Best Welding Practices – Ensuring Good TPO Welds

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Good welds are essential for trouble-free single-ply roofs. Installers are very important in ensuring welds are done well, both in the overlaps and in the flashings. While TPO is easy to weld, let’s take a look at some best welding practices. Strong welds are a key to making sure the final installation functions as a single sheet of membrane covering the entire roof.

Before Welding

  • Make sure equipment is properly set up and in working order.
    • Use a proper size generator for your automated welder (Required min. 10K Watt). No other equipment should use the generator.
    • Use an extension cord that meets requirements of the welder (recommend a 12-gauge cord).
    • Adjust the shoe position in accordance with the manufacturer’s recommendations.
  • Make sure each sheet is pulled straight and flat before fastening.
  • Install fasteners in line with the markings on the sheet.
  • The second sheet should be installed with the edge up against the overlap line on the first sheet.
  • Good welds require clean and dry TPO. Don’t lay out the TPO until before welding is about to start. If the TPO does get dirty, use TPO cleaner. For detail work, always use TPO cleaner to prepare the area.
    • For TPO that’s been exposed for more than 12 hours, we suggest cleaning the area to be welded.
    • Allow solvent-based cleaner to “flash off” prior to welding; in colder temperatures it will take longer for solvents to flash off.

Remove any excess contamination with a common household cleaner before using the solvent-based cleaner—any soap residue must be removed prior to welding.

When to Do a Test Weld

  • Test welds must be done—it’s the only way to check that the power supply and welder are operating properly, and that the settings are right.
  • Test welds should be performed:
    • Just before welding in the morning
    • Upon returning from lunch in the afternoon
    • When there’s been a significant change in weather (e.g., air temperature, wind speed, cloud cover, etc.)

How to Do a Test Weld

  • Take two pieces of “bag fresh” TPO approximately 18” long
  • Set your automatic welder’s speed and heat
    • For full-size welders, such as the BAK LarOn, I suggest starting at the following settings:
      • Temperature between 800˚F (427˚C) and 1,100˚F (593˚C)
      • Speed 10–16 feet (35 m) per minute
    • Weld the 18” (457 mm) pieces together and then allow the membrane a few minutes to cool. Cut a 1” (25 mm) wide strip across the welded material.
    • The welds are tested by pulling the weld apart by hand. Good welds require a lot of force — but it’s the way the weld comes apart that’s the key.
    • A satisfactory weld will:
      • Fail by exposing the scrim reinforcement, called a “film-tearing bond”
      • Be between 1” (25 mm) and 1.5” (38 mm) wide
    • A poor weld fails by separating between the two layers of the membrane
  • Remember, settings required for a good weld will change based on weather conditions and membrane thickness
  • The substrate can have an effect. For example, concrete decks act as heat sinks that draw heat away from the membrane. These typically require higher temperatures or slower speed.


Caption: In this picture of three test welds, the one on the right shows a 100% film-tearing bond (this is the goal). The sample on the left is the result of a cold weld — the membranes have separated. The center sample is somewhere in between, with about a 50% film-tearing bond.

Adjust Welder Settings

  • If the test weld looks good, with a 100% film-tearing bond, then membrane installation can begin. If not, the welder settings need to be changed.
  • Only make one change at a time and never change heat and speed together.
  • If you are welding at 1,148˚F (620˚C) and do not get a good weld, do not automatically adjust the speed because the temperature may be too high. Lowering the temperature or increasing speed may be a necessary adjustment.
  • If the weld is greater than 1.5” (38 mm), you may have the temperature too high and this could lead to a failed weld over time.
  • If you have bleed out, the welder is too hot. Unlike PVC, you don’t want to see bleed out on TPO.

While Welding

  • Make sure the welder is tracking along the edge of the top sheet.
  • Don’t allow the welder to push the top sheet, as this will create wrinkles. It may be necessary to change the weight — follow the manufacturer’s instructions.
  • Always walk on the unfastened sheet; this keeps it from creeping off the welding line. If it appears that the top sheet isn’t flat, stop and pull the sheet to take out any looseness and wrinkles.
  • Be alert for weather and temperature changes. You’ll need to be adjusting the welder after any change; do another test weld to make sure settings are right.
  • Pay attention to the wind. If it picks up, this will pull heat away from the welder. Do another test weld in case there’s any doubt.

After Welding

  • Use a seam probe to make sure there are no gaps in the welds. This should be done after the membrane has cooled (it’s typically done at the end of a day).
  • Make sure the seam probe tip is rounded off and won’t scratch or tear the membrane.

Hand Welding

  • With practice, hand welders can be dialed in just as quickly as the automatic units.
  • Remember that membrane will take higher temperatures but items like T-patches or prefabricated corner flashings will take lower temperatures. Only hand weld when you can’t run an automated welder.
  • All hand welds should be done in two passes.


