Propex. The surface of Opus Roof Blanket synthetic steep-slope roofing underlayment is more slip-resistant than traditional synthetic and felt products, the firm says, and the product is durable enough to be walked on, stays cool in the summer, is lightweight, and can be marked with a chalk line or pencil. The material can remain exposed to UV rays for up to six months. Opus installs with staples or nails, but requires caps if left exposed. 800.621.1273.