By William Decker, Nick Gromiko and Kenton Shepard 

An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow from draining. As water backs up behind the dam, it can leak through the roof and cause damage to walls, ceilings, insulation and other areas.

What causes ice dams? 

Ice dams are formed by an interaction between snow cover, outside temperatures, and heat lost through the roof - more specifically, from heat loss through the poorly insulated ceiling of the house into the attic area and under the roof.  The increased use of recessed lighting (also called "can lights" is greatly contributing to this problems.  When there is snow on the roof, warm  portions of the upper roof (warmer than 32° F) allow water to drain down to the  and cold portions of the lower roof (at freezing or below).  The melted snow from the warmer areas will re-freeze when it flows down to the colder portions, forming an ice dam.  The ice dam actually forms a dam, keeping the liquid water from draining into the gutter and allowing it to flow, upwards, between the shingles and through the roof, causing water damage on the ceiling and walls.

Gutters do not cause ice dams to form, contrary to popular belief, and heating wires in the gutter or at the edge of the roof do not help the condition, and can actually make it worse!  Gutters do, however, help concentrate ice from the dam in a vulnerable area, where parts of the house can peel away under the weight of the ice and come crashing to the ground.   Hopefully, there will be nobody under the ice when this happens.

Problems Associated with Ice Dams. 

Ice dams are problematic because they force water to leak from the roof into the building envelope (ceiling and walls). This may lead to:

• rotted roof decking, rafters and exterior and interior wall framing and sheathing;

• respiratory illnesses (allergies, asthma, etc.) caused by mold growth;

• icicles form and can grow to gigantic sizes.  These icicles, and the large mass of ice behind them, break off and can cause roof, gutter and siding damage, as well as injure, and even kill, people who are unlucky enough to be underneath them when the fall off, and;

• reduced effectiveness of insulation. Wet insulation doesn’t work well, and chronically wet insulation will not decompress even when it dries. Without working insulation, even more heat will escape to the roof where more snow will melt, causing more ice dams which, in turn, will lead to leaks; and

• peeling paint. Water from the leak will infiltrate wall cavities and cause paint to peel and blister. This may happen long after the ice dam has melted and thus not appear directly related to the ice dam.

Prevention

• Keep the entire roof cold.  This can be accomplished by implementing the following measures: 

• Install a metal roof.  Ice formations may occur on metal roofs, but the design of the roof will not allow the melting water to penetrate the roof's surface.  Also, snow and ice are more likely to slide off of a smooth, metal surface than asphalt shingles.

• Seal all air leaks in the attic floor, such as those surrounding wire and plumbing penetrations, attic hatches, and recessed ceiling light fixtures leading to the attic from the living space below.  Recessed "can" lights on the top level of the house should be sealed.  This not only guards against heat loss into the attic, but also stops humidity from entering the attic area from the house.  Humidity in a cold attic condenses and can lead to water leaking from the lights as well as promote mold formation.

• Most older houses have the insulation installed on the floor of the attic and the attic area ventilated.  The modern insulation technique is to insulate the underside of the roof and extend the building envelope to include the attic area.  With this method the attic area is sealed, not ventilated, and has been found to virtually eliminate ice damming.  A word of caution:  When insulating the underside of the roof, only an open-cell spray foam insulation product should be used.  Trying to insulate the underside of the roof with fiberglass insulation can cause moisture accumulation and, eventually, rot the rafters and the roof decking.

• Increase the thickness of insulation on the attic floor, ductwork, and chimneys that pass through the attic.  More important that insulation, in many houses, is the presence of a vapor barrier between the insulation and the ceiling surface.  This barrier will keep moisture and water vapor from entering the attic area.

• Move or elevate exhaust systems that terminate just above the roof, where they are likely to melt snow.  Also make sure that all ventilation systems (bathroom exhaust fans, stove hoods, clothes dryer vents) vent, directly, to the exterior of the house and not into the attic area.

• Remove snow from the roof. This can be accomplished safely using a roof rake from the ground. Be careful not to harm roofing materials or to dislodge dangerous icicles.

• Create channels in the ice by hosing it with warm water. Because this process intentionally adds water to the roof, this should be done only in emergencies where a great deal of water is already flowing through the roof, and when temperatures are warm enough that the hose water can drain before it freezes.

Prevention and Removal Methods to Avoid.

• Electric heat cables. These rarely work, they require effort to install, they use electricity, and they can make shingles brittle.  In most situations, electric heat cables cause more damage than they prevent.

• Manual removal of the ice dam using shovels, hammers, ice picks, rakes, or whatever destructive items can be found in the shed.  The roof can be easily damaged by these efforts, as can the homeowner, when they slip off of the icy roof.

In summary, ice dams are caused by inadequate attic insulation, but homeowners can take certain preventative measures to ensure that they are rare.

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