A roof decking membrane welding system and method comprises first and second roof decking membranes having edge portions thereof welded to each other so as to define a seamed connection therebetween. The edge portion of a first one of the membranes is disposed beneath a seam plate and folded over the top of the seam plate so as to be secured upon itself either at a location adjacent to the seamed connection with the second membrane or between the first membrane and the edge portion of the second membrane. In either case, the folded edge portion of the first membrane forms a closed loop portion which presents multiple regions for load-sharing in resisting uplifting wind load forces. Alternatively, the edge portion of the first membrane is folded over itself but disposed beneath the seam plate so as to present dual plies of the first membrane for engagement with the seam plate so as to again provide load-sharing characteristics under uplifting wind load forces.