How to Hot-Air Weld TPO Seams: Test Welds, Settings, Probing and Fixing Cold Welds

Step by Step

1. 1Step 1: Prepare the lap and the weather

   Wipe the overlap of both sheets with a clean, dry rag. On dirty, weathered or greasy membrane, use the manufacturer's cleaner and let it flash dry. The welder cannot fuse over dust, water, adhesive or oil.

   Pull the lap flat. Wrinkles, fishmouths and folds at the lap are the usual place a seam opens, so cut relief and patch them later rather than welding through a crease. Check the weather: wet membrane cannot be welded, and a strong wind changes how much heat reaches the sheet. A small tent or windbreak helps.

2. 2Step 2: Check the power and warm up the welder

   Hot-air welders are sensitive to voltage. A long, thin extension cord or an overloaded generator drops the voltage, which lowers the nozzle temperature even when the dial says otherwise. Use heavy-gauge cord, keep it short and avoid sharing a generator circuit with heavy tools.

   Let the machine warm up for several minutes before making a test weld. A welder that is still heating gives a test weld that does not represent the seam to come, and a nozzle with old melted membrane stuck to it should be cleaned with a brass brush.

3. 3Step 3: Make a test weld on scrap

   Cut two strips of membrane from the roll being installed, lap them as on the roof and weld them with the machine set to the manufacturer's recommended temperature and speed. Typical starting points for a robotic welder are nozzle temperatures of several hundred degrees Celsius and travel speeds of several feet per minute, but those are starting points only. The working value is what the test weld proves on that day, with that sheet, in that weather.

   Make a new test weld at the start of each session, after every break, after any change of membrane roll, when the air temperature or wind changes noticeably, and whenever the power supply changes. Write down the settings that worked and keep the test samples for inspection.

4. 4Step 4: Peel test the sample

   After the sample has cooled, cut a strip about 1 inch wide across the weld and pull the two sheets apart by hand. A good weld fails by the top or bottom sheet tearing, or by the scrim showing through, which installers call a film tear bond. A poor weld separates cleanly along the weld line, leaving two smooth surfaces.

   If the test peels, adjust and repeat. If the membrane is scorched, brown or glossy, back off the heat or increase the speed. If it is too cold, turn the heat up or slow down. Change one thing at a time and do not start the roof until two consecutive tests pass.

   Tip: Most roofs get a quick hand peel at each restart. Some manufacturers and inspection agencies also require field peel tests or pull tests of actual seams at intervals, so check the warranty requirements before the job starts.

5. 5Step 5: Weld the seam with a steady pass

   A robotic welder does the long straight laps. It clamps to the membrane, drives at a set speed and feeds hot air into the lap while a roller presses the sheets together behind it. Start with the wheels on the membrane, the nozzle seated in the lap and travel straight. Stopping mid-seam burns the membrane, so keep moving and set it down at the end of a run.

   A hand welder does the details, tie-ins, and T-joints. Insert the nozzle of about 1-1/2 inches wide between the sheets, and follow immediately with the silicone roller at an angle, pressing from the centre of the weld outward. Work at a steady pace with a consistent nozzle angle. Hand welds are slower and more variable than machine welds, which is why they are tested and probed just as carefully.

6. 6Step 6: Check the width and appearance of the weld

   The standard minimum finished weld width on many products is 1-1/2 inches, measured from the edge of the top sheet. Confirm it on the manufacturer's specification, since some products require more. A small bead of melted material along the edge, called squeeze-out, tells you the sheet reached fusion temperature.

   Brown, blistered or shiny membrane next to the weld means scorching. A dull, flat edge with no bead usually means too little heat. Either way, the seam needs to be probed, and any doubtful length is cut back or reinforced with a cap strip.

7. 7Step 7: Probe every seam when it has cooled

   Wait until the weld has cooled completely. Use a blunt, rounded-tip probe, not a sharp screwdriver, and draw it along the edge of the top sheet with light pressure. The edge should not lift, and the probe should not slide into the lap at any point. Probe every hand-welded detail, every corner and every tie-in too.

   Mark every defect with a crayon or chalk and fix it before the crew moves on. Do not leave a probed seam to be checked tomorrow. Unmarked, untested seams are where a roof leaks for years before anyone finds the cause.

8. 8Step 8: Fix cold welds, open edges and T-joints

   A gap in a weld can be reheated with a hand welder and pressed with the roller. Bad scorching or a long cold weld is cut out or covered with a cap strip of the same membrane that extends at least 1-1/2 inches past the defect on each side. Clean the surface first, weld it, and probe it.

   The T-joint, where the end of one lap meets the side of another, is the most common leak point. The step from two sheets to three leaves a small gap. Many manufacturers require a round or oval T-joint patch of unreinforced or reinforced membrane over each one. Patches need rounded corners, because a sharp corner lifts. Follow the manufacturer's patch size.

Robotic Versus Hand Welding

What Goes Wrong and Why

Cold and Damp Conditions

Common Mistakes

• Not testing. A new sheet, a change in wind and a cooler afternoon each move the settings. One morning test does not cover the day.
• Welding over dirt or moisture. The two sheets appear to stick and do not fuse.
• Stopping the robotic welder on the seam. The membrane under the nozzle burns through or scorches and the lap is weakened.
• Not probing. Probing is the only field check for a continuous seam, and skipping it is how open seams stay undiscovered.
• Unwelded T-joints. The step between sheets leaves a channel that leaks if not covered.
• Power issues. A long cord, shared generator or low voltage drops the nozzle temperature and the cold welds follow.