On Feb 19, 2:43*pm, wrote:
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?

Foam board comes in thicknesses from 1/4" to 2", maybe more, so it
should be possible to lay a decent slope for what you need by
transitioning from one thickness to another. A right angle grinder
would take care of any transition junctions that are too abrupt. HTH

Joe