A seam is anywhere on a garment that two components are joined by means of stitching. It would, for example, occur where two fabric panels meet or even where elastic is joined at a leg line. Seams can be straight stitched, overlocked (with or without a stay stitch), cover stitched, zig-zagged or any number of specialty stitches (e.g.; embroidery). Seams and stitching methods affect the way a garment sits and moves on the body and therefore they need to be considered before you make the pattern. Different machines may also require a different seam allowance to be added to the pattern.
A straight stitch is the simplest type of seam. The domestic sewing machine is primarily a straight stitch machine. A raight stitch is not automatically locked and both threads can slip. Because it’s essentially a straight thread top and bottom it doesn’t have any room to stretch. You can lengthen your stitch length and pre-stretch your fabric as you sew in order to build in “give” but all you are really doing is loosening the seam. I’m often told by home sewing enthusiasts that this can be done successfully however I don’t recommend it. A straight stitch really has no place in stretch work. The photo is of my industrial straight stitch machine .. it doesn’t get much work in my studio unless I’m doing non-stretch sewing.
A zig zag stitch is similar to the straight stitch except that each successive down of the needle moves left to right a preset distance. A straight stitch is essentially a zig zag with a stitch width of zero. A zig-zag stitch does stretch. It is use is fairly limited to top stitching elastic. The width of the elastic seam (and hence the seam allowance) depends on the width of the elastic. The stitch length is a matter of style, but should not be as long as it is wide before it loses strength, stretch and aesthetics.
The most utilised stitch in stretch construction is the overlock (serger). There are many different types, each with varying amounts of stretch from zero to as much as 400%. An over locker can have between three and five threads with four being typical. The overlocker also has a blade for cutting the seam allowance neatly to its preferred width before it wraps the seam edge with its loopers. I tend to add a seam allowance 2mm greater than the width between the blade and left needle and cut this off during construction. If you use an exact seam allowance sometimes it gets undercut on the cutting table and loopers may drop stitches … it’s places like this that will catch and break stitches, resulting in a client that thinks less of your garment quality.
If you utilise one overlocker needle and both loopers (3 threads) the seam has the greatest stretch and the edges are wrapped. This is the preferred format for swimwear seams with negative ease. If you drop a looper (2 threads) the underside looks like a chain and the edges are not wrapped. This is called a chain stitch and can be used with or without the blade. It’s uses are limited to areas where the extra looper thread might cause unnecessary seam bulk. An example of this might be the seam joining the top and bottom fabric covering a preformed bra cup. Obviously the unwrapped edges won’t show and the seam doesn’t stretch significantly on the foam cup.
If you utilise two or three needles and both loopers you have an strong seam that resists stretching but won’t easily break if you try to stretch it. This is not very useful in swimwear but very useful for zero or positive ease stretchwear (e.g.; t-shirts, ribbed collars, sleepwear).
The cover stitch is a very clever stitch that works on a similar principle to the over locker except that on top you have two or three lines that appear to be straight stitched and underneath you have the zig zag loops of an overlocker. Some cover stitch machines have a top looper that makes both top and bottom stitches look the same (used mostly in active sports wear for strength). Cover stitchers are used exclusively for top stitching leg elastic and for bound seams. Again, the width of the elastic seam depends on the width of the elastic while the bound seam requires no seam allowance.
A cover stitch machine can also create a chain stitch the same way as an overlocker does. This is most often used for narrow bound edges and does not require seam allowance.
Seam strength is a product of the number and strength of threads used. However in a stretch-fit garment one should give consideration to the stitching not being put under any stress at all (i.e.; the fabric should reach its stretch limit before the seam). Do not use natural fibers for swimwear as salt and chlorine will cause them to rot. Only use synthetics like spun polyester and kevlar.
Wooly nylon thread was long considered the only thread to use in loopers as it sat flat and soft. This is really a side effect of low powered domestic overlockers and coverstitch machines that dropped stitches when you crossed one seam with another. The excess fabric bulk acted like a tensioner on the looper thread causing it to miss the needle. Industrial machines do not suffer from this problem, indeed they can burn wooly nylon at high speed causing it to tear (or worse the heat can weaken the thread without any visual evidence).
Ok now we come to a topic that is as serious as the battle between Macs and PCs or if you’re as old as me, the battle between VHS and Betamax. At what point do you add seam allowance? Some people add it directly to the block and some add it only to the pattern. Let’s consider the advantages and disadvantages of each.
Seam allowance added at the block stage:
- You only need to add it once and then each time you make a pattern it’s automatically transferred saving enormous time if you draft on paper.
- You still need to add seam allowance where the block is cut into smaller pattern pieces.
- You have to constantly keep track of where you have and haven’t added seam allowance so mistakes are easily and very often made. There are notching systems to help prevent these problems to some extent but my experience has been that this is not a fool proof system (e.g.; a simple nick with scissors or fabric cutters might end up looking like a notch).
- You might need to change the seam allowance if the seam type changes meaning a modification that needs to be tracked for future reference.
- If you work as a team this is a constant nightmare of who did or didn’t add seam allowance where and who is responsible for the error that’s just been transferred to a thousand pieces on the cutting table.
Seam allowance added at the pattern stage:
- You need to add the seam allowance to every edge each time you draft a new pattern. This is tedious if you draft on paper or easy and instant if you use a CAD package.
- If you leave adding any seam allowance to the last task you don’t need to track where it might be and hence you don’t make mistakes.
- As you are physically adding it to an existing marked line you can easily see where and how much seam allowance you have on every edge everytime without looking for notches. This is especially useful because errors can be picked up on the cutting table before the fabric is cut as the seams are visible.
What is the right system? If I had to do it by hand each time I might be tempted to add it to the block as a short cut but I’d soon change my mind once an expensive mistake was made … and it’s only a matter of time before it does.
As I use a CAD program it’s a no-brainer that my blocks are seamless. At the very last stage of making the pattern click a button, the program asks me how much to add and then does it in a nanosecond. Perfectly curved or straight, precise and labelled every time. No errors. Eventually paper blocks will become a thing of the past and so will this argument. These days almost everything is done on computer and emailed from designer to pattern maker to printer to cutter, sometimes across international borders, often where each person doesn’t meet the other.
Either method, make sure you employ a system where you consistently mark your seam allowances on the final pattern the same way every time. Call your designers, cutters and manufacturers to find out which system they use and discuss what they require of you.