An apparatus for welding a synthetic strip to a roofing membrane that is attached to a roofing construct, the synthetic strip having a central raised portion that extends along the longitudinal length of the synthetic strip, the synthetic strip having flanges extending widthwise from the central raised portion, and the synthetic strip having a substantially planar underside portion, the apparatus having a self-propelled housing having a drive wheel for movably supporting the housing on the membrane; a first guide-wheel element mounted to the housing and positioned in or underneath the housing and the first guide-wheel element having a peripheral channel adapted to engage and receive the underside flat-planar portion of the synthetic strip and guide the synthetic strip, wherein the guide-wheel receiving channel has a channel width adapted to receive the total width of the synthetic strip.