Well, the manufacturer says that it "will adhere to wood, metal,
plastic, glass, fiberglass, rubber, masonry, brick, smooth surface
built up
roofs, non-granular roll roofing, non-granular "half lap" roofing,
painted
surfaces, wood fiberboard, lightweight concrete, and
polyisocyanurate."

Besides wedging the underlayment, would there be any other way to
slope it?

Mike