Rooftop Ice
Dams:
About every fifth winter in mild weather areas
and more often in colder climates a phenomenon
occurs which leads to a considerable amount of
bizarre homeowner behavior. Water suddenly
begins dripping inside the house, usually
occurring around windows on exterior walls. In a
panic, homeowners arm themselves with brooms,
shovels, and even water hoses and attack the
perceived source: snow and ice on roof edges and
in gutters. Their attacks are almost always
futile, often damaging the house and causing
personal harm. This is a situation that can be
prevented but cannot be readily and practically
relieved once it occurs.
The phenomenon is called "ice damming," a much
misunderstood problem. Ice damming or the
accumulation of snow and ice on the roof edges
and in the gutters is not the problem. Instead
the true problem and the source of the water
damage is "snow-water." The damages from this
seeping water can be devastating. Plaster
crumbles, paint peels, and a mildew odor
lingers. Even structural failure can occur due
to rotting within the wall itself. Effective
corrective measures must be directed at stopping
the formation of "snow-water."
The role of the roof-snow is complex. It acts
both as insulation and the source of water which
may freeze into a dam or seep into the house.
Heat rising into the attic space warms the roof
sheathing and melts the covering snow from the
underside. This "snow-water" then flows down the
roof under the protective insulation of the
covering snow. This will occur harmlessly unless
the snow-water passes over an area which is cold
enough to freeze it into ice. During severe
weather the lower edges of roofs that project
beyond the outside walls or the gutters will be
cold enough since they are not affected by heat
rising within the house. Ice that forms here may
gradually build into a considerable mass which
blocks or "dams" subsequent snow-water flowing
down the roof. If enough water is trapped behind
the dam of ice it can build up behind overlaying
shingle pieces and drip into the attic itself.
As it wets the attic floor insulation it ruins
its effectiveness and accelerates the formation
of snow--water by permitting more heat from the
attic to reach the underside of the roof
sheathing.
Air bypasses which allow warm household air to
reach the attic spaces are also thought to be a
primary cause of ice damming problems.
Removing snow from the roof would be expected to
solve the problem, since it is the source.
Unfortunately there is no known safe method of
doing this. Likewise clearing snow from the
lower, more accessible edges of roofs often
proves unsuccessful as it quite often merely
relocates the dam upward on the roof surface.
The snow-water will freeze when it exits from
beneath the insulating blanket of snow. Dams can
be avoided only by eliminating the heat which
causes snow-water formation.
It takes a combination of both adequate
insulation and ventilation to prevent the
formation of snow-water. If your attic floor has
only four inches of insulation (regardless of
type) you would be well advised to add more. The
most practical material for the do-it-your-selfer
is rolled blankets of fiberglass six inches
thick. Use foil-faced blankets only if your
attic floor has no insulation whatsoever;
otherwise use the un-faced type. This is to
avoid a vapor barrier between insulations. Tuck
the blankets between the floor joists. Allow
three inches of air space around any lights or
heating fixtures that may project up through the
lower ceiling. Trapping the heat from the
lighting fixtures with your newly installed
insulation may create a fire hazard. If the
attic is floored the job is usually best left to
a contractor.
With luck your house will have a projecting
overhang. Large overhanging roof projections,
also called soffits, can lessen damage since
seeping water rarely enters the house. They also
provide space for "soffit vents."
Check the underside of the projecting overhang
supporting the gutters for screened vents or
louvers. If no vents exist, create them. Buy a
three inch "hole saw" and a quantity of three
inch diameter louvers from a hardware store.
Attach the hole saw to your electric drill and
cut three inch diameter holes every six inches
all along the underside of the overhang. Your
finished job should display holes down the
center of the soffit from one end to the other.
Push your louvers up into the holes and you are
done.
This "soffit" ventilation allows air to enter
under the lower edge of the roof sheathing and
exits via the higher "gable" vents. The gable of
a roof is the "A" shaped high area at each end
of the house.
A house with no projecting overhang must turn
elsewhere for ventilation improvements since
there is no space for soffit vents. These houses
will respond well to the installation of "ridge"
vents. The ridge or peak of a roof is the
highest point or line where the two slopes come
together. Two inches of roof sheathing is cut
away on each side of the ridge and along most of
its length. A "ventaridge" louver is then
installed which allows attic heat to escape and
prevents rain and snow from entering the attic.
This type of ventilation is best installed by
professional roofers and generally costs
approximately $25 per lineal foot installed.
The best ventilation possible for roofs is a
combination of both soffit and ridge vents. It
will prevent ice damming and will keep you
cooler in the summer time.
Remember the concept, a "cold-roof" prevents the
formation of "snow-water" and this is
accomplished by sealing up air bypasses and a
combination of adequate insulation and
ventilation.