How to Use a Hot-Air Welder on TPO and PVC Roofing Membranes
Clean, heat, roll and test. The seam is only as good as the test weld you made before you started.
- Professional
- 20 minutes to set up and test, then the welding
- Updated
The short answer
A hot-air welder fuses overlapping TPO or PVC sheets by heating both surfaces until they melt together under a roller. Clean the seam, set temperature and speed on a scrap test weld, run a consistent weld of at least the manufacturer's minimum width (commonly about 1-1/2 inches for automatic welds), then probe every seam edge once it cools. Always follow the membrane maker's settings.
TPO and PVC single-ply roofing sheets are joined by heat welding, not adhesive. A hot-air welder blows air at a precise temperature between two overlapping sheets, melting the surfaces so they fuse when pressed together by a roller. A good weld is as strong as the membrane itself. A bad one can look fine, yet peel apart under the first winter's thermal movement.
Welding quality depends on three things: clean surfaces, the right temperature and speed for the conditions, and a consistent technique. This guide covers how to set up an automatic welder and a hand welder, how to do a test weld, how to weld and roll the seam, and how to probe it afterward. For the membrane and the seams themselves, see how to weld TPO seams, how to install mechanically fastened TPO and how to repair a TPO roof.
Tools
- Automatic hot-air welder with a wide nozzle
- Hand-held hot-air welder with a 1-1/2 inch nozzle
- Silicone seam roller
- Seam probe
- Clean cloths and approved cleaner
- Generator or properly rated power supply with cord
- Scrap membrane for test welds
Materials
- TPO or PVC membrane
- Membrane cleaner approved by the manufacturer
- Seam cover or cut-edge sealant where required
- Heat resistant gloves
Safety first
- The nozzle reaches temperatures high enough to cause a serious burn in a second. Wear heat resistant gloves and never touch the nozzle or a freshly welded seam with bare skin.
- Keep the welder's hot air away from flammable materials, insulation, wood and people. Put the welder on a heat-proof surface when it is hot and never leave it running unattended.
- Use a properly rated power supply and cord. Welders draw a lot of current, and an undersized cord or generator overheats. Keep cords out of puddles and protected from edges.
- Be careful on wet or windy days. Moisture under the seam makes a poor weld, and wind cools the membrane and can change the required settings.
Step by Step
Step 1: Check the power, the machine and the weather
An automatic welder commonly draws several thousand watts. Use a generator or circuit sized for it, with a heavy gauge cord of the shortest practical length. Voltage drop causes the temperature to swing and weld quality to suffer. Inspect the nozzle for cracks and build-up, and the rollers for wear.
Do not weld in rain, on a wet membrane or with standing water. Wind and cold change the setting needed, so be ready to retest when the weather changes during the day.
Step 2: Clean the seam area
Dirt and film on the surface stop the two sheets from fusing. New membrane may carry a factory release or dust. Aged membrane may need a scrubbing and cleaning pass. Use clean white cloths and change them often. Let the cleaner flash off before welding.
Check the overlap width against the membrane instructions. Most seams need an overlap of at least 3 inches for automatic welds, but follow the product data.
Step 3: Set the temperature and speed from the manufacturer's data
TPO and PVC weld at different temperatures, and thicker membranes need more heat or slower travel. Manufacturers publish starting ranges, and they change with weather. A general rule is that a cold or windy day needs more heat or a slower speed, and a hot sunny day needs less. Your test weld is the real answer.
Let the machine warm up until the temperature stabilizes before you weld or test.
Step 4: Make a test weld and check it
Cut a one inch wide strip across the test weld and pull it apart by hand, or peel it with pliers. A good weld tears the membrane or leaves a clean film transfer, with no separation at the weld line. A cold weld peels apart cleanly. An overheated weld shows a brittle, scorched or bubbled surface. Adjust the temperature or speed and repeat.
Make a test weld at the start of each day, after any break, and whenever the weather, the membrane or the machine changes.
Tip: Keep your test welds as a log. Tape each one to a card with the settings, the time and the temperature so you can refer to it, and so anyone can see it was done.
Step 5: Weld with the automatic welder
Line up the machine along the overlap with the nozzle entering the seam and the front of the machine guided by the edge. Start it moving, and keep it moving. Stopping with the nozzle in the seam burns the membrane. Watch for a small bead of molten material at the seam edge, which is called a squeeze out. A consistent squeeze out shows the weld is getting proper heat and pressure.
