How is the Weld Strength of TPO Tested?

One of the selling points of heat-weldable single-ply membranes is that the final result is a single sheet of membrane covering the roof. There are no adhesive seams to pop loose and the weld area is often said to be twice as strong as the rest of the membrane since it’s twice as thick. This all sounds good, but how does the manufacturer test weld strength? Here is a look at what we do at GAF.

The first step is to weld some TPO with the same equipment and settings that are used on a roof. In fact, we make many welds at varying speeds and temperatures to cover most conditions.


We recommend roofers do a test weld at the start of each day, after lunch, and whenever conditions have changed. A strip is then cut and pulled to check quality. Here’s a cross section of what a weld cut actually looks like:

At GAF, we do the exact same thing. However, instead of doing a manual pull, we use some sophisticated test equipment, which enables us to measure the actual strength. (We use a physical property tester made by Instron.) The sample gets mounted between two steel jaws and pulled as shown here:

The machine records the force needed to make the pull. If you look at the center picture, you can see that the cap layer has just broken. After that, the cap peels away from the core, as can be seen in the right-hand picture. If the weld itself opens up, then it’s a bad weld. The picture on the right shows what we call a film tearing bond. We’ve torn the cap or core film right off the membrane, exposing the reinforcement fabric.

Our tester reports the force as a graph that looks like this:

This shows that it took a peak force of around 48 pound-force to break the cap or core. Then, to peel the cap from the core, it took around 20 pound-force. These are strong numbers and the reason why our sheets never fail due to cap-to-core delamination.

At GAF, we test our membrane this way at the start of every production shift and after every product change. This ensures that the product welds and is the same strength day after day, mile after membrane mile!

Learn more about TPO and which brands perform best at and watch our how-to video on hand welding.

Related blog:

What is a Weld Window?

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