Ice Dam Refresher

The Green Building Advisor recently released a helpful article on ice dams. Ice dams are caused by the combination of 1) a sub-freezing outdoor temperature, 2) snow on the roof, and 3) a roof/attic that is above freezing. With this combination, the heat from the living space escapes out through the attic, melting the snow, which then runs down and re-freezes at the edge of the roof. This can cause damage to gutters and shingles, and can cause water leakage into the wall underneath the bottom edge of the roof. This image from the CMHC website illustrates the problem well.


How do you prevent ice dams?

1. Seal air leaks in your attic.
2. Increase your attic insulation (this depends on your climate zone, but for us the target is R-50).
3. Improve ventilation between the insulation and roof sheathing.
4. Cover the roof sheathing (i.e. underneath shingles) with rubberized membrane underlayment.

It is important to note that the first two steps (air sealing and insulation) will also help with the energy efficiency of your home. The third step (ventilation) might help with ice damming if your air sealing and insulation is not sufficient. The fourth step (ice and water shield) will do nothing to prevent ice damming, but will help to prevent water leakage caused by the ice dam.

If none of these options are available to you at this time, you can attach electrical cables to melt the snow/ice on the bottom portion of the roof, or use salt (which would need to be regularly replenished) to melt ice dams.