The finished weld should be at least as wide as the manufacturer requires, commonly 1-1/2 inches or more for automatic welds. Keep the machine clear of wrinkles, and smooth them before the welder reaches them.
Step 6: Weld details with the hand welder and roller
A hand welder gives control at details where the automatic machine will not fit. Hold the nozzle at the edge of the seam, angled to push hot air into the gap between sheets, and follow with the roller right behind it while the membrane is molten. Use a steady, even motion, rolling perpendicular to the seam, and lift the roller off at the end. A short stop leaves a cold spot.
For the details themselves, see TPO walls, curbs and corners and TPO penetration flashing.
Step 7: Probe every seam edge after it cools
Use a rounded probe and push gently along the edge of the top sheet, feeling for any place where it lifts. Mark every defect with a marker. Repair it by re-welding with a hand welder, or add a patch, depending on the membrane and the defect. Check every inch, including the T-joints where three sheets meet, where most leaks start.
Step 8: Shut down and store the equipment
Switch off the heat and let the fan run until the machine cools, so the element does not burn out. Clean the nozzle of residue with a brass brush while warm. Wipe the rollers. Store it in its case, and keep the cord coiled loosely. Keep the test welds, settings and probe results as a record of the day's work.
What a Good Weld Looks Like
- A continuous, even bead of squeeze out along the edge, without gaps.
- A weld width at or above the manufacturer's minimum along the whole seam.
- No scorching, brown staining, blisters or shiny melted patches.
- No lifting when probed, and no open channels at T-joints or corners.
- A test weld that tears the membrane rather than separating at the weld.
Cold Welds and Burnt Welds
A cold weld has not received enough heat. It may look normal and even have a squeeze out, but the surfaces have not fused, and it peels apart in a test. Causes are speed too fast, temperature too low, dirt or moisture. A burnt weld has too much heat, and the membrane becomes brittle and thin. Causes are speed too slow, temperature too high or stopping with the nozzle in the seam. Both are failures, and both are found by test welds and probing, not by looking.
Common Mistakes
- Skipping the test weld. The seam looks fine until it peels in winter.
- Dirty seams. Dust, oil and moisture stop the sheets fusing.
- Stopping the welder in the seam. It burns through the membrane.
- Not probing. Voids at T-joints and corners go unnoticed until they leak.
- Undersized generator or cord. Voltage drops and the temperature swings.
- Welding wet membrane. Moisture turns to steam and leaves voids.
When to Call a Professional
Hot-air welding is a skilled trade, and manufacturers of TPO and PVC roofing usually require installers to be trained for their warranties. A seam that fails is expensive to find and repair once the roof is in use, so a roofer with welding experience is the right call for anything other than a small patch.
If you find cold welds, open seams or scorched areas on an existing roof, have a roofer inspect it. See how to repair a TPO roof.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Use a Hot-Air Welder for TPO and PVC: Questions
What temperature do you weld TPO at?
It depends on the membrane, the machine, the speed and the weather, so there is no single figure. Manufacturers publish starting ranges, and the real setting is found with a test weld that is pulled apart and checked before welding starts. Cold or windy conditions need more heat or slower speed, and hot sunny conditions need less.
How wide should a TPO weld be?
Membrane manufacturers set a minimum weld width, commonly about 1-1/2 inches for automatic welds, and seams need a larger overlap to allow for it. Hand-welded details may have a slightly different requirement. Always follow the product's installation instructions, because weld width is part of what the manufacturer's system depends on.
How do you test a TPO seam?
Run a rounded probe along the edge of the welded seam once it has cooled, pressing lightly to find any open spot or void, and mark and repair every one. Before welding, make a test weld on scrap and pull it apart to check it fails in the membrane, not at the weld line.
Can you weld TPO in cold weather?
Yes, but settings must change. Cold air and a cold membrane draw heat away, so temperature must go up or speed down, and test welds must be repeated as conditions change. The surface must be dry and free of frost or ice. Below the manufacturer's stated limit, stop and wait for better conditions.
What is the difference between a hand welder and an automatic welder?
An automatic welder is a machine that drives itself along the seam at a set speed and pressure, which gives a consistent weld in the field of the roof. A hand welder is used for details, corners and patches where the machine does not fit, with a roller following the nozzle. Most jobs use both.
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This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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