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Ice dams and their resultant leaks are often a chronic and expensive problem in those regions where snow-covered roofs are a common occurrence. To understand their detection, cause, and prevention requires some understanding of the dynamics of a structure's construction, insulation, and ventilation.

Ice dams most commonly make their presence known when the owner or tenant of a shingle-covered dwelling detects a water leak in the ceiling near its corner with an exterior wall while snow covers the roof. Unaddressed, the leaks produced by a roof dam can result in significant structural decay and extensive cosmetic damage.

Ice dams form when heat from the structure's interior rises and warms the underside of a snow-covered roof slope. This warming melts the bottom layer of snow, and the water begins to flow down to the gutters. If the structure's roof overhangs its exterior walls, the flowing water will encounter a section of the roof above the overhang that is no longer warmed by heat escaping from the interior. The water then refreezes on the roof above the overhang. As more and more water freezes, the resulting ice creates a barrier that stops or significantly retards the flow of the water, and it begins to pond. Because the ponding water will be over the area where the roof is still being warmed from below, it does not freeze and remains liquid.


Structures without roof overhangs are not immune from ice dam formation. In such cases, the melt water will freeze and accumulate on and/or in the gutters, particularly if leak guards are installed, and thus an ice dam is formed. Large and numerous icicles are often a clue to the existence of such an ice dam.

Shingle-covered roofs are not impervious to water. Their overlapping design and the roof's slope are intended to shed water quickly before it can penetrate though the nail holes, seams, and other potential leak pathways that are common to a shingle-covered roof. Therefore, when water becomes ponded on a roof slope as the result of an ice dam, the water has ample time to find its way through these pathways, and interior damage can result. Additionally, the formation of an ice dam can lift shingles and exacerbate the problem.

Several steps can be taken to prevent or mitigate ice dam formation. The best method is to design or modify the structure so that problem never occurs in the first place. Insulating an attic's floor will minimize the heat that warms the roof and has the added benefit of lowering energy costs. Hand in hand with insulation is good attic ventilation, which will further prevent heat from accumulating in the attic and thus minimize the creation of water on the roof surface.

Ice and water shield is an impermeable membrane that is placed below the shingles at appropriate locations to prevent ponded water from making its way into the structure's interior if an ice dam forms. Its thick, rubbery consistency seals around any nails that necessarily must penetrate its surface. Of course, if the water ponding extends beyond the shield's location, leaks can still occur.

In those locations where difficult-to-control chronic ice dams occur, the solution is often a combination of ice and water shield and heat tape. Several manufacturers provide cables that use electricity to keep an area of the roof warm enough to prevent the formation of ice dams. Some systems use sophisticated sensors that automatically activate the heat tape when temperature and moisture conditions are conducive to the formation of ice dams.

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DONAN, a family owned company founded in 1947, provides full-service Forensic Engineering and Fire Investigation Services. For over 60 years, Donan Engineering has been providing unbiased answers to their client's complicated questions. Their team of licensed and experienced forensic experts provides investigative services in the fields of engineering investigations, fire and explosion origin & cause, roofing investigations, and component testing. The diversity of their multidisciplinary staff allows them to provide not only a complete service, but a complete service whose conclusions can be successfully and completely supported.

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