Abstract: A ply joint bar for casting a seamless ceiling from a hardening material such as plaster or concrete, poured over the surfaces of adjacent molding panels, has a bridge with a lower surface resting on and flush with two such adjacent panels and an upper gable-like portion adapted to be anchored in the hardened casting material, an optional base member with two adjacent coplanar surfaces which grasp the underside of the panels, and a leg frangibly connected at one of its ends to the bridge and permanently connected at its other end to the base member, so as to separate adjacent panels sandwiched between the bridge and base member. After the casting material hardens, the molding panels are removed and the frangible leg separated from the bridge leaving a substantially seamless ceiling surface with the bridge embedded therein and its lower surface flush therewith.