Ice Dam Creation:
Buildings in Minnesota with roofs that slope to unheated eaves may experience ice dams and icicles
along the eaves and valleys in the winter. Water that ponds up slope of ice dams increase the risk of
leaks into the building as most sloped roofs are designed to shed water, not hold standing water.
Roofs that contain dormers, valleys or other features are prone to ice damming in the region of these
features and require extra attention.
Studies have confirmed that the base of snow can start melting based on a 9.5” snow cover of the roof,
even if the attic temperature is at 22° F.
As the water against the roof deck melts, the snow cover absorbs the water by capillary action, wicking
the water up and away from the roof coverings. As the water migrates through the snow pack it will get
colder and freeze, creating a thin ice lens above the roof deck. This process will continue until the ice
lens is large enough to match the storage capacity of the snow layer. Once this happens any new snow
melt becomes liquid water that flows down the roof underneath the ice lens. When the water reaches
the unheated eave edge the water will freeze, eventually sealing the gap between the ice lens and roof
deck. This causes any new snow melt (water) to back up, getting underneath the shingles and increasing
the risk of leaks in the home. As the water continues to back up and pool at the shingles and flashing it
can cause damage to the roof water-resistant barrier system including, but not limited to:
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Reducing and eliminating the adhesion of the shingles and ice-and-water shield.
It can get underneath the flashing (which is not designed to resist pooling water, just shedding
water) and if it freezes it can reduce the effectiveness of the flashing, allowing water into the
building envelope.
When the pooling water from ice dams leak into the building, it can create a moist environment
allowing rot to grow. The environment created by the ice dams can cause the wood members to
lose 40% to 100% of their strength in as little as 12 weeks according to a study by the USDA
Forest Service.