Membrane Age and Cleaning

No matter how carefully you work, most jobs require some additional welding after the project has been inspected. For example, some patches over punctures might be needed. When coming back to a job to do repairs and finishing touches, make sure you clean the area before welding.

Final Thoughts

Always refer to the membrane supplier’s specification manuals for more details. Also, review the welder manuals and keep the welders and any power generators serviced. Finally, I have provided more details on how GAF makes sure our EverGuard® membrane weld window is large and how we test our membrane weld strength.

Learn more about TPO and which brands perform best.

There are 16 comments

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  1. Danny

    Hi i had a question on how much pressure should be applied to a weld to keep a consistent water tight weld with my expirence with tpo and pvc ive Noticed that alot more presure is needed when welding pvc. Is there any guides or information out there that says how much pressure should be applied via hand rolling when welding the diffrent materials?

    • Thomas J Taylor, PhD

      Danny – thanks for checking in with us. Hand welding takes a little experience – we recommend practicing with some strips of TPO or PVC first, to get your technique and settings down. Its best to start with the hand-welder set a little over mid-way, say 5 or 6. After welding, try to pull the two pieces apart. You need to be aiming for the weld to stay solid, and the adjacent membrane splitting down the center and exposing the fabric reinforcement. Welding is a combination of time, temperature, and pressure. When hand welding everyone’s idea of moderate pressure is different. With PVC you can look for a small amount of bleed-out from the seam. TPO doesn’t do that but with practice you’ll be getting good welds. Remember to do a test weld before starting, just to get the settings right. You can go faster with a higher heat setting, but then you risk overheating the membrane (the fabric will start to show through). If the temperature is too low, or you go too fast, the test weld will open up when you pull on it.

    • Thomas J Taylor, PhD

      I think you are asking if PVC and TPO can be welded together. The answer is a firm No! Its like asking if aluminum and steel can be welded together; again, No! TPO and PVC are different materials and won’t fuse together. For more information on welding thermoplastics, take a look at this video. Thanks for checking in with us.

  2. Nathan Martin

    I am so appreciate for this blog because there are some interesting points about best welding practices ensuring good tpo welds. I am so thankful for you and your blog. Thanks for sharing such a useful blog.

    • Thomas J Taylor, PhD

      Bryan – thanks for checking in with us. We want every roofing project to be successful and offer great training programs for contractors. Contact your local GAF sales person to find out how to sign up.

  3. Muy

    This is good topic about GAF. I have been roofing for 16 years and going on 17 next month. I like GAF materials but I do not like the detail membrane since it too tough to detailed in certain area. I was wondering if you guys ever consider in making material similar to a competitor’s detail membrane?

    • Thomas J Taylor, PhD

      Allen – yes, the thickness of the membrane can impact how you set up the welder. Always do some test welds at the start of a project – beginning with settings you normally use for those conditions. Look for a good film tearing bond when you try to pull apart a one inch wide strip across the test weld. If you haven’t installed say 80 mil before, then try to reach out to a manufacturer’s trainer by contacting your local membrane sales rep. They’ll be able to give you some tips on how to adjust your settings depending on what you are used to.

  4. George

    I love TPO and have installed to date over 50000 square feet of the material. I prefer the Leister heat gun for welding and i usually use two of the heatguns when weldings 400 feet or longer seams . I can usually do 50 feet a minute.
    I have a clamp I made that connects the guns perfectly , one set a inch or two ahead of the other .
    I am 58 now but when I was in my 20s. I was full of so much hot air I could just blow on it and it would melt perfectly.

    • Thomas J Taylor, PhD

      Jeff – thanks for the comment. Done properly, the welds are actually stronger than the main field of the membrane. During production of our TPO, we check the weld strength many times each day to ensure consistency and we’re proud of the wide weld window that GAF EverGuard TPO has. That is because we’ve formulated the sheet to give good welds over as wide a range of welder speed and temperature settings as possible. Of course, as said in the article, its critical that test welds are done each day and whenever conditions change. Take care!

  5. mike

    i am new to t p o roofing and i have to say everyone makes it look good but so far my experience has been mostly 5 to 8 year old tpo i think . one cannot even tell if its tpo or pvc also no one on u tube ever tells how to fix old t p o you cannot weld it and i would like to know what kind of glue if any one could use to make a seam repair. best thing we have done so far is to use contact glue on the old seams and butal tape as a cover and caulk the edges. if you can enlighten me i would be thrilled as of now i tell everyone its only good when its new if you can get it installed correctly real world tells me it is very difficult to work on and all though some people can install it most are lost.

    • Thomas J Taylor, PhD

      Mike – I’m pleased that you are interested in TPO and how to successfully install and repair roofs. Get in touch with your regional GAF commercial roofing sales person and ask about training. We run many classes and can also, for larger jobs, provide on-site training and guidance. We want roofs to be installed well – so we can cover new and repair situations with you in a class. Good luck in the business for 2017.

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