What is a Weld Window?

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Note: Second image below corrected 4/26.

I recently described how TPO manufacturers measure weld strength. Strong welds are a key to making sure the final installation functions as a single sheet of membrane covering the entire roof. But, just as important is the ability of the membrane to weld under a wide range of conditions. That’s where the weld window comes in. First, let’s go over the basics of a good TPO membrane:

  • A strong sheet will give a strong weld. We optimize sheet formulation for high strength and solid ply adhesion.
  • Good sheet consistency is essential — a TPO membrane should be the same roll to roll, lot to lot. As I outlined in the blog post on weld strength, we check weld strength with every lot.

A membrane’s weld window needs to be as large as possible — meaning there’s a wide range of heat settings and welder speeds that will all give strong welds. The installer will still need to adjust to ambient conditions, but a wide weld window translates into a membrane that is easier to “dial in.”

The latest welders can operate between 600°F 1,148°F, with speeds between 5.5 – 39 feet per minute (fpm). The fastest speeds are likely too fast for an installer to even walk at, but if the installation needs to be done quickly then it could be necessary. So, manufacturers test welds at many combinations of heat settings and speeds. It’s a lot of work, but we weld at low and high speeds and low and high temperatures — and everywhere in between.

So what are good and bad welds? At the start of each day, after lunch, or if weather changed substantially throughout the day, we encourage the installer to weld two 18-inch pieces of TPO together. Welder settings depend on conditions, but will be somewhere in the range of 800 – 1,100F at speeds between 10 and 16 fpm. With experience, it gets easier to dial in to the right settings fairly quickly. Allow the test weld to cool and then try to pull it open. Here’s what you are testing:








Here are the three types of failure that can occur:

The sample on the right is showing an excellent weld — the weld hasn’t failed at all, but one of the membrane pieces has split along the reinforcement layer. This is known as a film-tearing bond. The sample on the left shows what happens when the welder is operating at too low a temperature and/or at too high of a speed.

So, how do we measure weld windows and what do they look like? At GAF, we do a large series of test welds at lots of temperature and speed combinations. The test welds are then pulled apart, exactly as you would on a roof, looking for perfect film-tearing bonds. We show all the combinations that give good welds in a table like this:






The combinations shown in green, the “weld window,” give good welds. But remember, this is in an indoor setting — we formulate to give successful welds in as many combinations as possible. As the installer has to adjust for actual conditions, a large weld window allows the membrane to be capable of being dialed in easily.

When we’ve developed new membranes such as our EverGuard Extreme®, or any of the colored membranes, we always make sure the weld window is identical to our regular EverGuard® membrane. That lets the installer know what to expect and how to dial in a GAF TPO membrane.

Learn more about TPO and which brands perform best.

There are 3 comments

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  1. Thomas J Taylor, PhD

    Great question. I cover that in a blog that we recently posted, called What is a Weld Window?. I hope this helps explain how GAF ensures our membranes weld well in many different conditions. Also, look for an upcoming blog about welding tips – guidance on how to achieve good welds.

  2. Gerald B. Curtis

    Information about the TPO Weld Window and GAF’s testing was most interesting and helpful. Such candor is appreciated.

    However, nothing was said about the middle example that showed part of the lap area having failed, but not across the entire lap area.

    In my experience, such an example of failure has more to do with inconsistent robot roller-pressure occurring side to side, bad robot tracking, substrate-flatness irregularities, or even incorrect diagonal hand-rolling across the lap than not having welded within the Window.

    As a QAO, I ask the contractor to perform the periodic Test Weld EXACTLY conforming to the field conditions by placing the two pieces of TPO to be welded on EXACTLY the substrate that the robot and laps will see as field laps are welded, and in or out of shade as the case may be.

    So this means, NOT performing the Test Weld with the Test Weld placed on a layer of already-applied TPO, but instead, on the substrate of the deck, insulation, or cover-board so that the heat-absorbing characteristics of the substrate are EXACTLY the same as those of the field laps, and so that the quality of the Test Weld will not be deleteriously affected or be different from that of the field laps.

    • Thomas J Taylor, PhD

      Gerald – you make some good observations and recommendations. I fully agree that test welds should be done in exactly the same conditions that will be used – so they should be done adjacent to the actual area being roofed and over the same substrate. You are correct that insulation and cover boards can absorb heat differently – that needs to be taken into account.

      It’s good to share best practices – at the end of the day we all want to see successful roof installations.

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