Expandable, one-piece sill pan flashing

A sill pan flashing is provided with a width-extendable accordion section enabling the flashing to be manufactured, shipped and installed in one piece regardless of significant variations in the actual width of a framing sill from a designed nominal width. The flashing includes opposite side end caps connected by a horizontal sill cover. The sill cover includes one or more accordion sections configured with a plurality of integrally formed accordion elements which may be of V-shape, U-shape, sinusoidal or other configuration, allowing the one-piece flashing to be elongated in the width direction during installation, in order to completely cover a sill member. The new flashing is manufactured in one piece for a minimum sill width and expanded as necessary during installation at the job site to fully cover the entire surface of a sill of greater width. Substantial savings can be realized while providing a superior moisture seal.