Built-in apron simplifies installation on existing curbs and ensures watertightness. Engineered with compression spring operators to provide smooth, easy, one-hand operation, regardless of size. Automatic hold-open arm locks the cover in the open position to ensure safe egress. Overlapping cover design, full welded corners on cover and curb, EPDM rubber gasket, and fully insulated cover and curb ensure weathertightness and energy efficiency. Constructed with corrosion resistant materials to provide many years of trouble free, dependable service. Heavy gauge construction and positive latching mechanism maintain building security. Designed for installation over steep or ship type stairs.