The use of thermoplastic roofing membranes has grown dramatically over the past years. This has occurred for a number of reasons including the proven track record of some products, the move to light colored “cool” materials, and the variety of systems available. A key benefit of thermoplastic membranes is the ability to hot air weld their seams, creating a permanent seal. As these single-ply membranes are installed in a one-ply configuration, a properly executed seam is critical to their long-term performance. Welding properties of membranes from five different manufacturers of thermoplastics: two polyvinyl chloride (PVC), two thermoplastic olefins (TPO), and one KEE (ketone ethylene ester) were studied. All materials were welded at twelve different combinations of welding temperature and speed. Each of these “welding windows” was completed with material as received, after 4 days immersed in water and after 30 days of exposure to severe soiling. Finished welds were assessed by peel testing in a tensile test apparatus. Differences in welding properties of specific materials and the generic types of products were evaluated, including sensitivity to changes in weld parameters, the effectiveness of manufacturer recommended cleaning/seam preparation, as well as the impact of the conditioning processes on weld quality. Integrating a previous study by others, a weld safety factor concept was developed which is a useful metric for assessing weld quality. This work demonstrated the need for clear, product specific welding guidelines for both new materials and for roof membranes exposed to the elements so as to ensure a proper weld.