Ice dams
Ice dams can be a persistent problem in regions with significant snow accumulations
during extended periods of below-freezing temperatures. Ice dams form when roof
areas become warm enough to melt snow, typically because of the combination of the
insulating effect of heavy snow accumulation and heat loss from conditioned spaces.
Water from melted snow runs down the roof under the snow cover and refreezes along
roof overhangs where the roof surface temperature is lower. As more water from the
melting snow becomes trapped behind the dam of ice, it can back up under shingles
and infiltrate the underlayment, leading to leaks.
Figure: Ice damming
Melting snow that produces ice dams can be controlled if a roof surface temperature
is maintained below freezing. To this end, NRCA recommends minimizing the amount
of heat reaching the underside of the roof deck by adding a continuous ceiling air
barrier or alternatively sealing interruptions in the ceiling vapor retarder with
air-impermeable materials such as closed-cell spray polyurethane foam.
Secondly, a balanced attic ventilation system will help remove stray heat so temperatures
throughout a roof deck are maintained close to the outside temperature. In areas
with a history of ice dam formation, vent opening area and ventilation space height
(in cathedral ceilings) should be appropriately sized to establish uniform rising
air flow, which will cool a roof deck to below freezing when outside temperatures
are below freezing. Guidelines for designing cathedral ceiling ventilation to prevent
ice dam formation have been developed by researchers working at the Cold Regions
Research Engineering Laboratory (CRREL) of the U.S. Army Corps of Engineers and
are available by
clicking
here.
Measures that prevent stray interior heat from reaching a roof deck may not always
be sufficient to prevent ice dam formation. Sometimes, ice dams occur even when
attics are correctly insulated and ventilated because of solar heat and variation
in snow cover thickness. A watertight barrier of self-adhering membrane underlayment
or layers of underlayment felt cemented together with asphalt bitumen may be applied
to a roof deck along the eaves as protection against ice dam leaks.
2012 IRC requires the installation of a water and ice dam protection membrane in
place of typical underlayment along the eaves as part of asphalt shingle, metal
shingle, slate, and wood shake and shingle roof systems in areas with a history
of ice dam formation. The code requires the ice dam protection membrane extend at
least 24 inches past the exterior wall line of a building.
NRCA recommends installation of water and ice dam protection membrane in locations
where the average temperature for January is 30 F or less. The drawing below illustrates
the approximate areas in the 48 contiguous United States.
Figure: Areas of the U.S. with an average January temperature below
30 F, composite of 1983-2007 data. Map is based on data provided by the NOAA/ESRL
Physical Sciences Division, Boulder, Colo., from
www.cdc.noaa.gov/USclimate/USclimdivs.html. Contours represent U.S. climate
divisions.
NRCA recommends water and ice dam protection membrane extends at least 24 inches
past the exterior wall line of a building when installed on roof slopes of 4:12
and higher. For slopes less than 4:12 and in heavy snowfall regions, NRCA recommends
water and ice dam protection membrane extends at least 36 inches past the exterior
wall line.
Figure: Installation of water and ice-dam protection membrane
Where ice damming presents an urgent problem and demands immediate action, it may
be advisable to remove the snow accumulated on the roof and to prevent additional
water backups. Snow removal from a rooftop should be attempted only if it can be
done safely from the ground level. An extended-handle broom or roof rake specifically
designed for snow removal from sloped roofs may be used to pull snow down and off
a roof. This action may damage the roof and should be performed with care to avoid
hitting the roof surface, dislodging roof covering materials or pushing snow up
the roof.
NRCA advises against attempting to break up ice buildups on roofs by chipping away
with shovels, picks, etc., or applying high-pressure, high-temperature water. The
impact of sharp or hard tools or thermal shock is likely to damage a roof covering
and aggravate any existing issues from ice damming.