Ice dams and their resultant leaks can be an expensive and even chronic 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 at the interface of the ceiling and exterior wall, or at the top of a window opening while snow covers the roof. Unaddressed, the leaks produced by a roof dam can result in fungal growth and structural decay in addition to cosmetic damage.
Ice damming is initiated by fluctuations in temperature above and below freezing following snow accumulation, or when heat from within the dwelling escapes into the attic and melts snow on the roof surface. Heat enters the attic principally through any penetration in the ceiling of the occupied space below. Heat warms the underside of the roof deck, melting the snow. The lower portion of the roof (at the gutters and above the eaves soffit) is typically colder and the water re-freezes, progressively building up a dam of ice and trapping any water resulting from further melting.
Because shingle roofs are designed to shed water but are not actually waterproof, water pooled behind the ice dam can flow under the shingles and onto the roof decking. Having breached the weather protection normally provided by the roofing, the water disperses down walls and through ceilings into the space below. Installation of a self-adhering ice and water barrier membrane to the top of the roof decking, which is required by the 2012 International Residential Code in much of the northern United States, can prevent meltwater from reaching the roof sheathing. However, this is only effective if the ice dam does not exceed the height of the membrane. At the exterior of the property, ice damming can be observed as an accumulation of ice near the eave. Even if there has been a partial melt prior to the site visit, the gutters can remain filled with ice. Because ice dams, like snow or sleet accumulation, build up on the top surface, shingles are rarely damaged by the accumulation of ice. Ice damming can contribute to the failure of shingle sealant strips, though this only occurs at shingles in contact with the accumulated ice dam- not higher up on the roof slope. Excessive ice accumulation can cause damage to slate roofing at the edges of valleys, eaves, and those slates adjacent to snow clips, but slates in the field of slopes are rarely affected.
Interior indicators of water intrusion due to ice damming include wetness, water stains, or bubbled paint at the exterior walls and/or adjacent portions of ceilings, and at the underside of window openings. Meltwater penetration into the structure may follow paths and channels difficult to recognize as the source of interior moisture. Penetrating water often seeps down rafter tails, along plates, and into wall cavities, saturating the wall insulation and reducing its efficiency. Bay windows beneath the eaves soffit are often the first area to display water intrusion. Even without passing inbound of the exterior wall, water that has entered the eaves soffit can penetrate the top of an under-soffit bay window.
Within the attic, indicators of water intrusion due to ice damming include damp or stained roof decking and/or damp, stained, or flattened insulation near the eaves. These areas can be difficult to access in attics with shallow roof slopes. Wetting of insulation compromises its effectiveness.
Ice dams, and the resulting water intrusion, can be one-time events caused by very specific conditions or can be recurring problems aggravated by poor insulation and poor ventilation. Proper air circulation within the attic is required by modern building codes and reduces the potential of ice dam formation. It is best that air be allowed to circulate by natural thermal convection, entering from intakes at a low point and exiting, or being exhausted, from vents at a high point. This is most effectively done with eave vents (intakes) and ridge vents (exhaust). The underside of the roof deck should be bathed in outside air by natural convection from the eave to the ridge. Insulation often blocks the free flow of air along the roof slope, allowing for the buildup of heat and the eventual warming of the roof surface.
Inadequate provision of ventilation area, improper combinations of power and passive ventilation systems, and some overlapping roof designs are other issues that can also result in inadequate attic ventilation. Water entering the attic produces an environment conducive to fungal (mold) growth. Code-specified levels of attic ventilation are intended to control the attic temperature and relative humidity when water has not breached the roof cladding system. Even code-compliant levels of attic ventilation can be inadequate to sufficiently dry the attic and prevent fungal growth following water intrusion from an ice dam or other causes. Drying of wetted materials in the attic as well as in the living spaces below should be performed to minimize the potential for growth.
If the temperature of the roof decking is cold enough and the air in the attic has sufficient moisture (in the form of water vapor), condensation on the roof sheathing and roofing nails can also cause water to leak into the rooms of the house. The water vapor within the attic can be solely due to interior sources (such as cooking, bathing, or improperly vented laundry or bathroom exhausts), or may be exacerbated by water intrusion from ice dams or other leaks. Interior damage from this condensed water vapor can range from an annoying dripping and stains on the insulation or ceiling, to much more severe water damage. If the condition is severe enough, left unchecked over an extended period of time, it can result in fungal growth and wood rot.
In locations where chronic ice dams occur, a combination of ice and water barrier membrane and heat tape are often used to minimize the potential for damage. Heat tape consists of cables that use electricity to warm an area of the roof, gutters, and downspouts. While not intended to completely prevent snow and ice accumulation, the heat trace melts the immediately adjacent ice providing a path for melted snow and ice to flow from the roof and gutters. Heat tape systems should be checked regularly to ensure they are operating properly.
The application of de-icing salt and the removal of accumulated snow with shovels or long-handled snow rakes are direct methods of combating ice dams in existing winter conditions. Extreme caution should be used if choosing to employ these techniques as a fall from the roof or snow sliding off of the roof could cause injury or death. Careless use of metal tools to remove snow and ice can result in damage to the roof cladding and gutters. The application of good construction practices based on understanding of the principals discussed above are the best way to minimize the potential of damage from ice dams without the need to employ these more dangerous or destructive measures.
- Ice damming is initiated by fluctuations in temperature above and below freezing following snow accumulation, or when heat from within the dwelling escapes into the attic and melts snow on the roof surface. The lower portion of the roof (at the gutters and above the eaves soffit) is typically colder and the water re-freezes, progressively building up a dam of ice and trapping any water formed from further melting.
- Ice dams can result in water intrusion affecting the roof decking, attic insulation, exterior walls and adjacent ceilings. Unaddressed, the leaks produced by a roof dam can result in fungal growth, structural decay, and cosmetic damage.
- Good construction practices including provision of code-specified self-adhered ice and water barrier membrane, attic ventilation, and attic insulation are the best ways to prevent damage from ice dams. In locations where chronic ice dams occur, the additional provision of electric heat trace at the eaves, gutters, and downspouts can further minimize the potential for damage.