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APPENDIX
BASIC INFORMATION ABOUT ICE DAM FORMATION
AND PREVENTION
How ice dams form
Snow and ice on a roof can thaw because of
heat loss through the roof due to inadequate
roof venting and/or insufficient ceiling insulation.
In these cases, the roof surface above heated
areas of the building is warm enough to melt the
snow, while the roof surface above the overhang
is cool enough to refreeze this melt water as
it trickles down the roof. As a result, a layer of
ice forms on this cooler, lower roof surface and
in gutters. As snow continues to melt, the melt
water flows downward and re-freezes at the
roof edge. As this happens, ice builds up to
several inches in thickness forming an ice dam.
In addition to having ice dams along the roofline,
structures such as skylights and dormers often
have poor venting, and ice dams may form in the
surrounding areas.
Signs of ice dams
Ice dams can be identified at points where snow
has melted on an upper roof surface, but the roof
area further down is still snow or ice covered.
Icicles are also a sign of ice dams.
Damaging results
Ice dams prevent water from flowing off the roof.
The melt water will pool at the ice dam instead,
which can result in water seeping between shin-
gles. Water that has seeped under the shingles can leak through roof penetrations,
such as nail holes and sheathing seams, and inside the building walls and ceiling.
The water can also flow along ceiling beams or wiring. Eventually, water can leak into
the building interior at light fixtures or through the ceiling finishing at places other
than below the ice dam’s origin.
In addition, ice buildup in gutters can put significant stress on the gutter resulting in
gutter damage.
How to avoid ice dams
To avoid ice dams, the entire roof surface should be kept at about the same
temperature as the outside air. The best way to do this is to ventilate the space under
the roof. However, this is often costly or impractical. In these cases, de-icing cables
can be a cost-effective solution to ice dam problems. Placing de-icing cables on
the roof edge above the overhang and in the gutters and downspouts prevents melt
water from refreezing. This allows the melt water to have a clear path to flow to the
ground and prevents water from leaking inside your home and damage to your gutter
system.
ICE DAM FORMATION
AND THE CONSEQUENCES
ROOF HEAT CREATES POTENTIAL
FOR ICE DAMS TO FORM
