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The CertainTeed Roof Runner is a synthetic polymer-based underlayment that is reinforced with gauze and is water-resistant. This underlayment layer is meant to be placed beneath asphalt roofing shingles. Compared to standard asphalt felt underlayment when wet Roof Runner has remarkable dimensional stability that eliminates the possibility of moisture-related expansion wrinkles. The standard asphalt felt underlayment can cause distasteful wrinkles on the finished roof because it can become very wrinkled when it picks up moisture. The wrinkles can then sometimes transfer through the shingles applied over it. Roof Runner has an extra-large length and width for a speedy application and it has a special top surface treatment that provides superb slip resistance even when it is wet.