Abstract: The scaffold unit comprises a pair of towers, each having a pair of rails held together by crossties. Each rail has a gear rack on the front face thereof. The towers are held in fixed lengthwise parallel alignment by cross braces removably attached to the sides of the rails facing each other. A self-elevating, platform-supporting carriage projects outwardly from each tower. The carriage has a frame which supports a pair of upright shoes, each defining a U-shaped channel for slidably receiving a rail therein. The shoes fit over and guide the carriage on the rails when the carriage is in motion. A shaft is rotatably mounted on the frame and carries a pair of gears which mesh with the racks through slots in the shoes. A controllable motor is adapted to rotate the shafts independently of, or in synchronism with, each other.