It's been a long, cold, and snowy winter here in Massachusetts and over the past months of performing home inspections I have run into numerous instances of ice dam formation and the subsequent questions from prospective buyers and sellers.
So what exactly causes ice dams? Simply put, ice dam formation is caused when melting snow runs down the roof and then refreezes at the roof edge. The water runs down the roof slope underneath the blanket of snow and then refreezes into a band of ice at the roof edge creating a "dam". Additional snow melt can then pool against the dam and leak into the building through the roof or roof trim. Ice dams can actually form with as little as 1 or 2 inches of snow accumulation given the right weather circumstances.
The question I receive most often is, "what can I do to prevent ice dams?" The answer, while it may sound contradictory at first, is increased amounts of insulation and increased attic ventilation. In order to understand why, one needs to fully understand the mechanisms of ice dam formation: The upper roof (or attic) surface is typically at a temperature that is above freezing - this is what causes the melting at the upper roof surfaces. The lower part of the roof (or attic) surface (along the eaves) is typically below freezing - this is what causes the refreeze. It is heat lost from inside the house is the source of the melting at the upper roof surfaces! Because the lower roof surfaces (particularly the overhangs) are not warmed by indoor heat-loss, these regions can remain at below freezing temperatures, especially during periods of very cold outdoor temperatures.
Check your home carefully when ice dams form, even when there doesn't appear to be a leak. Get in your attic and look at the underside of the roof sheathing and roof trim to make sure they haven't gotten wet. Check the insulation for dampness. And when leaks inside your home develop, be prepared. Water penetration often follows pathways difficult follow. You may wish to hire an inspector that is equipped with an infrared or thermal imaging camera, as this equipment may be able to spot water penetration that is not readily visible with the naked eye. Ultimately, however, you will want to correct the problem to prevent future occurrences:
Insulation: Houses in this region of the country should have attic insulation of at least R-38, or about 12 inches depth of fiberglass or blown in material. The most notable problem area is at the far eaves, or right above where the exterior wall is.
Ventilation: A 'ridge/soffit' ventilation system is currently the most effective ventilation system, and is the system found in most new construction. If you have an older home, you may still be using a power fan, static roof vent(s) and/or gable end louvers - these simply aren't as good and will have a more difficult time retarding ice dam formation. Ridge/soffit systems should also include foam or plastic baffles at the far eaves to prevent insulation from blocking the soffit intake vents. If you have soffit baffles currently installed, make sure they are not crushed or displaced - they are relatively flimsy items yet they perform a very important function.
Air Leakage: Although insulation is what primarily keeps warm air in our house, small (or large) holes can allow significant volumes of warm indoor air to pass into attic spaces. Some of these sources can include: pull-down staircases, doorways leading to attic staircases, recessed lighting, bathroom exhaust fans, and various holes for cable TV, internet, etc. Take the appropriate measures to seal up or insulate these areas, it may be the difference maker!
Remember: Always wear personal protective equipment when making repairs of any nature. And when in doubt, always hire a professional.
Winter is almost over!