TPO seam welding temperature is not a fixed number, and treating it as one is how seams fail. Published manufacturer weld windows for reinforced TPO run from roughly 790°F to 1,150°F, and the correct setting inside that band depends entirely on the travel speed you pair it with, plus the ambient temperature, wind, membrane surface condition and substrate on the day you are welding. Every major TPO manufacturer resolves this the same way: you establish the setting each morning with a destructive test weld, not from a chart.
This page covers the field procedure: how the three welder variables interact, how to run and read a test weld, what a passing peel looks like, how to probe a cooled seam, minimum seam widths by method, and the T-joint and patch details that get skipped. Figures below come from installation guides published by Carlisle SynTec, Versico, GAF, Johns Manville and Holcim Elevate, plus ASTM D6878 for the membrane itself.
By The Roofing Brief Team. Last reviewed: August 2026.
What temperature should a TPO hot air welder be set to?
Start in the 900°F to 1,100°F range for a robotic welder on reinforced TPO, then confirm by test weld. Carlisle lists roughly 1,000°F as the operating temperature for its TPO at 10 to 13 feet per minute with a Leister Varimat. Versico publishes 1,004°F at 12.5 feet per minute at 100 percent air flow for the Varimat and BAK LarOn, and 986°F at 18 feet per minute at 90 percent for the Leister V2. Elevate lists 1,000°F at 11.5 feet per minute for the Varimat and 1,100°F at 12.5 feet per minute for the V2.
Those numbers are starting points that assume a specific machine, a specific air flow setting and a specific weight on the welder. Change any of them and the number moves.
| Manufacturer guide | Machine | Temperature | Speed | Air flow |
|---|---|---|---|---|
| Carlisle SynTec TPO Welding Guide | Leister Varimat | about 1,000°F | 10 to 13 ft/min | not stated |
| Versico TPO and PVC Welding Guidelines | Leister Varimat | 1,004°F | 12.5 ft/min | 100% |
| Versico TPO and PVC Welding Guidelines | Leister V2 | 986°F | 18 ft/min | 90% |
| Versico TPO and PVC Welding Guidelines | BAK LarOn | 1,004°F | 12.5 ft/min | 100% |
| Elevate UltraPly TPO Application Guide (2025) | Leister Varimat | 1,000°F | 11.5 ft/min | 80%, plus 15 lb |
| Elevate UltraPly TPO Application Guide (2025) | Leister Varimat V2 | 1,100°F | 12.5 ft/min | 70%, plus 15 lb |
| GAF Technical Advisory TAB-C-39 | BAK LarOn (starting band) | 800°F to 1,148°F | 10 to 16 ft/min | not stated |
Note the spread: two guides for the same Varimat V2 differ by more than 100°F because they pair it with different speeds and different air flow. That is the whole point of the next section.
Temperature and travel speed are one setting, not two
Nozzle temperature and travel speed trade against each other almost linearly, because what actually fuses the seam is total heat delivered per inch of lap. Run faster and you must run hotter. Elevate publishes this relationship as a “weld window” table, and it is the clearest illustration in any TPO manual of why a single temperature number is meaningless without its speed.
| Speed (ft/min) | Varimat V temperature | Varimat V2 temperature |
|---|---|---|
| 2.2 | 789°F | 858°F |
| 4.2 | 836°F | 911°F |
| 6.2 | 883°F | 963°F |
| 8.2 | 930°F | 1,016°F |
| 10.2 (V2: 10.1) | 977°F | 1,066°F |
| 12.1 | 1,022°F | 1,118°F |
| 14.1 | 1,069°F | not listed |
| 16.0 | 1,113°F | not listed |
Source: Elevate UltraPly TPO and TPO XR Membrane Weld Windows, Table 2, application guide dated January 2025. Varimat V at 80 percent air flow with 15 lb of added weight, V2 at 70 percent with 15 lb.
