Ice Dams

An illustrated guide to what they are, how they form, and what to do about them.


It's that time of year again. Multiple snows, freezing rain, and oscillating temperatures begin a freeze-thaw cycle. Perhaps some see the elegant icicles as a silver lining to the unpleasant weather - but all we see are ice dams.

Ice dams are serious, and can cause permanent water damage to the interior of your ceilings, walls, and windows.


Below is an illustrated guide to what causes them. Read on.


This is a fairly common, if simplified view of what most wood-frame buildings look like at their eaves. (Eaves = overhang of the roof).


This is a diagram of the air temperatures at different points under the roof. You will notice that the air changes dramatically from warm-to-cold as the roof slides over the exterior wall.


As snow hits the warm roof, it melts and drains to the gutters.


As this water passes over the cold end of the roof, it begins to freeze, building up in layers and filling the gutters.


Soon, the ice builds up so high that it begins to prevent the water from reaching the gutter, creating the "dam." Meanwhile, the snow falling on the warm part of the roof continues to melt and drain down toward the dam. Roof shingles are lapped so that water moving downward never penetrates the layers, however the dam redirects the water up, freezing and pushing the shingles up and out of place.


Soon, the water infiltrates the roof under the shingles. This infiltration happens at a very vulnerable location in the building - over the exterior wall cavity. Water can fall to the interior side - wetting the ceiling and interior wall, the center of the cavity - eventually appearing at a window penetration or causing mold, or back on the exterior side - usually showing up as icicles within the eaves.


The diagram above describes the warning signs that your ice dams are beginning to cause permanent damage.


What you can do.


If you recognize the icy build-up along your eaves as an ice dam, you need to take immediate action. Water begins to infiltrate well before you will see it appear on the inside.


Do not panic and get outside on a ladder and try and shovel or break them off. You will very likely damage your gutter or break a roof shingle. It is also dangerous. Instead, fill a few porous socks, such as a cut panty-hose, with rock salt and toss them up sporadically into the problem areas. This will melt the ice and essentially provide a passage for the water to flow through. You don't need to melt the full dam so long as their are periodic outlets.


A long-term solution requires a trip to our attic to evaluate your insulation. Ideally, you want the insulation on the floor of your attic. It more efficiently keeps the heat inside the living area and also keeps the roof cold. A cold roof stops the stream of melting water that layers into the ice dam.


Secondly, check that your roof is well ventilated. If your insulation runs along the underside of the roof, and not the attic floor, you should check that baffles were installed and are not blocked at the eaves. Good insulation keeps that roof surface cold.


Lastly, you can purchase and install heated tape in your gutters or tack it along the roof over the eaves. When it's cold, a switch turns these on and keeps anything from freezing above.