RoofRunner™ lightweight synthetic polymer-based underlayment outperforms felt and outclasses other synthetics. Designed for use on roof decks as a water-resistant layer beneath asphalt roofing shingles, this scrim-reinforced underlayment includes a special top surface treatment that provides excellent slip resistance, even when wet. Follow finished roofing manufacturer's instructions and all local building code requirements. It has exceptional dimensional stability compared to standard asphalt felt underlayment when wet. Its stability eliminates the possibility of wrinkles caused by moisture related expansion. Standard asphalt felt can become so wrinkled when it picks up moisture that it can sometimes "telegraph" its wrinkles through to the shingles applied over it, creating visually objectionable wrinkles on the finished roof.