Across the Varimat V range, every additional foot per minute of speed costs about 23°F of temperature. A crew that speeds the robot up to make the day without touching the heat is welding cold, and a crew that turns the heat up because the seam looked weak without slowing down is welding hot. Both produce a defective seam from the same mistake.
The third variable: pressure
Heat only softens the two faces. Pressure is what fuses them, and on a robotic welder that pressure comes from the drive and pressure wheel loading the seam immediately behind the nozzle. Elevate specifies its published weld windows with two additional 15 lb weight plates on the machine, which means the temperature and speed pairings above are only valid at that loading.
GAF’s technical bulletin flags the failure mode directly: too much weight on the automatic welder combined with too high a speed setting can cause wrinkles in the weld area. On hand welds, pressure comes from the roller instead. Versico directs installers to roll across the hot seam immediately at a 45 degree angle with a two inch silicone roller. Carlisle specifies the same two inch silicone roller and warns against turning it up on edge. If you are choosing a tool, our seam roller comparison by membrane type covers the difference between silicone and steel.
Johns Manville adds a geometry rule that gets ignored on flashings: weld so the finished seam carries shear stress, not peel stress. A seam loaded in peel weakens and may eventually rupture, which is why membrane is welded to the horizontal face of coated metal rather than left to hang off a vertical edge.
How to run a TPO test weld
The test weld is the only thing that converts a manufacturer’s suggested range into a correct setting for the roof you are standing on. GAF calls it one of the most important steps in getting a properly welded roof. Run it on scrap so you are not cutting into finished work.
- Cut two pieces of clean scrap membrane. GAF specifies two pieces of bag fresh EverGuard membrane about 18 inches long. Johns Manville specifies a 4 to 5 foot test weld run.
- Set a starting speed from ambient temperature. GAF publishes a rule of thumb: ambient temperature divided by 10, plus 2, equals feet per minute. At 80°F ambient that gives 10 feet per minute.
- Find the top and bottom of the weld window. GAF’s method is to start at 600°F at that speed, test weld, then raise temperature in 100°F increments at the same speed until the machine maxes out (typically 1,148°F), noting where welds pass and where they burn.
- Set the machine in the middle of the window. Not at the edge. The middle is what gives you tolerance when a cloud crosses the sun or the wind picks up.
- Let the sample cool. GAF allows 10 minutes before peeling a test sample. Carlisle’s TPO welding guide calls for approximately 30 minutes before a test cut is peeled or a seam is probed.
- Cut strips across the seam and peel them. Carlisle, Versico and GAF all specify one inch wide strips cut across the weld. Johns Manville specifies two inch wide samples taken 6 inches either side of the seam, and a minimum of three samples per day.
- Change one variable at a time. GAF is explicit: never change heat and speed together. If a weld fails at maximum temperature, the answer may be to lower the temperature, not to slow down.
Frequency. Carlisle calls for a test weld before starting work each morning and again each afternoon, plus any time the substrate or weather changes. GAF adds “whenever there has been a significant change in weather,” naming air temperature, wind speed and cloud cover. Elevate requires destructive tests at the start of each welding day and after every interruption, including power failures, welder shutdowns and lunch.
What a passing TPO weld looks like when you peel it
A correct TPO seam does not come apart at the weld. It fails inside the membrane, tearing the TPO film away from the polyester scrim reinforcement underneath. GAF names this a “film tearing bond,” or FTB, and it is the pass criterion. Carlisle uses the same test in different words: delamination of the membrane from the scrim reinforcement indicates a properly welded seam. Johns Manville puts it plainly, saying good seams will be virtually impossible to peel and should delaminate the TPO film from the reinforcing scrim.
