Hot-Air Welder

A hot-air welder joins thermoplastic roofing membranes such as TPO (thermoplastic polyolefin) and PVC (polyvinyl chloride) by heating the overlapping surfaces until they melt and then pressing them together so they fuse into a single, homogeneous seam. Both hand-held and automatic welders are used on most thermoplastic roofs.

Types

• Hand-held welders have a heating element, a fan and a flat nozzle. The operator directs the nozzle into the seam and follows immediately with a silicone hand roller. They are used for flashings, details, corners, patches and short seams.
• Automatic welders are wheeled machines that travel along field seams at a set speed, combining a hot-air nozzle with pressure rollers. They give consistent results on long seams.

Settings and test welds

Weld quality depends on the balance between temperature, speed and pressure, and on ambient conditions. Membrane manufacturers publish starting settings for specific machines; one manufacturer's guide gives starting temperatures for automatic welders of roughly 990-1,000 °F (about 530-540 °C) for TPO and around 1,090 °F (about 590 °C) for PVC, at speeds of several feet per minute, adjusted for membrane thickness. Because sun, shade, wind, substrate and temperature change through the day, manufacturers recommend making test welds several times daily and peel-testing them (cutting a strip about 1 in or 25 mm wide across the weld and pulling it apart) to confirm a proper bond.

Common problems

• Cold welds: too little heat or too fast travel leaves an unfused seam.
• Burned or overheated welds: too much heat degrades the membrane.
• Contamination: dirt, moisture or oxidised membrane surfaces prevent fusion and may need cleaning or preparation.
• Wrinkles and fishmouths at seam intersections and step-offs.

Seams should be probed with a seam probe after cooling, typically at the end of each work day.

Power and safety

Automatic welders need a stable power supply, often from a dedicated generator of adequate size and heavy-gauge extension cords. Hot nozzles and air can cause burns and ignite combustible materials, so welders are rested on stands and kept clear of debris.