Provided herein is a curb-mounted skylight having a rigid plastic curb frame, a mechanism for firmly securing the curb frame to an opening in a building, a transparent or translucent covering element, and a mechanism for retaining the covering element on the frame. The curb frame includes a fixed base frame (having an interior frame and a counter-flashing element) and a moveable sash frame (having an accessory shelf and a sash). The base frame and the sash frame may be connected to one another by a first hinge member integral with the counter-flashing element and a cooperative and complementary-shaped second hinge member attached to the sash. The hinge members form a watertight seal and may be disengaged to permit the sash to be removed from the skylight assembly during installation, if so desired. Because the frame and hinge components are made of plastic, the skylight assembly is relatively lightweight.