Flatlock Seams Explained
In brief: A flatlock seam joins two pieces of fabric so the join lies completely flat, with the edges butted together and stitched over rather than overlapped and bulky. The result is a smooth, low-profile seam with no raised ridge to chafe the skin, which is why it is the standard for activewear, base layers and anything worn close to the body during exercise. Flatlock seams are also flexible and strong, moving with stretch fabric without popping.
What is a flatlock seam?
A flatlock seam is a method of joining fabric so that the seam sits flat against the skin rather than forming a raised ridge. Instead of folding the two edges together and sewing them so they stand up inside the garment, a flatlock machine butts the edges close together and stitches across the join with interlocking thread on both sides. The two pieces of fabric end up lying side by side on the same plane, held by a flat band of stitching. You can usually recognise a flatlock seam by the distinctive flat, ladder-like rows of stitching visible on both the inside and the outside of the garment.
Why flatlock seams reduce chafing
The whole point of a flatlock seam is comfort against the skin. A conventional seam leaves a raised ridge of folded fabric and thread on the inside of the garment, which can rub and chafe during the repetitive movement of exercise, especially once sweat is involved. Because a flatlock seam lies flat with no bulky ridge, there is far less to irritate the skin. This is why you find flatlock seams throughout performance base layers, running kit, cycling wear and anything designed to be worn tight to the body for long periods. They keep the seam from becoming a hot spot.
Flatlock seams and stretch
Activewear fabric stretches, and the seams have to stretch with it. A rigid seam on a stretch garment would either restrict the fabric or pop under tension. Flatlock stitching is designed to flex: the interlocking thread structure allows the seam to extend and recover along with the cloth, so it moves with the body and resists bursting. This combination of flat comfort and elastic strength is exactly what stretch performance wear needs, which is why flatlock has become the default construction for joining stretch panels in technical sportswear.
Flatlock vs overlock seams
It helps to compare flatlock with the more common overlock seam. An overlock, or serged, seam wraps thread around the raw edges of two overlapped pieces of fabric, creating a neat but slightly raised seam on the inside, the kind you see in most everyday t-shirts. It is quick, strong and tidy, but it leaves a small ridge. A flatlock seam goes further by butting the edges and stitching them flat, removing that ridge entirely for next-to-skin comfort. Overlock is fine for relaxed casual wear; flatlock is the upgrade for close-fitting activewear where seam comfort matters.
Flatlock construction at OD's Designer Clothing
At OD's Designer Clothing we stock activewear and base layers from premium sport brands that use flatlock seams for next-to-skin comfort, chosen for construction that holds up to hard wear without rubbing. Flatlock seams pair naturally with stretch and moisture-wicking fabrics in well-built performance kit. We offer next-day delivery and free click and collect, and customers in the North West are welcome to examine the construction in person at our St Helens store.