by Nick Gromicko and Rob
An ice dam is a ridge of ice that forms at the edge of a
roof and prevents melting snow from draining. As water backs up
behind the dam, it can leak through the roof and cause damage to
walls, ceilings, insulation and other areas.
How do ice dams form?
Ice dams are formed by an interaction between snow cover,
outside temperatures, and heat lost through the roof.
Specifically, there must be snow on the roof, warm portions
of the upper roof (warmer than 32° F), and cold
portions of the lower roof (at freezing or below). Melted snow
from the warmer areas will refreeze when it flows down to the
colder portions, forming an ice dam.
Gutters do not cause ice dams to form, contrary to popular
belief. Gutters do, however, help concentrate ice from the dam in
a vulnerable area, where parts of the house can peel away under
the weight of the ice and come crashing to the ground.
Problems Associated with Ice Dams
Ice dams are problematic because they force water to leak from
the roof into the building envelope. This may lead to:
- rotted roof decking, exterior and interior walls, and
- respiratory illnesses (allergies, asthma, etc.) caused by
- reduced effectiveness of insulation. Wet insulation
doesn’t work well, and chronically wet
insulation will not decompress even when it dries. Without
working insulation, even more heat will escape to the roof where
more snow will melt, causing more ice dams which, in turn, will
lead to leaks; and
- peeling paint. Water from the leak will infiltrate wall
cavities and cause paint to peel and blister. This may happen
long after the ice dam has melted and thus not appear directly
related to the ice dam.
- Keep the entire roof cold. This can be accomplished by
implementing the following measures:
- Install a metal roof. Ice formations may occur on metal
roofs, but the design of the roof will not allow the melting
water to penetrate the roof's surface. Also, snow and ice are
more likely to slide off of a smooth, metal surface than asphalt
- Seal all air leaks in the attic floor, such as those
surrounding wire and plumbing penetrations, attic hatches,
and ceiling light fixtures leading to the attic from the
living space below.
- Increase the thickness of insulation on the attic floor,
ductwork, and chimneys that pass through the attic.
- Move or elevate exhaust systems that terminate just above the
roof, where they are likely to melt snow.
- A minimum of 3" air space is recommended between the top
of insulation and roof sheathing in sloped ceilings.
- Remove snow from the roof. This can be accomplished
safely using a roof rake from the ground. Be careful not to
harm roofing materials or to dislodge dangerous
- Create channels in the ice by hosing it with warm water.
Because this process intentionally adds water to the roof, this
should be done only in emergencies where a great
deal of water is already flowing through the roof, and
when temperatures are warm enough that the hose water can drain
before it freezes.
Prevention and Removal Methods to Avoid
- electric heat cables. These rarely work, they require effort
to install, they use electricity, and they can make shingles
- manual removal of the ice dam using shovels, hammers, ice
picks, rakes, or whatever destructive items can
be found in the shed. The roof can be easily damaged by
these efforts, as can the homeowner, when they slip off of the
In summary, ice dams are caused by inadequate attic
insulation, but homeowners can take certain preventative measures
to ensure that they are rare.