Ice Dams and Steep Slope Roof Ventilation

December 16th, 2011

Winter is fast approaching and with it the prospect of roof damage due to ice buildup. For many owners and managers of commercial structures with a steep slope roof, ice dams are a way of life in the cold seasons. While thousands of dollars of insurance claims make it through the system each season due to structural and water damage caused by ice build up around eaves, overhangs, gutters and underneath asphalt shingles, there is an opportunity to avoid much of this seasonal headache.

Some careful attention to roof temperature in a steep slope roof system can minimize the results of winter blasts. This is actually accomplished through two components in the roof system design - insulation and ventilation.

Proper insulation is a first step to controlling roof temperatures. In the particular relative seasons, the heat or the cool temperatures need to be confined to the dwelling without impacting the temperature significantly in the space between the dwelling and the roof deck. Too much heat finding its way up from the dwelling causes snow and ice on the roof surface to melt unevenly. This can lead to the buildup at the roof edges, eaves and gutters causing ice dams.

Once desired insulation R-value is achieved in the ceiling, there will still be a certain amount of heat finding its way into the free area between the roof deck and the ceiling. Ventilation is the key to making sure this temperature doesn’t reach extremes in the winter or in the summer. Consistent ventilation is accomplished through balanced air flow.

Balanced air flow refers to the relationship between intake and exhaust. Roofs that are under-configured will sometimes create an imbalance, so too much air enters the system and can’t escape, or not enough enters the system. In either direction, the cooling function of continuous air flow needs to work with the installed insulation and outside climate, to keep the roof temperature constant and cool enough not to promote rapid snow melt.

When designing a steep sloped commercial roof, the principal consideration in selection of engineered components is whether they will adequately facilitate balanced ventilation to achieve consistent temperature on the roof decking. Using the Net Free Area, the recommended ridge opening size and eave vent width can be calculated for a specific roof system configuration. Using eave and vent components which are designed and manufactured to work together and sized correctly, helps create optimum airflow for the roof ventilation system. When the roof maintains a consistent temperature as the result of balanced air flow; snow melt will be uniform and ice dams can be avoided.