Why is the hem of my T-shirt wavy?

Short answer: A wavy hem is usually a sewing setup problem rather than an unavoidable feature of knit. When jersey is stretched as it feeds through the coverstitch machine, the hem is sewn longer than the body and ripples when it relaxes. When the fabric is held back too much or the thread tension is too tight, the hem gathers and puckers. The differential feed, presser foot pressure, stitch length, needle and looper thread tension and the operator's handling all affect the result. Hem depth and fabric matter too: very narrow hems on light jersey and fabrics that curl at the edge are harder to sew flat, and if the hem fold shrinks differently from the body, waves can appear after washing. Fix it by adjusting the machine on the actual bulk fabric, setting a measured hem width tolerance, and checking hems flat and after washing.

Common causes

• Fabric stretched during sewing, giving a rippled hem.
• Fabric held back or thread too tight, giving puckering.
• Wrong differential feed or presser foot pressure.
• Stitch length too short for the fabric.
• Very narrow hem on light or curling fabric.
• Uneven shrinkage between hem and body after washing.

How to fix it

• Set differential feed and foot pressure on the bulk fabric, not on a test scrap.
• Lengthen the stitch slightly and balance thread tension.
• Use a hemming guide or folder for even depth.
• Consider a slightly deeper hem for light jersey.
• Train operators not to pull the fabric.

How to check

• Lay garments flat and look along the hem for waves or gathers.
• Measure the hem sweep against the body sweep; a hem much longer than the body indicates stretching.
• Wash a few garments with an ISO 6330 procedure and recheck.
• Measure dimensional change of the fabric with ISO 5077 if waves appear only after washing.

A worked example

On the brand's shirts, measuring the sweep shows rippled hems are about 3 cm longer than the body just above. The factory had moved the style to a new line with a different machine setting. After adjusting differential feed and retraining operators, the hems measure within 1 cm and lie flat.

Limits

Some slight waviness on light jersey can relax with washing and pressing, and some looks are designed with a lettuce edge. Agree acceptable appearance with photos.

Facts, recommendations and uncertainty

Established: wavy or puckered hems mainly come from feed, tension and handling during sewing. Recommendation: set machines on bulk fabric and measure hem sweep. Uncertain: how much waviness your customers tolerate on light fabrics.

Sources and further reading

• ISO 6330:2021 — Domestic washing and drying procedures for textile testing
• ISO 5077:2007 — Determination of dimensional change in washing and drying

Standards and regulations are referenced by their published scope. Test parameters, cycles and pass levels should be agreed with your laboratory for the specific product; legal requirements should be confirmed for each destination market.