Ice Dams, their causes and dangers

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 shingles (if you don't have properly installed ice and water shield membrane) and cause damage to walls, ceilings, insulation and other areas.  Ice dams, in an of themselves, are not a real problem and usually cause no damage.  But if the roof was not properly installed and/or the attic area was not properly insulated, the backed up water will enter the house and cause significant damage.

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 intoX-Ray diagram to ice dam formation. the attic area and under the roof.  The increased use of recessed lighting (also called "can lights") and the resulting improper insulation in higher end houses has greatly contributing to this problems.  When there is snow on the roof, the warm portions of the upper roof (warmer than 32° F) melt the snow and allows water to drain down to the cold portions of the lower roof (at freezing or below).  These lower proof areas are usually at the roof edge, called the eave, and is colder because the over air, not a warm house leaking heat.  The melted snow re-freezes on the colder edges, forming an ice dam.  The result is an actual dam, keeping the liquid water from draining into the gutter backing it upwards where it flows between the shingles.  If the roof was properly installed (and they rarely are) the 3 feet of roof under the shingles and inside the house's exterior wall will be covered with a rubber membrane (ice and water shield).  This membrane will not allow the water to enter the house.  But most roofers either do not install this membrane or they install it incorrectly.  The backed up water enters through the roof, causing water damage on the ceiling and walls.

Most ice dams form, mainly, because of improper insulation and air sealing of the house.  The old method of insulating the roof of a house called for insulation of the ceiling of the house and ventilation of the attic space, between the ceiling and the roof.  It was thought that by keeping outside air circulating in the attic space, that the air would cool the attic area in the summer (also keeping humidity down) and keep the roof cold and stop ice damming.

Roof and attic ventilation

BUT... (isn't there always a but?), new construction products have changed this.  The use of ridge vents (which I have never seen installed correctly, according to their manufacturer's instructions) do not allow for proper ventilation.  The use of "blown in" attic insulation (many times without any vapor or air barrier) does not provide proper thermal insulation.  Sure, it may be "rated" with a high R value per inch, but R value has very little meaning in actual field use, as opposed to controlled laboratorySpray foam attic insulation conditions.  Even if the insulation is proper, and properly installed with a vapor and air barrier, along come the electricians, piercing through all that with recessed can lights.  These lights not only allow heat to leave the house and enter the attic, they also allow humidity from the house to flow into the attic, where it condenses and caused mold formation.

The solution is to insulate the underside of the roof structure with a closed cell spray foam product (right).  This not only provides a great deal of insulation to the house, but it will stop ice dams from forming.  I have seen this type of insulation save homeowners 30 - 40% on they heating bill and some 20 - 30 % on their air condition bill in the summer.

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!  The heating wires are, at best, a band-aid approach and do not solve the problem.  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.



Keep the entire roof cold.  This can be accomplished by implementing the following measures: Regula and Thermal image picture of an ice dammed house


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.  Trying to insulate the underside of the roof with fiberglass insulation (shich provides no air sealing) 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.  A neat trick for this is to fill some old panty hose with ice melt (calcium Chloride, not Rock Salt) and throw it up on the roof, behind the ice dam.  It will melt its way through the dam and allow the water to flow off the roof.

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.