Among the many key components to a successful thermoplastic single-ply roof installation, properly executed hot-air welding of field seams, head laps, and flashing detail remains crucial to the overall long-term performance of the system. Advancements in both welding equipment and membrane technology have helped professional contractors achieve greater command of this process, however, it still boils down to mastering three basic welding principles: management of the temperature, speed, and welding pressure parameters.

TRIAC ST roofing 6

The earliest hot-air welders were simple hand-held tools designed to deliver heat into the seam or lap, where installers would utilize a hand roller to provide adequate pressure to bond the heated upper and lower surfaces within the seam on a molecular level – with the goal of creating a monolithic roof system. When properly performed, the 1.5″ welded seam becomes stronger than the adjacent field membrane. Seam inspection to verify weld integrity can be accomplished in the field through two primary methods: A blunt-tipped probe can be used to check for voids along the edge, and destructive cross-seam peel tests can be utilized to check uniformity throughout the full weld. If done correctly, the upper or lower surface will delaminate, or pull away, exposing the reinforcing scrim.


While still an approved installation method, the hand-welding process was – and still is – greatly impacted by outside factors that can affect the overall quality and reliability of the weld. This would include external influences like changes in ambient air temperature or incoming line voltage, and additional human factors such as, “how fast or slow is the technician welding?”, or “is the roller pressure applied consistently over every lineal foot of seam?”… just to name a few. Although infrequent, such factors can lead to deficient, partial bonds that are subject to failure over time.

VARIMAT V2 overlap welding

Over the past twenty-five years or so, the development and progression of automatic welders allowed installers to gain additional control over the process and mitigate some of the external factors that could negatively affect hand-weld quality. Now, with advanced digital technologies, roof technicians can regulate temperature and speed, as well as access other valuable features and data. After performing test welds at the beginning of each day, contractors possess the ability to pinpoint specific temperature and speed settings and save recipes for easy retrieval. The machines digitally adapt to field conditions in order to maintain desired settings and the weight of the machine provides the consistent welding pressure that is often varying or lacking in the hand-welding process. The result is an increased probability for uniform, predictable, and reproducible welding results.

Industry advancements continue to diminish the reliance on traditional hand-welding techniques used in performing most detail work. Today’s newest innovative equipment, adaptable to changing rooftop requirements, allows installers to automate the welding process over more lineal feet of production than ever before. Through this evolution, the industry has realized greater quality control capabilities, as well as enhanced ergonomics and worker productivity.

While equipment manufacturers are developing the next generation of automatic welder to meet future needs, thermoplastic and membrane manufacturers are continually improving formulations and advancing new products to market that allow welding in a greater range of temperatures and conditions – better known as a wider “installation or welding window.” All of this, combined with industry initiatives to expand contractor training and certification programs, will continue to help reduce warranty issues related to weld integrity and create higher-performing roof systems.