Hinckley Roofing, Medina OH

Ice Dams

What is an ice dam?

An ice dam is a ridge of ice that forms at the edge of a roof and prevents

melting

snow

(water) from draining off the roof. The water that backs up

behind the dam

can leak into a

home and cause damage to walls, ceilings,

insulation, and other

areas.

What causes ice dams?
There is a complex interaction among the
amount of heat loss from a house, snow cover,
and outside temperatures that leads to ice
dam formation. For ice dams to form there
must be snow on the roof, and, at the same
time, higher portions of the roof's outside
surface must be above 32° F while lower
surfaces are below 32°F. For a portion of the
roof to be below 32°F, outside temperatures
must also be below 32°F. When we say
temperatures above or below 32°F, we are
talking about average temperature over
sustained periods of time.

The snow on a roof surface that is above
32°F will melt. As water flows down the roof it
reaches the portion of the roof that is below
32°F and freezes. Voila!—an ice dam.

The dam grows as it is fed by the melting snow
above it, but it will limit itself to the portions of
the roof that are on the average below 32°F.
So the water above backs up behind the ice
dam and remains a liquid. This water finds
cracks and openings in the exterior roof covering and flows into the attic space. From the
attic it could flow into exterior walls or through the ceiling insulation and stain the ceiling
finish.

What causes different roof surface temperatures?

Since most ice dams form at the edge of the roof, there is obviously a heat

source

warming the roof elsewhere. This heat is primarily coming from the

house. In rare

instances solar heat gain may cause these temperature

differences.

Heat from the house travels to the roof surface in three ways: conduction,

convection,

and radiation. Conduction is heat energy traveling through a

solid. A

good example of

this is the heating of a cast iron frying pan. The

heat

moves from

the bottom of the pan to

the handle by conduction.

If you put your hand above the frying pan, heat will reach it by the other two

methods.

The air right above the frying pan is heated and rises. The rising

air

carries heat/energy

to your hand. This is heat transfer by convection. In

addition,

heat is transferred from the

hot pan to your hand by

electromagnetic

waves and

this is called radiation. Another

example of

radiation is to stand

outside on a bright

sunny day and feel the heat from the

sun. This heat is

transferred from the sun to

you by radiation.

In a house, heat moves through the ceiling and insulation by conduction

through

the

slanted portion of the ceiling (Figure 1). In many homes, there is

little space in

regions

like this for insulation, so it is important to use

insulations with high R-value

per inch to

reduce heat loss by conduction.

The top surface of the insulation is warmer than the other surroundings in the

attic.

Therefore, the air just above the insulation is heated and rises, carrying

heat by

convection to the roof. The higher temperatures in the insulation's

top

surface

compared

to the roof sheathing transfers heat outward by

radiation.

These two

modes of heat

transfer can be reduced by adding

insulation. This

will make the top

surface temperature

of the insulation closer

to surrounding

attic temperatures

directly affecting convection and

radiation

from this

surface.

There is another type of convection that transfers heat to the attic space and

warms the

roof. In Figure 1, the winding arrow beginning inside the house

and

going through the

penetration in the ceiling, from the light to the attic

space,

illustrates heat loss by air

leakage. In many homes this is the major

mode of heat

transfer that leads to the

formation of ice dams.

Exhaust systems like those in the kitchen or bathroom that terminate just

above the

roof

may also contribute to snow melting. These exhaust systems

may have to be

moved or

extended in areas of high snow fall.

Other sources of heat in the attic space include chimneys. Frequent use of

wood

stoves

and fireplaces allow heat to be transferred from the chimney

into

the attic

space.

Inadequately insulated or leaky duct work in the attic

space

will also be a

source of heat.

The same can be said about kneewall

spaces.

Photo 1 shows a single story house with an
ice dam. The points of heat loss can be clearly
seen as those areas with no snow. The ceiling
below this area needs to be examined for air
leakage, missing or inadequate insulation,
leaky or poorly insulated ductwork, and the
termination of a kitchen or bathroom exhaust
into the attic space.

Photo 2 illustrates unusually high heat loss
from the roof. There is very little snow left on
the roof and at its edge is both an ice dam and
a "beautiful" row of icicles.

Dealing with ice dams

Immediate action:

Remove snow from the roof. This eliminates one of the ingredients
necessary for the formation of an ice dam. A "roof rake" and push
broom can be used to remove snow, but may damage the roofing
materials.
In an emergency situation where water is flowing into the house
structure, making channels through the ice dam allows the water
behind the dam to drain off the roof. Hosing with tap water on a warm
day will do this job. Work upward from the lower edge of the dam. The
channel will become ineffective within days and is only a temporary
solution to ice dam damage.

Long-term action:

Increase the ceiling/roof insulation to cut down on heat loss by
conduction.
Make the ceiling air tight so no warm air can flow from the house into
the attic space.

Preventing ice dams in new homes

The proper new construction practices to prevent ice dams begin with

following or

exceeding the state code requirements for ceiling/roof insulation

levels.

The second absolutely necessary practice is to construct a continuous,

100%

effective

air barrier through the ceiling. There should not be any air

leakage

from

the house into