The width of that failure matters too. GAF specifies an acceptable weld peels between 1 inch and 1.5 inches wide. Carlisle specifies a consistent 1.5 inch minimum weld, free of voids and wrinkles.
| Result | What you see on the peeled strip | Cause | Correction |
|---|---|---|---|
| Acceptable weld | Film tears away from the scrim; the seam itself never separates | Correct heat, speed and pressure | None. Record the settings. |
| Partial weld | Partially separates between the two membrane layers | Marginal heat input | Slow down or raise temperature by one increment |
| Cold weld (false weld) | Cleanly separates between the two layers, scrim intact | Too fast, too cool, contaminated or damp surface | Reduce speed or raise heat, clean and dry the lap |
| Burned weld | Discolored or melted membrane, bleed out from the lap edge, weld wider than 1.5 inches | Too hot or too slow | Increase speed or lower temperature; cut out and patch the scorched area |
The burned case is the one crews misjudge. GAF warns that if the weld peels wider than 1.5 inches the temperature may be too high, and that this “could lead to a failed weld over time.” A seam can look strong on day one and still be damaged.
Why hotter is not safer: the scrim is the strength
Reinforced TPO gets its tensile strength from a polyester scrim sandwiched between two layers of TPO compound, not from the compound itself. ASTM D6878, the consensus specification for thermoplastic polyolefin based sheet roofing, sets a minimum breaking strength of 220 lbf by the ASTM D751 grab method for that reinforced sheet. Melt the scrim and the number is gone, no matter how well fused the polymer looks.
There is very little polymer protecting it. ASTM D6878 requires a minimum thickness over scrim of 0.015 inches measured per ASTM D7635, and since the 2013 revision the coating over scrim on the weathering side must be at least 30 percent of the sheet’s nominal thickness. That works out to 18 mils on a 60 mil sheet and 24 mils on an 80 mil sheet, per NRCA’s technical reporting on the standard. The material standing between a 1,100°F nozzle and the reinforcement is measured in thousandths of an inch.
That is the whole argument against defaulting to a hotter setting. An overheated seam looks fused, peels wide, and has had its reinforcement cooked at the exact line where the roof concentrates thermal movement stress. Johns Manville requires burned membrane to be cut out rather than welded over: remove the damaged material at least 1 inch beyond the burned edges, then cover with a patch extending at least 3 inches beyond all damaged edges, allowing a minimum 1.5 inch weld width on all sides, cut square or rectangular with rounded corners.
Minimum TPO seam width by welding method
Automatic welder seams and hand welder seams do not share a minimum. Hand welds are wider because the heat and pressure are less consistent, so the specification builds in margin.
| Weld type | Minimum finished width | Source |
|---|---|---|
| Automatic (robotic) welder, field seam | 1.5 inches | Carlisle, Elevate, Johns Manville |
| Hand welder, all locations | 2 inches | Elevate UltraPly TPO application guide |
| Membrane to TPO coated metal, automatic | 1.5 inches | Elevate |
| Membrane to TPO coated metal, hand | 2 inches | Elevate |
| Wide weld (Varimat wide weld kit) | 4.5 inch nozzle | Elevate |
| Typical sheet overlap that produces these | about 2 inches (nozzle inserted fully into the weld zone) | Carlisle, Johns Manville |
Probing seams: the daily quality check
Probing finds voids that a peel test on scrap cannot, because it inspects the actual installed seam along its entire length. Every manufacturer reviewed here requires it, and requires it daily.
- Wait for the seam to cool. Carlisle specifies a minimum of about 30 minutes. Premature probing damages a warm seam.
- Use a blunt tool. Johns Manville names a blunted scratch awl or cotter key puller. Elevate specifies a dull cotter pin puller. Carlisle sells a dedicated seam probe.
- Draw the tip along the seam edge with firm, steady pressure. Probe the seam junction, not down into the bottom sheet. On a correct weld the tool will not penetrate the lap.
- Mark every penetration. Use a water soluble marker at the start and end of each void or wrinkle.
- Repair the same day. Carlisle recommends repairs be made the day they are discovered, and requires deficiencies be corrected no later than the end of the workday. Johns Manville requires every seam and T-joint tested before the end of each work day.
