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North Hills Monthly

Icicle Light Sabers Signal a Bigger Problem

Dec 30, 2014 12:11PM ● By North Hills Monthly Magazine

By Matthew Schlueb

"Oskar, why are there snowballs stuck on the side of the house?"

"I'm trying to knock off the icicles."

When my son was a few years younger, he liked to see how long icicles would grow. One year, we had one that reached the ground. Now that he is 12, he is more interested in breaking them loose from the edge of the roof so that he can wield them around in light saber duels with his younger brother, Olin.

Living in Pittsburgh, where big temperature swings in a single day are not uncommon, we are blessed with an icicle breeding ground, at least in the eyes of children. But from the perspective of a homeowner, who knows that icicles can cause structural damage to a house or worse, the hazard of bodily injury, they are far less admired for their beauty or fun.

How do icicles form on roof edges and more importantly, how can they be prevented? The answers to these questions are frequently misunderstood. To get to the heart of it, we must take a look under the hood (or in this case, under the roof).

In older houses and most new ones, attics are insulated with blown-in insulation that settles between ceiling joists, leaving the attic as cold as the outdoors. This can be a good strategy, as long as there is a sufficient amount of insulation and it is placed correctly. Quite often, however, the insulation around the perimeter of the attic is cut short by insulation installers. Even when it is properly installed, blown-in insulation frequently becomes dislodged during storms when strong winds blow through vents in the eaves.

These gaps in the insulation allow heat to leak from inside the house. Aside from wasting energy and raising winter utility bills, this heat loss warms the snow-covered roof. The snow melts on the underside and is insulated from the winter cold on the topside, causing snowmelt to run down the surface of the roof shingles. As this water reaches the end of the roof, the cold metal gutters refreeze it, forming ice. Over time, as snowmelt water continues to run down the roof, the ice grows into a dam along the gutter.

This ice dam, hidden from sight under the snow, causes a lot of damage to the house. As the ice grows up the roof, it works under the shingles, finding its way onto the roof sheathing where it melts for a second time as a result of attic heat loss. At this point, water runs down onto ceiling joists and wall studs, damaging interior finishes and eventually rotting away the structural integrity of the framing.

None of this sounds good, but what does this have to do with icicles? Well, to answer this, we need to return to the temperature swings that we get here in the 'burgh. As the thermometer flirts around the freezing point—below it in the early morning and above it in the late afternoon—ice dams melt on the top side that is exposed to the warming outdoor air. This time however, the water runs down over the ice dam and refreezes in and over the gutter as it fills up, eventually causing icicles to hang down.

Icicles are indicators of the bigger problem—ice dams growing out of sight up on the roof under the snow. You can use these icicles, not just for light sabers, but to signal what parts of the roof need a closer investigation for insulation deficiencies. Ice dams and icicles can form by other factors (snow drifts in roof valleys, as an example), but with the right amount of insulation properly placed, the vast majority of ice-related problems on the roof can be avoided.

Here are a few additional, cost-effective measures that can be taken to improve the quality of roof construction and performance against ice damage:

  • Add insulation baffles at the eaves along the bottom of the roof rafters.
  • Switch to fiberglass batt insulation at the perimeter of the attic.
  • Use a rubberized asphalt, adhesive-backed ice and water shield underlayment on the first 36 to 72 inches of roof sheathing along the gutter line.

In the end, money spent will be recouped in heating utility savings and your roof will last much longer; though your kids will need to go to your neighbor's house for light sabers.


Matthew Schlueb is a registered architect and owner of SCHLUEBarchitecture. For questions or comments, contact Matthew at This article is part of an ongoing series addressing architectural issues for homeowners.