Ice dams may be hidden under the snow on sloped roofs with inadequate insulation. Two
circumstances combine to make an ice dam. First, snow melts on the part of the roof over the
building interior (inside the perimeter of the outside walls) because heat and air that leak from the
warm interior raise the roof temperature above the freezing temperature. Second, on the part of
the roof covering the overhang (beyond the perimeter of the outside walls), the roof is cold and
the runoff refreezes. The refrozen runoff forms a dam, further backing up melt water.
Backed up water can get under the shingles and through the roof deck. It can drip on the ceiling
insulation or run down the underside of the deck to the connection between the roof and the
walls. It then makes its way into the building in the form of damaging leaks. Melt water under
the unmelted snow can decrease the friction between the snow and the roof and cause a snow
slide, like an avalanche. Melt water often refreezes as icicles hanging from the gutters or edge of
the roof. These icicles eventually break off when they get too heavy. Both snow slides and falling
icicles endanger passersby.
Energy efficient roofs minimize problems with ice dams because they keep the entire roof cold.
There is little difference in temperature between the part of the roof inside the perimeter of the
outside walls and the part covering the overhangs. Thus, melting and refreezing is minimized.
Insulating to prevent heat leaks and sealing against air leaks between the inside of the
building and the attic are the best ways to achieve a cold roof.
Increasing the level of
insulation from R-11 to R-38 in a 1000 square foot attic should cost about $500. Accumulated
dollar savings for heating and cooling, beyond the cost of the installed insulation, should be more
than $1000 in the Washington DC area. Costs to repair damage from a leak caused by an ice dam
could easily exceed $5000. Ventilation of the attic may help to achieve a cold roof. Its primary
purpose, though, is to prevent moisture from condensing in the attic on the underside of the roof
and dripping down into the insulation. This moisture is in any warm air that leaks
from the inside of the building. Sealing the air leaks is more effective than increasing the
ventilation.
To stop dammed up water from leaking under the shingles use a waterproof membrane under the
parts of the roof where ice dams occur and melt water backs up. The membrane is usually placed
from the edge of the roof up beyond where the walls intersect the roof. This membrane is
installed when old shingles are replaced. If ice damming is a recurrent problem, heaters along the
edge of the roof can be used to break up ice dams as they form. But these heaters use a lot of
expensive electrical energy; they need to be used whenever it snows until air temperatures are
about 45ºF.
If ice dams are building up and no heaters are in place, building owners may want to take
emergency action. Hammers, hatchets, ice picks or even salt used at the edge of a roof to attack
ice dams and icicles do more harm than good and are not recommended. If snow is piling up to
the point where the roof seems or sounds vulnerable to collapse, some snow can be removed but
safety is the first concern. The object of snow removal from roofs is to reduce the snow load to
safe levels, not to clean it off entirely. Regardless, there is danger of damage to the roof surface
from using implements such as rakes or shovels. There are hazards to people who climb up to the
roof on slippery ladders and stand on them. Walking on an already stressed roof may cause local
failures in the structure. Snow removal from a roof is a risky proposition. Use a licensed
contractor in most cases.
A well-designed, energy efficient roof should tolerate the loads of once every 50 year events like
the recent snowstorms in the northeastern U.S. Nature eventually melts snow from roofs more
cheaply and safely than artificial means. Best to let nature take its course and observe deficiencies
in a particular roof, such as lack of insulation and unwanted air leakage paths from the interior
space to the roof, so corrections can be made before the next emergency.
The U.S. Department of Energy, through its Buildings Technology Center at the
Oak Ridge National Laboratory in Tennessee (http://www.cad.ornl.gov/kch/demo.html),
is working to provide
builders, contractors and owners of buildings with research results to facilitate
cost effective, energy efficient roofs and/or attics in every building. Much
of the relevant information is
collected in an upcoming Attic Handbook. Energy information is available
from the Energy Efficiency and Renewable Energy Clearinghouse. The information
presented
here was prepared by the Buildings Technology Center and has focused on ice
dams, how to eliminate their cause, and what to do and not do about ones that
appear as a result of the blizzard
of '96 and future snowstorms. This information is summarized from the Attic
Handbook and from results of research at the U.S. Army Corps of Engineers' Cold
Regions Research and Engineering
Laboratory as well as in the Division of Building Research of the National
Research Council of Canada.
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