
The photo on the right shows heat loss around a plumbing vent stack, again into the attic.

The warm weather and rain we've had the last couple of days has removed most of the existing snow from roofs, but this is only February ... we're sure to have more snow before spring arrives. Continue to be on the lookout for ice dams and roof leaks, and try to take care of any obvious potential for ice dams before next winter.  Be sure to repair any water damage from this winter as soon as possible
Jason Abbott

Jason Abbott

Sales Representative
CENTURY 21 Infinity Realty Inc., Brokerage*
Contact Me