This invention relates to an underlayment for use under a plurality of courses of roofing shingles. The underlayment includes a water impermeable membrane layer containing at least one flap extending horizontally across the membrane layer. The flap is movable between a flat position and a raised position. In the raised position, an upper edge of a roofing shingle can be positioned under the flap. The upper edge of the roofing shingle can then be subsequently overlapped by the flap when the flap is moved into the flat position. The flap can then subsequently be overlapped by another course of roofing shingles.