RoofRunner™ is a lightweight, synthetic polymer-based underlayment that 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. It's large roll size - 4 feet wide x 250 feet long - helps speed application.