WHAT IS AN ICE DAM?
An ice dam is a ridge of ice that builds up along the
edge of a roof. The ice creates a dam that backs water
up and under the roof shingles. Once the water is deep
enough, it penetrates the roofing system and creates
water damage inside the home.To recognize an ice dam, look for a bulge of ice
attached to the eaves or overhang of a roof. There
may be icicles (“Aren't they pretty!”) hanging from the
edge, and you may see stains on the siding. The rain
gutters may be overflowing with ice.
Often, the bulge of ice is covered with several inches
of snow, so you may not see it. Under the buildup of
frozen snow is the melting snow and ice—water that is entering your home. If interior damage has already
occurred, you will see a wet ceiling and wall or water
flowing into windows.
Beneath the ice dam, undetectable damage is occurring
in the attic and wall cavities. The wood framing is wet
and may be rotting. Insulation is soaked, which makes it
inefficient. Mildew and mold can grow in hidden spots,
causing odors and other problems inside your home.
Soaked framing and insulation will take a long time to
dry out and will continue to contribute to wall damage
and interior moisture problems. Uncorrected, the
water can cause serious structural damage.
UNDER COVER: A CLOSE LOOK AT
Ice dam problems are most common in snowbelt
regions. They begin when snow accumulates on a
roof. Generally, deeper snow and colder temperatures
increase the formation of ice dams. North or northwest
winds usually accompany snowfalls, so more
snow is deposited on north and west roof planes.
Complex roof structures that trap snow compound its
depth and the problems it creates.
Once the snow has built up on the roof, it acts as an
effective insulator. (Light snow has an insulation value
of about R-1 per inch.) Heat from the attic warms the
underside of the roof and melts the bottom snow into
a slush/ice/water mixture. This mixture slides under
the snow cover and runs down the roof till it meets a
cold surface like the overhang. The slush then refreezes.
As more slush accumulates, the layer becomes
thicker and thicker, creating an ice dam. All of this
action occurs hidden from view under the snow cover.
Once the ice dam is high enough to overcome the
pitch of the roof, water seeps under asphalt shingles.
Standard roof shingle construction is not designed to
resist the attack of water pooling on its surface. The alternate freezing and thawing that occurs under these
conditions can increase the magnitude of roof leaks.
Once the water has penetrated the shingles, it flows
under the siding and eaves and leaks through the
framing into your home.
“IT NEVER HAPPENED IN THE GOOD
What seasoned homeowners say is true: ice dams
were not a problem before the ‘30s or ‘40s. Back
then, builders used a totally different type of construction.
Homes had steeply pitched roofs. Wooden shingles
were installed over spaced boards for sheathing so
the shingles could breathe and dry. Spaces between
the shingles and deck ventilated the attic and cooled
the roof deck. Many families did not fully heat the
home's second story, or they heated it just enough to
keep water from freezing in a drinking glass. (I grew
up in one of those homes.)
In newer homes with good heating systems, the attic
was often excessively warm because energy was cheap
and so homes were not well insulated. This excessive
heat rapidly melted the snow on the roof. Usually a
small line of ice existed only at the gutter or eaves, and
even that cleared on warmer days. Water penetration
did not linger, and ice dams as we know them today
did not exist.
Then, in the 1930s and ‘40s, we began to tighten up
our homes and use new materials. Roofers began
applying asphalt shingles, and building paper, plywood,
insulation, and vapor barriers came into use.
Central heating was made very effective, and all the living
spaces were heated. These changes triggered
new problems with moisture and ice dams. Attic ventilation
slowly became the standard.
Tom Feiza's Tips For Operating Your Home