What is Ice Damming?
In the winter months when snow blankets the roof, ice damming can occur when the heat from the attic and/or roof deck starts to melt the first layer of snow. The melting snow will run down the roof until it reaches the cooler overhang of the roof where it will freeze again, creating a thick accumulation of ice along the bottom edge of the roof. The ice builds up, blocking the gutters and causing water to collect behind the dam while the cycle of thawing and freezing continues. This can lead to a multitude of problems to both the exterior and interior of the home.
What Causes Roofline Ice Dams?
- Insufficient or defective attic insulation – As heat is transferred through the frame it warms the roof deck and melts the snow.
- Ventilation – Attic intake and exhaust vents need to be functioning properly to remove warm, moist air.
- Household air leakage – See The Usual Suspects for a list of factors that can contribute to a warm roof deck.
- Snow – Snow is an excellent insulator and 10 inches of snow is roughly equal to a 6-inch layer of fiberglass insulation. If there is enough snow on the roof it can insulate the attic and warm the air inside, causing the snow to melt.
- Low pitch roofs – These are particularly susceptible to ice damming. The lower the slope, the easier it is for the ice and water to anchor.
What is the Potential Damage?
The collection of water behind the ice dam can back up under the shingles and underlayment, causing a leak into the attic or living area. If undetected or neglected, this can result in water damage to roof decking, wood studs, insulation, drywall, paint and may even attribute to mold growth in the home.
Large amounts of ice and icicles can damage skylights, gutters, soffits, fascia, windows, siding and stucco.
In some cases, ice and water can build its way back up the roof 5 – 10 feet, causing lifting of the shingles and underlayment. Roofing systems are designed to shed water down the roof, not up, and underlayment is overlapped accordingly.
Solutions & Preventions
Your attic temperature should be relative to the outside temperature. Hot in the summer and cold in the winter. If this is not the case, your home may be at risk for ice damming.
If you are experiencing reoccurring damming, a water-tight membrane is a great solution. There are a number of underlayment options on the market to seal the roof deck and prevent water from penetrating the sheathing, such as Ice & Water Shield.
If ice damming persists and you’ve exhausted all other options, heat tape for the bottom edge of the roof may offer additional protection.
See the Best Course of Action for additional attic related solutions.