A lapped zip is a regular zip sewn into a seam so that one side of the fabric folds over the teeth and hides them, held by a single row of topstitching about 1 cm (3/8 in) from the seam line. Sew the seam below the zip opening and baste the opening closed, press it open, stitch the underlap side to the zip tape close to the teeth with a zipper foot, then fold the other side over the zip and topstitch it down from the outside, turning the corner at the bottom. It is the traditional zip on skirts, trousers and the side seams of dresses, and it is sturdier than an invisible zip.

What is a lapped zip and where is it used?

In a lapped zip, only one line of stitching is visible from the outside, running parallel to the seam and across its bottom, and the zip teeth are covered by a flap of fabric. A centered zip has two visible rows, one on each side of the teeth; an invisible zip has none. The lap gives the opening a tailored look and keeps the slider from catching on skin or lining, which is why lapped zips are standard on wool skirts, trousers with a side opening, and dresses in fabrics too heavy for an invisible zip.

The lap faces the back of the body on a left side seam, so it opens from front to back; on a center back seam it laps left over right. Either way the overlap is the side the topstitching runs down.

What do you need to sew a lapped zip?

  • A regular zip, polyester coil or metal, the length of the opening. On a skirt that is typically 18–20 cm (7–8 in), on a dress 40–55 cm (16–22 in). If the opening ends in a waistband, the zip stops 1.5 cm (5/8 in) below the waist seam line so the slider clears the band. Too long a zip can be shortened; see how to shorten a zip.
  • A zipper foot, the kind that snaps on to the left or right of the needle so you can sew close to the teeth from both sides.
  • Seam allowances of at least 1.5 cm (5/8 in); 2 cm (3/4 in) along the zip is more comfortable and is worth adding when you cut, because the overlap uses the full width of its allowance.
  • A strip of lightweight fusible interfacing, 2.5 cm (1 in) wide and the length of the opening, for each allowance in light or stretchy fabric.
  • Basting thread in a contrasting color, and chalk or a water-soluble marker.

How do you prepare the seam for a lapped zip?

  1. Fuse interfacing to the wrong side of both allowances along the opening if the fabric is light, loosely woven or on the bias. It stops the zip area from rippling.
  2. Finish the seam allowances of the whole seam with a zigzag or serger, before anything else: finishing afterward is awkward around the zip.
  3. Sew the seam below the zip opening from the hem up to the zip notch, at 1.5 cm, and backstitch at the notch.
  4. Baste the opening closed on the seam line with the longest machine stitch, 4–5 mm, from the notch up to the top edge. The zip will be positioned against this basted seam.
  5. Press the seam open, including the basted part. Pressing the allowances open here is what makes the folds on both sides sit exactly on the seam line.

How do you sew a lapped zip step by step?

The underlap is the side that sits under the flap, against the teeth: the back allowance on a side seam.

  1. Form the underlap fold. Lay the garment wrong side up with the opening basted. On the underlap allowance, press a fold 3 mm (1/8 in) outside the seam line, toward the raw edge, so that when the allowance is turned back the fold extends 3 mm past the seam. The underlap is now a narrow lip beside the basted seam.
  2. Position the zip face down on the opened allowances, teeth centered over the basted seam, top stop 1.5 cm (5/8 in) below the top raw edge (or at the waist seam line), bottom stop at the notch. Pin through the zip tape and the allowances only.
  3. Stitch the underlap. Fit the zipper foot to the right of the needle. Open the zip, fold the garment away so only the underlap allowance and the zip tape are under the foot, and sew the pressed fold to the tape 2 mm from the teeth, from bottom to top. Close the zip for the last 5 cm (2 in) and move the slider past the foot.
  4. Press the overlap side. Turn the garment right side up. The overlap allowance now folds along its seam line and covers the teeth, lying against the underlap stitching with a 3 mm overlap. Press this fold, keeping the seam line straight.
  5. Mark the topstitching line on the right side with chalk or a strip of 1 cm (3/8 in) masking tape laid along the fold: 1 cm from the fold down the length of the zip, across the bottom to the seam, 3 mm below the bottom stop.
  6. Topstitch the overlap from the outside with the zipper foot to the left of the needle: start at the seam line at the bottom, sew across to the 1 cm mark with the needle down, pivot, and sew up to the top edge, catching the overlap allowance and the zip tape underneath. Use a stitch length of 3 mm; the guide to straight topstitching has the tricks.
  7. Remove the basting with a seam ripper, pulling the threads out from the wrong side, and open the zip to check that the slider runs freely and the lap covers the teeth.
  8. Press from the wrong side with the zip under a towel, and from the right side with a pressing cloth, keeping the iron off polyester teeth.

Finish the top as the pattern directs: a waistband attached across the zip tapes with a hook and bar on the overlap, or a facing turned down with its ends folded under at the zip.

Can you hand-pick a lapped zip instead of topstitching it?

Yes, and on wool, silk and velvet it looks better. Replace step 6 with a prick stitch: a tiny backstitch taken through all layers from the right side, with 6 mm (1/4 in) between stitches and only a dot of thread showing on each. Use a doubled thread run through beeswax, or a silk buttonhole twist in a matching color. It takes 15 minutes on a skirt zip and leaves a line of small beads instead of machine stitching.

Common mistakes with lapped zips

  • The teeth show under the lap. The overlap was not wide enough or the underlap fold was pressed on the seam line instead of 3 mm outside it. The 3 mm lip is what makes the fabric close over the teeth.
  • The lap ripples. The allowances were not interfaced, or the zip tape was stretched while stitching. Interface, and let the feed dogs take the layers at their own pace.
  • A puckered seam after washing. A cotton zip tape shrank. Prewash cotton tapes, or buy polyester coil zips.
  • The topstitching drifts toward the fold at the top. The slider got in the way. Open the zip for the first part and close it once the foot has passed the slider's position.
  • The bottom of the lap stands away from the body. The topstitching did not turn the corner close enough to the bottom stop. Sew the crossing line 3 mm below the stop so the lap is anchored.

In Sewly's sewing guide, the notions list gives the zip length for the skirt or dress you drafted, and the step for the zip, with the 1.5 cm seam allowance already on the pattern; you can add 5 mm along the zip edges when you cut if you want the roomier lap.

When should you use a lapped zip instead of an invisible or centered zip?

Choose a lapped zip on medium and heavy fabrics, on side seams and trouser openings that get strain, on garments with a waistband, and in a tailored style where one clean row of topstitching suits the look. Choose an invisible zip on light and medium fabrics where no stitching should show, especially in a dress's center back seam: the invisible zip guide walks through it, and invisible zip vs regular zip weighs the two. A centered zip is the easiest for a beginner and suits center back seams in casual fabric. For a decorative metal zip on display, see how to sew an exposed zip.