Ice Dam Cometh
If you’re fortunate enough to live in the Sunbelt or some other balmy region, chances are you’ve never heard of—or at least experienced—ice dams. I, on the other hand, live in New England, so every winter ice dams are as common—and as welcoming—as frostbite and sub-zero temperatures. That’s because ice dams can be destructive, problematic, and virtually impossible to remove once fully formed.
Ironically ice dams are often responsible for one of the most beautiful and iconoclastic of all winter images: glistening crystal-clear icicles hanging from gutters and eaves. Heck, even I marvel at the delicate beauty of icicles, but only when they’re hanging off someone else’s house. Here’s a brief explanation of how ice dams typically form:
It all starts with a roof blanketed in snow. The snow layer sitting directly on the roof begins to melt, and water runs down the roof underneath the blanket of snow. When the water hits the overhanging eave of the house, it begins to freeze. Some water drains into the gutter, where it freezes, as well.
As the snow continues to melt and water freezes at the eave, ice eventually builds up along the lower edge of the roof, forming a thick ridge or dam. Then, as water runs down the roof, it’s blocked by the ice dam, and forced up the roof. I know it seems to defy all laws of physics (and commonsense), but water will actually flow up the roof, working its way under the shingles. And that’s where the trouble really begins. Remember, roofs are designed to shed water running down the shingles, not up.
Over time water will work its way beneath the shingles and the underlayment, and into the seams between the plywood roof sheathing. From there, water drips directly into the attic, where it soaks through the insulation and drywall ceiling, before dripping into the room below. If not detected immediately, the damage can be very difficult and expensive to repair, especially if it ends up ruining the floor, walls, or furnishings.
Now, the above explanation is a rather simplified version of how ice dams form, the science behind this phenomenon is much more complicated, but if you go back to the very beginning, you’ll find a clue to solving this problem.
The entire process starts when snow sitting on the roof begins to melt. That’s key because ideally you want the interior attic temperature to be as close as possible to the outside air temperature. That’s why there’s usually only insulation on the attic floor, not between the roof rafters.
However, if the attic floor isn’t properly insulated or if heated air is leaking through the ceiling, via cracks, crevices, ductwork and light fixtures, then the attic will become warm. And it doesn’t take very much heat to raise the attic temperature enough to melt the snow on the underside of the roof sheathing. And since the overhanging eave extends past the house, it remains cold, which is why the water freezes when it hits the eave.
So, what can you do to prevent ice dams? First, use caulk, insulation and weatherstripping to seal all upstairs ceilings to stop heat from flowing into the attic. Next, measure the depth of the insulation on the attic floor, and add more, if necessary. Call the local building inspector or licensed contractor to find out how much insulation is required in your area.
Check to be sure that the attic ventilation system is operating properly. Most homes have soffit vents and ridge or gable-end vents, which allow air to flow into the attic along the eaves and out the attic at the ridgeline or gable walls. Be certain the attic-floor insulation isn’t blocking the soffit vents. And never staple plastic or anything else over attic vents. Remember, you want attic temperature to be as close as possible to the outside air temperature.
Finally, once the roof is covered with snow, use a snow rake to pull as much snow as possible from lower portion of the roof. That’ll at least remove the source of water, which will greatly reduce the chance of an ice dam forming. (See my tool test of the Garelick Roof Rake.)
If your home already has ice dams, there’s not too much you can do about it. Avoid installing electric cables or any other device to melt the ice; those products don’t solve the problem that’s causing the dams to form in the first place.