December Renovation Tip: What Icicles Tell You About Your Roof

Thu, December 13, 2018

As you’re enjoying this festive season, you’ll likely appreciate some icicles to add to the wintery ambience. Unfortunately, icicles are not as welcome if they’re forming on your roof! 

In the winter, warm air from within your home rises, which—under the wrong circumstances—can warm the top of your roof and melt snow. As this melt travels down the roof to the colder eaves (overhangs), it cools and re-freezes, causing a horizontal ridge or “dam” of ice to form. The more that snow melts, the more it continues to pool above the dam, getting behind the shingles and penetrating through the roof deck into the home. To be clear, minor icicles are not an issue, but when the diameter of icicles reaches an inch or thicker, or are forming all along your roof, then they may indicate a larger ice backup scenario. 

Ice backup, in turn, indicates an airflow issue. A properly built roof has good ventilation on the underside, as this keeps the temperature under the roof close to that of the cold exterior and prevents a freeze-thaw cycle in the first place. For a roof that has the typical airflow features (e.g. vents that allow air to travel from the soffit to the peak ridge), a common culprit of poor airflow is the roofing insulation being installed butting against the underside of the roof deck. Rather than preventing heat from reaching the roof, insulation can only resist—not eliminate—the transfer of heat, so the roof still has a temperature differential. The better practice is to have a distance of at least 2” between the insulation and the roof deck so that cold air can circulate through this gap. 

Depending on the type of insulation, a DIY solution can remediate poor airflow. If moveable fiberglass insulation was used, it may be possible to simply pull the insulation away from the roof. Products such as Moore vents, available at most home improvement stores, can be used to act as a spacer between the insulation and the roof (image below; image source: Home Depot).

moore vents

On the other end of the spectrum of fixes, there have been times when it simply wasn’t possible to access the underside of the roof to increase ventilation, and we’ve had to build a new roof deck over the existing one (creating a gap in between old and new decks)! 

Prevention is always the best solution. Next time you have your roof redone, you can also make sure that all low-lying zones where ice is likely to build up—the eaves and valleys—are protected by an ice-and-water shield: a rubber membrane attached to the roof deck under the shingles, so that moisture can’t penetrate the roof even if ice backup occurs.