Just a few comments I made in the Fashion Incubator forum when making patterns to accommodate stretch (knits, stretch wovens) and/or shrinkage.
Have to be careful when scaling patterns in CAD. Shrinkage/stretch is not proportionally the same in both directions. Many pattern making books advise to add the same amount of change to the length as the width. Don't do this. The only way to know is to know the fabric. Things shrink more in length than width. I am so sick of washing pants and long sleeve shirts and having them end up too short. If anything, remove width ease and add length on knits. It sounds counter-intuitive but prevents high-waters. Not sure on stretch denims, so test, test, test.In my Etsy shop I have a simple tech pack kit that includes a wash testing form. The wash testing form includes instructions and formulas to figure out shrinkage in both the length and width of a fabric. It is well worth the investment of time and money to test your fabrics for shrinkage, particularly knit fabrics. Once you know the approximate shrinkage of a knit fabric, I recommend adding the extra width and length needed to accommodate expected shrinkage.
Adding a shrinkage allowance to a pattern piece may cause the finished garment to look proportionally wrong. If you do not pre-wash the finished item before shipping to the retailer, the item may look wrong on the hanger and the customer may decline purchasing the item because it does not appear to fit. So there is a limit to the amount of shrinkage you can include in a pattern before proportions, fit, and hanger appeal are impacted. Generally, you can include about 3-5% shrinkage in a pattern without too much of an impact. If your wash testing results return a shrinkage greater than 5%, then you either need to reject the fabric or pre-shrink the fabric before cutting and sewing.
Knits in particular add an added layer to product development because they are more likely to shrink. There are various types of knit fabrics with different types of finishing. Some knits are pre-shrunk at the factory and some are not depending on whether the fabric will be printed or dyed at a later date. When sourcing knits it is important to ask about shrinkage.
Accounting for stretch in knit fabrics is also another added layer. Stretch influences fit. Should the item fit close to the body? If so, the pattern needs to be reduced in width and length. This is another area where the amount to reduce will be different for the length and width. As an item is stretched in one direction, the other direction also changes. You may notice this when you stretch a knit t-shirt in the width direction - the length of the t-shirt will shorten. This is a complex topic and the answers depend entirely on the style and chosen fabric.
