The Kansas City area has been experiencing the classic conditions for roof ice dams. Snow and ice accumulate on roofs. Cold air temperatures keep it there. Occasionally, the sun breaks out and warms the roofs to the melting point. Water runs down to the roof edge until it starts to drip off into the gutter. It does not get far because the air temperature is well below freezing. It freezes, the gutters and downspouts freeze solid and things get worse from there.
Ice builds up under melting conditions. More snow falls. Soon the roof has a tight cap of snow and ice. Heat from the home rises up and accumulates. Just the heat of the furnace will do, but fireplace chimneys, furnace flues, and dryer vents are also contributors. The underside of the ice melts, but has nowhere to flow. Water is invisibly trapped and ponding, even on steep roofs. Eventually, it finds its way into the house.
Sloped residential roofs are made to be completely watertight like a swimming pool. It is assumed that some moisture will get in. Ideally, water flows with gravity and eventually gets out without causing damage. Ice dams prevent gravitational flow and evaporation.
Current codes require an impervious sheeting at the edges of the roof extending 24” inside the line of the exterior wall below. The idea being that the ice dam water will be trapped in this area. If the problem area is greater, the sheeting will do no good.
Why then, is not the entire roof covered with this waterproof sheathing? Good question. As noted above, residential roofs are never perfectly constructed and the materials need to breath. A watertight cap all over the roof would trap moisture resulting in mold and rot.
The best ways to prevent ice dams are: (1) Insulate the floor of the attic; (2) Insulate flues; and (3) Vent the attic properly.
Conventional wisdom would say, “The warmer the attic is, the warmer my house will be in the winter. Why would I want to bring in cold air?” You want your roof the same temperature as the outside air. Temperature differentials and lack of ventilation will result in condensation, moisture accumulation and ice dams. In the summer, you will want that ventilation even more.
Install adequate high and low vents to create airflow. The residential code requires a minimum clear vent area 150th of the area of the attic. This minimum area should be divided evenly between low and high vents to create airflow.
There is one other thing to do, but I hesitate to mention it. That is removing the snow from your roof manually. Extension shovel rakes are sold. And people sometimes climb up and shovel off the snow. This is done more in more northern areas where the weight of accumulating snow is a concern. Unless you want to end up on YouTube, America’s Funniest Home Videos, or the emergency room’s Frequent Visitor list, DON’T GO UP THERE!