Abstract: Apparatus for heat welding together thermoplastic membranes mounted on a surface includes a frame, a support member connected to the frame, a nozzle mounted on the support member to receive heated air, weld wheels mounted on the support member adjacent to, and immediately following, the nozzle for pushing a heated portion of an upper membrane toward a lower membrane to effect a weld, wherein the support member is adapted to be fixed to the frame member in a first position proximate a first side of the frame for welding the two membranes together, is releasable from the first position, is movable to a second position nearer a second side of the frame, and adapted to be releasably fixed to the frame at the second position for welding a membrane to an underlying edge piece mounted on a free edge of the surface.