- Re-probe repairs after they cool. Then wipe off the marker if the repair passes.
- Blunt the probe tip regularly. Johns Manville notes that continuous probing sharpens the tip, which then scores the membrane it is supposed to be inspecting.
Probing is also a feedback loop on the welder, not just a defect hunt. Carlisle’s guidance is that seams must be probed throughout the day specifically to check quality and make adjustments to the welding equipment. Ongoing seam condition is the single biggest variable in TPO roof maintenance and long term seam care.
T-joints, patches and cut edge sealant
A T-joint is where three layers of membrane overlap and the edge of the middle sheet creates a step. That step leaves a void the robot cannot close, which is why T-joints are the most common leak origin on an otherwise sound TPO roof.
- When a T-joint patch is required. Elevate requires T-joint patches at reinforced membrane seam intersections whenever membrane thicker than 0.045 inches is used, and wherever seams pass through angle changes of 1:12 or greater. Carlisle requires a TPO T-joint cover or a non-reinforced flashing surface splice over all T-joint intersections on 60 mil membrane and heavier.
- Patch size. Johns Manville recommends patching all T-joints, including base flashing, with a 7 inch rounded piece of reinforced field membrane or a manufactured T-joint patch.
- Prepare the step. Elevate requires the edge receiving a T-joint cover to be chamfered by heating and rolling to minimize the step down. Johns Manville’s hand method is to lift the upper sheet, heat both faces, then roll and fuse with the edge of a silicone roller until a crease forms along the intersection, which indicates a proper weld.
- Round every corner. Square patch corners peel. Carlisle, Johns Manville and Elevate all specify rounded corners on patches and flashing overlaps.
- Seal exposed scrim daily. Elevate requires all cut edges with scrim exposed to be sealed daily with cut edge sealant. Carlisle specifies a one eighth inch bead after probing is complete, and notes vertical splices do not require it. Johns Manville notes the purpose: preventing water entering the welded area by wicking or capillary action.
How weather and contamination shift the correct setting
The setting that produced a film tearing bond at 7 a.m. will over-weld by mid afternoon. Every condition below changes the heat that actually reaches the lap, which is why the test weld is repeated rather than performed once.
| Condition | Effect on the weld | Adjustment |
|---|---|---|
| Cold ambient or cold membrane (early morning) | Membrane absorbs more heat before reaching melt | Slow the welder down; morning speeds run slower than midday |
| Rising surface temperature (midday sun) | Membrane arrives at the nozzle already warm | Speed may be increased as surface temperature rises |
| Moving from sun into shade | Shaded membrane surface is cooler than sunlit membrane | Re-check splice results and adjust heat or speed |
| Dew, mist or a passing shower | Evaporating moisture pulls heat out of the splice | Wipe water off before welding, then raise heat or slow down |
| Wind | Cools the surface and disrupts air flow inside the lap, reducing nozzle effectiveness | Increase heat or reduce speed |
| Plywood or concrete substrate | Acts as a heat sink | Higher temperature or slower speed than over insulation |
| Cool damp substrate | Same heat sink effect, amplified | Higher temperature or slower speed than dry substrate |
| Surface contamination or exposure | Dirt and oxidation block fusion regardless of setting | Clean per manufacturer before welding, no setting change |
Contamination is the one variable you cannot weld through. Elevate requires cleaning with splice wash across at least 6 inches of both sheets if membrane has been exposed more than 12 hours or picks up dirt, debris or moisture, and requires the cleaner to flash off completely because residual cleaner itself contaminates the bond. Johns Manville sets its threshold at 24 hours of exposure and treats cutting out and replacing the section as the last resort. Versico directs installers to use weathered membrane cleaner on TPO and clear all residue before welding. Elevate also states flatly that solvent welding is not acceptable on TPO, which distinguishes it from PVC roof installation, where welding practice and settings differ.
