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,
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.
