It makes sense to choose the strongest thread possible. Especially when mending, you want the seam to hold up well. But in many cases, it is precisely thread that is too strong that causes the seam to burst or the fabric to wear out faster.
A seam is always an interaction between thread and fabric. If that balance is upset, wear and tear will end up where you don't want it.
The thread should give in to the fabric, not fight against it.
When a garment is worn, the fabric is constantly moving. It is stretched, folded and loaded in different directions. If the thread is significantly stronger and stiffer than the fabric, it is the fabric that has to take all the load.
Instead of the thread giving way a little, it starts to act like a thin knife cutting between the fibers. The seam looks solid, but the fabric around the stitches starts to give way.
Common in repair and reinforcement
This problem often occurs when mending older or thinner garments. In this case, you may want to choose a heavy thread to compensate for wear and tear. The result is often that the repair lasts, but the fabric next to it tears.
This is why holes in pants or shirts often move after a repair, instead of disappearing.
Polyester thread is stronger than many people think
Modern polyester threads are very durable. In many everyday garments, the thread is stronger than the fabric itself, especially after a few years of use and washing.
This does not mean that polyester thread is wrong, but that it must be used with the correct stitch length, needle and seam allowance. In some cases, a slightly thinner thread can give a better end result.
Too strong thread results in stiffer seams
A heavy thread makes the seam stiffer. In garments that are supposed to follow the body's movements, this can be a problem. The movement then ends up just outside the seam, where the fabric is weakest.
This is often noticeable in:
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branches and knees
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shoulders and armholes
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side seams in tight garments
How to choose the right thread for the fabric
Instead of relying on the strength of the thread, it is better to rely on the quality of the fabric and its intended use. The thread should be strong enough to hold the seam together, but not so strong that it takes on all the load.
A good rule of thumb is that the seam should be able to move with the fabric without anything feeling stiff or tight.
A sustainable seam is all about balance
When thread, needle, stitch length and fabric work together, you get a seam that lasts over time. Choosing the strongest thread possible rarely solves the problem, and in many cases can create new ones.
Small adjustments in material selection often make a bigger difference than you think.
