Hot-Air Welding

Hot-air welding is the standard way to join thermoplastic single-ply membranes, mainly PVC and TPO (and related materials such as KEE and FPO). A stream of heated air softens the top and bottom sheets in a lap, and a roller presses them together so they fuse into a single homogeneous layer. Properly done, the seam is as strong as or stronger than the sheet itself. Thermoset membranes such as EPDM cannot be welded and are joined with tape or adhesive instead.

Equipment

• Automatic (robotic) welders. Wheeled machines that travel along long field seams at a set temperature and speed, with a built-in pressure roller.
• Hand welders. Hand-held heat guns used for flashings, corners and details. Canadian guidance uses a 38 mm (1 1/2 in) nozzle for straight seams and a 19 mm (3/4 in) nozzle for corners.
• Seam rollers. Silicone or brass rollers used with hand welders.

Hand welding technique

Canadian guidance describes preheating the nozzle at full setting for at least a minute, then adjusting heat and making test welds. The back edge of the lap is first tacked with a thin continuous bead; the seam is then welded with the nozzle at about 45 degrees and the roller held at 90 degrees to it, moving steadily as the material begins to flow.

Test welds and settings

There is no single correct temperature: settings depend on the membrane, thickness, air temperature, wind and machine. Canadian guidance calls for test welds at the start of each work period and after significant weather changes. Too little heat gives a weak bond; too much damages the membrane.

Signs of a good weld

For PVC, a good weld shows a glossy surface, a little smoke while welding and a continuous bead of melted material at the seam edge. After cooling, every seam is checked by seam probing, and test cuts may be taken for peel testing. Seams must be clean and dry; dirt, moisture or chalking on aged membranes can prevent a bond, and older membranes may need cleaning or preparation before repair welds.