Your roofing system must include protection from wind-driven rain.
This is accomplished by installing Ice & Water Barrier directly beneath the shingles. Owens Corning offers WeatherLock® Self-Sealing Ice & Water Barrier Products and our Fiberglas™ Reinforced Felt Underlayment product. All are designed for both new construction and reroofing projects. Also use WeatherLock® Self-Sealing Ice & Water Barrier Products on areas of the roof where water has a tendency to collect or flow. These areas include valleys, vents, chimneys and skylights. It's important to prevent water collection on a roof because that can cause an ice dam to form. This ridge of ice causes water to back up and sit on a roof, instead of properly draining off. This water can work its way under the roof covering and flow into the attic—and cause water damage to a home's interior.