Finishing seam allowances requires planning and thought
Decide what type of seams will be used, and how they will be finished BEFORE the garment is cut.
This post answers Ginny H’s question, Is the amount seam allowances are modified because of the finish that will be used? The answer is YES! Here are examples.
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How the seams are used that are addressed in this post:
Flat-felled seams – used to finish men’s shirt sleeves and armholes.
French seams – used to finish high-end blouse sleeves and armholes.
Bias-bound seam allowances – often used to finish wool skirt seams if the skirt either has no lining or a free-hanging lining (home sewers call this a Hong-Kong finish).
Selvage – used down straight seams of high-end garments.
Hand overcasting – may be used to finish high-end garments cut from fragile fabrics.
Hand rolling – used to finish hems and scarves cut from silk and other thin fabrics.
Reduce (or increase) the pattern’s seam allowances BEFORE cutting the garment
This is ALWAYS done in industry.
The pattern must fit before the fashion fabric is cut. If this is the first time you are using the pattern, cut a muslin from the shell patterns, then check the pattern for style and fit. Make any fit corrections in the pattern BEFORE cutting the fashion fabric.
Note: This post assumes the reader is using home sewing patterns with 5/8 inch seam allowances.
Overlocked (serged) seams – Measure the bite of your overlock (serger). The bite is the distance between the needle and the knife. Most overlocks have a 3/8 inch bite. The knife is meant to cut away stray threads, not seam allowance. Reduce all seam allowances 1/4 inch that will be overlocked to give a 3/8 inch seam allowance.
Flat-felled/single-needle seams – Reduce the pattern’s seam allowances that will be flat-felled, 1/4 inch to give a 3/8 inch seam allowance. The seam is sewn with the top ply 1/4 inch from the needle, the bottom ply 1/2 inch from the needle (diagram 3).
The seam is then turned around and sewn back up with the wider seam folded over and then under the narrower seam allowance. This moves the seam 1/8 inch, but maintains the garment’s fit (diagram 4).
Single-needle identifies this flat-felled seam when it is used to sew higher-end men’s shirts. The name applies to this flat-felled seam because the seam is sewn on a sewing machine that uses only one needle. The single-needle flat felled seam can be sewn on any home sewing machine.
FYI: Double-needle is a flat-felled seam that uses two needles to sew the flat-felled seams found in less-expensive men’s shirts. The seam is sewn on an industrial sewing machine that can sew the double-needle seam at high speeds. Because far more low-end clothing is sold, manufacturers can easily justify the cost of this expensive, complicated sewing machine.
French seams – Reduce the pattern’s seam allowances that will be French seamed, 1/8 inch to give 1/2 inch seam allowances. The seam is first sewn on a scant 1/4 inch on the outside of the garment, then the seam is turned face-to-face and sewn again on a 1/4 inch seam allowance (diagram 5).
Bias-bound seams – The preferred seam allowance for bias-bound seams is 3/4 inch. INCREASE 5/8 inch seam allowances 1/8 inch. (diagrams 6 and 7).
Selvage – Utilizing the selvage as a seam finish eliminates any need to finish the seam allowance because the selvage is a natural finish. The selvage can be used as a seam finish if the selvage is presentable – not thick or ragged. Use to finish straight seams. Increase or decrease the seam allowance 1/8 inch to give a 1/2 or 3/4 inch seam allowance.
Hand overcasting – Use to finish seam allowances in high-end garments made from fragile fabrics. Reduce the 5/8 inch seam allowance 1/8 inch to 1/2 inch width.
Hand rolling – this seam finish requires a 1/4 inch seam allowance (diagram 8).
All diagrams, with the exception of the first diagram, are from Sewing Techniques from the Fashion Industry.
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© Laurel Hoffmann, 2019.