CertainTeed RoofRunner Series 404987 Roofing Underlayment, 250 ft L, 4 ft W, Synthetic Polymer. Ct Rf Run Syn Underlay 4'X250'. 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 ft W x 250 ft L helps speed application.