For background on the membrane itself, see our overview of TPO roofing membrane, its properties and where it is used, and our review of Versico TPO, EPDM and PVC membranes.
Frequently asked questions
What temperature do you weld TPO seams at?
Most manufacturer guides put reinforced TPO between roughly 900°F and 1,150°F on a robotic welder, paired with 10 to 16 feet per minute of travel. Carlisle and Versico list about 1,000°F for a Leister Varimat, Elevate lists 1,100°F for the Varimat V2. The correct number for your roof is set by a test weld at that morning’s actual conditions, not by the chart.
How do you know if a TPO weld is good?
Peel a one inch strip cut across the seam after it has cooled. A passing weld produces a film tearing bond: the TPO film tears away from the polyester scrim and the seam itself never separates. GAF specifies the failure should occur across 1 to 1.5 inches. If the two membrane layers separate cleanly, that is a cold weld and the setting is wrong.
What is a cold weld in TPO roofing?
A cold weld, also called a false weld, is a seam that looks closed but never fused. On a peel test it separates cleanly between the two membrane layers with the scrim intact. Causes include excessive travel speed, insufficient nozzle temperature, wind cooling the lap, moisture on the surface, or dirt and oxidation on membrane that was not cleaned before welding.
Can you over-weld TPO?
Yes, and it is more damaging than it looks. Excess heat burns through the TPO coating and degrades the polyester scrim that carries the sheet’s strength. ASTM D6878 allows as little as 18 mils of TPO over the scrim on a 60 mil sheet. GAF warns that a weld peeling wider than 1.5 inches may indicate temperature is too high and can lead to a failed weld over time.
How wide does a TPO seam weld have to be?
Automatic welder field seams must finish at a minimum of 1.5 inches wide per Carlisle, Elevate and Johns Manville. Hand welded seams must be a minimum of 2 inches wide per Elevate, because hand welding delivers less consistent heat and pressure. Sheets are typically lapped about 2 inches so the nozzle can be inserted fully into the weld zone.
How often should test welds be performed on a TPO roof?
At minimum, before welding starts each morning and again after lunch. Carlisle specifies morning and afternoon plus any change in substrate or weather. GAF adds any significant change in air temperature, wind speed or cloud cover. Elevate requires a destructive test at the start of each day and after every interruption, including power failures and welder shutdowns.
Why do TPO T-joints need a patch?
A T-joint is where three membrane layers overlap, and the edge of the middle sheet creates a step that leaves a void a robotic welder cannot close. Elevate requires T-joint patches on membrane thicker than 0.045 inches and at angle changes of 1:12 or greater. Johns Manville recommends a 7 inch rounded patch at all T-joints, including base flashing.
How long should a TPO seam cool before probing?
About 30 minutes. Carlisle specifies allowing hot air welded seams to cool thoroughly for approximately 30 minutes, and warns that premature probing can damage a warm seam. Test weld samples on scrap can be peeled sooner: GAF allows 10 minutes. All seams must be probed and any deficiencies repaired before the end of the same workday.
Sources
- Carlisle SynTec, TPO Welding Guide (document 603160) and TecTopics, “Focus on Test Welds and Seam Probing,” 2023.
- Versico, “TPO and PVC Welding Guidelines.”
- GAF Technical Advisory Bulletin TAB-C-39, “Test Welding Thermoplastic Membranes.”
- Johns Manville, JM TPO Installation Guide.
- Holcim Elevate, UltraPly TPO Roofing Systems Application Guide, January 2025, including Table 2 weld windows.
- ASTM D6878/D6878M, Standard Specification for Thermoplastic Polyolefin Based Sheet Roofing; ASTM D751 grab method; ASTM D7635 thickness over scrim. Requirement details as reported by NRCA’s Professional Roofing.
Reviewed by The Roofing Brief Team. Last reviewed: August 2026. Manufacturer settings cited here are starting points from published installation guides. Always follow the specific written instructions and warranty requirements for the membrane and welding equipment on your project.