Most people think that it takes snow to cause ice dams on a house roof. However, storms consisting of mostly sleet and freezing rain can result in ice dams as well.
What are Ice Dams?
You may be wondering what exactly are ice dams. If a winter storm that consists mostly of sleet and freezing rain hits your community, the sleet can form ice dams on your roof. You don’t need snow. Low temperatures for the days preceding the sleet or ice storm combined with low temperatures following the storm make for perfect conditions to create problematic ice dams on your roof.
As the sleet falls, some of it freezes on the shingles.
However, much of the sleet slides down the roof and collects at the eaves of the roof (the overhang that extends beyond the exterior walls), at the gutters, in the roof valleys, around dormers, behind chimneys and at roof penetrations. This collection of ice pellets freezes into a mass of ice at these roof locations.
If your roof looks like any of the photographs below, then you have ice dams.
Why are Ice Dams Bad?
The thin layer of sleet and freezing rain that accumulates on the upper roof shingles will melt quickly because the temperature in most attics is warmer than the outside temperature. I will explain more about this later. Also, once the storm is over, the sun may come out and help to heat asphalt shingles causing the sleet to melt on the upper shingles.
By contrast, the temperature at the eaves and soffits is lower than the attic temperature. The soffit is the area underneath the eaves of the house that overhangs past the exterior walls. Some soffits have vents. Some do not. Soffit vents help with attic ventilation. Regardless of whether there are vents or not, soffits have no insulation. The temperature at the soffits is will be much lower than the temperature in the attic and be much closer to the outside temperature. As mentioned already, sleet accumulates at the eaves because of this lower temperature.
As the ice on the upper roof melts and turns into water, it runs down the roof toward the eaves of the house where the soffit and gutters (if you have them) are located, but is blocked from exiting the roof through the roof drainage system (the gutters) where any ice dams are located.
Because of the ice dam, the melted sleet cannot reach the gutter. The melting of ice dams will likely occur from the bottom first where the shingles are warmer due to the attic temperatures being warmer than the outside air. Water will be moving underneath the mounds of ice. The water will then re-freeze overnight because of the low temperatures. The water will begin moving to the path of least resistance. Since water increases approximately 9% in volume when it freezes, the ice will expand and can exert pressure on the roof covering.This water may very well push under shingles and other building materials into the structure and unfortunately into the interior of your home.
This process can repeat itself over and over depending on weather conditions.
Many of my home inspection clients will understand why I harp over raised roofing nails and flashing that is not sealed. These are areas where water from ice dams and other weather conditions such as blowing rain can cause water to enter the structure causing roof leaks.
How long will it take for Ice Dams to Melt?
As long as the weather conditions remain favorable for ice dams with low outside temperatures and little sun, ice dams will melt very slowly. Any water remaining on the roof will re-freeze each night as temperatures drop.
Sleet as tiny ice pellets are a good insulator because of its density. Sleet is therefore much slower to melt than typical snowflakes because the density does not allow air to penetrate as freely as it can with most snowflakes. According to NOAA’s National Sever Storm Laboratory, on average 13 inches of snow is equal to one inch of rain, whereas two inches of sleet is equal to about one inch of rain. Combine this fact with low outside temperatures following a sleet or ice storm and ice dams may take a several days or longer to completely melt, much longer than snow would take.
Ice dams in shaded areas or on the north side of the roof will likely take longer to melt.
Why are Attic Temperatures Warmer than Outside Temperatures?
Winter attic temperatures are warmer than outside temperatures because the warm interior air rises. The warm air in the house is the air that you have paid to heat. This hot air is drawn into the attic because of temperature differentials between the house and the attic. Pathways exist between the house and attic that allow the warm house air to enter the attic because of an incomplete air barrier between the conditioned space in the house and the unconditioned attic. This knowledge is part of building science and within the purview of energy efficiency experts and home performance contractors.
What Can You do if there is an Ice Dam on your Roof?
- First, understand that if ice remains on your roof for an extended period, there is a chance that you will have an interior leak. You aren’t being singled out. Chances are most of your neighbors are in the same boat.
- If you happen to own a roof rake, scrap off any snow, sleet or ice that you can. During a storm or if you live in parts of the country that do not experience lots of snow, roof rakes may not be available. DO NOT improvise and use axes, picks, hammers or other tools. More damage will be done to your roof with these tools than any benefit you might gain. You may be able to use a spade and remove some of the ice from the gutters.
- Monitor the attic areas just below and in the general vicinity of the ice dam. Remember that if water finds its way into the structure it may travel and not be exactly beneath the ice dam. If you have vaulted ceilings, there may be no attic and the first sign of a leak will be wet drywall. Also, depending on house design, you may not be able to access attic areas near the ice dam. There may be closed roof sections or areas where the pitch is too low to access.
- Be vigilant and continue monitoring your attic as long as the ice dam remains on your roof. It can take time for water to work its way into the structure.
- Take a flashlight and shine it on the ceilings of all rooms in the house to look for water stains.
- Place a box fan in your attic aimed toward the area with the ice dam to help circulate the warmer attic air to the edges of the roof. This circulation may promote melting.
- Try to catch any leaks that you find in the attic if possible to prevent drywall damage.
- Call in your professional roofing contractor to assist you if you have damage. Be sure and check credentials, insurance and references of any contractors that you consider hiring.
- Don’t do anything foolish by risking your safety to deal with an ice dam or leak. In the big scope of problems, your health and well-being are more important than a home repair.
Author: Rick Nipper is a NC Licensed Home Inspector, a NC Licensed General Contractor and BPI Certified as a Building Analyst Professional and Envelope Professional. He is the owner of Nipper Home Inspection Services, Inc. and NC Radon Experts in the Raleigh, Durham, Chapel Hill area of North Carolina.