a weld wheel assembly for disposition on a membrane welding machine , and for positioning a weld wheel adjacent a nozzle hot air outlet for pressing an overlying roof membrane heated by the nozzle against an underlying roof membrane heated by the nozzle , for welding the membranes together . the assembly includes a weld wheel rotatably mounted on an axle which is moveable toward and away from the membranes , and pressure exerting structure mounted on the machine and operable to bias the axle toward the membranes during operation of the machine .