An ice dam is a buildup of ice and water that works its way under the roofing.
When snow and ice of the roof melt (caused by warmth from the attic or the sun) the melted water runs down the roof. When the water hits the cold area of roof over the eaves, it refreezes. As more melting water hits the cold spot, it begins to "stack up" and eventually forms an ice dam.
In a worst case scenario, water building up behind the ice dam may back up under the shingles and seep through the roofing felt around nails and staples. This can warp or decay sheathing, cause excessive moisture buildup in the attic, degrade insulation performance, cause interior leaks and stains, and over time cause structural damage to trusses, ceilings and walls.
In northern climates, where homes are at the highest risk for ice dams, an extra layer of “waterproof underlayment” (also know as “ice and water protection”) is usually required along the eaves. This material generally seals better around nails and staples than roofing felt and can help prevent leaks.
Preventing Ice Dams
Heat loss from the house to the attic tends to be the primary cause of ice dams. Escaping heat from interior rooms enters the attic and warms the underside of the roof, causing the ice dam cycle to begin. Reducing or eliminating the heat on the underside of the roof is the most effective way to prevent ice dams.
Having a well-insulated attic reduces the heat under roof, and it is an excellent way to increase energy efficiency and comfort. Also, sealing places where warm air leaks from the interior into the attic (bypasses) is an effective step.
Bypasses are typically found around vent pipes, exhaust fans, chimneys, attic hatches, wall/ceiling framing joints, wiring and lighting fixtures. Use silicone caulk, poly plastic as fill material, foam sealant or a combination of all three to stop air flow around these areas.
A properly ventilated attic deters ice dams by keeping the underside of the roof cool and preventing the snow from melting prematurely. Ventilation also prevents moisture buildup and keeps the underside of the roof cooler in the summer, which helps lengthen the life of the shingles.
Signs of inadequate ventilation include rusty nails or rust spots on insulation caused by moisture forming and dripping off nails, frost buildup during the winter, or possibly a lingering musty smell.
The very best way to create ventilation is with continuous soffit vents and a continuous ridge vent. Warm air leaves through the ridge vent at the top of the attic and cool air is drawn in through the soffits at the bottom of the attic.
A ridge vent is easiest to install during a complete roofing project. As an option, multiple individual roof vents (situated within a few feet of the ridge) can be installed into an existing roof (as can individual soffit vents).
As a general guideline, the ideal vent-to-attic-space ratio of 1:300 means 1 square foot of venting for every 300 square feet of space. The attic venting should be split between the ridge and the soffits.
In areas with vaulted ceilings, cardboard or polystyrene “rafter chutes” keep insulation from pushing up against the underside of the roof deck and allow airflow from the soffits to the ridge.