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Seam Allowances According to Seam Types

In my last post, I wrote about the benefits of not adding seam allowances until AFTER certain adjustments and processes are done. I lamented on the blanket use of a single width of seam allowance every where on commercial patterns from the usual popular pattern companies and I explained a little on why this is problematic. In professional practice, dressmakers, tailors, and ready-to-wear garment producers will use different widths of seam allowances at different parts of the pattern because this is actually more efficient in garment making.

Custom/bespoke dressmakers and tailors will likely have their own preferred widths (for different seams and seam types) in consideration of alterations required during the fitting process and for potential future need (to lengthen the garment or increase width). I have seen tailor-made dresses with 1-inch seam allowances on the side seams. This is not surprising when considering that custom-made usually equates with expensive, so one must provide for long-term wear in terms of accommodating a client's possible expanding girth. 



Techniques in the bespoke industry also tend to be more specialised and refined, allowing for the use of wide seam allowances at certain areas. For example, long side seams are usually hand-tacked (basted) first, then fitted on the client, and then machine-sewn. The tacking (basting) significantly reduces slippage when passed under the machine. It is then removed afterwards. The attachment of set-in sleeves is also given the same treatment. Having a wide seam allowance on the armscye facilitates the attachment of sleeve-heads in suits and jackets. It also allows for fit adjustments to the shoulder width during construction in 'made-to-measure'  (which is not the same as bespoke in that an order is first made with standard-sized templates based on a particular measurement given by the customer. The garment is then fitted on the customer before further alterations for fit are made).

No such considerations are ever given in the RTW production industry, where speed and economy is the rule. Seam allowance standards are set according to the most efficient and cost-effective way to sew up an item, based on the type of seam required by the designer/brand, and the needs of the item itself. There are MANY types of seams prescribed in the mass production industry, some of which are for very specialised items which are not within the purview of this post (like sails and tents). Every seam is given a coded classification for industrial reference which may vary by country.

For the purpose of this post, I'm just listing 4 seam types commonly used by the home sewer and the corresponding widths used in RTW production. Please not that these are terms used in tailoring and in home-sewing and not officially used in the sewing factories where the classification codes are referenced instead:

BUTTERFLY / OPEN SEAM

 SEAM TYPEWIDTH TO CUT FINISHED WIDTH  APPLICATION
 *Butterfly/Open  - ½" (1.3cm)  - ⅜" (1cm) ; trimmed during overlock  Side seams, shoulder seams


Butterfly/open/flat seams are the most straightforward and most common. They require only one pass under the machine to seam. Overlocking is just an additional step taken to encase the raw edge, trimming away ⅛" of the cloth as you do, either before (open seam) or after (flat seam) sewing up the seam.


FLAT SEAMS
    

 SEAM TYPE WIDTH TO CUT    FINISHED WIDTH APPLICATION
 *Flat seam                                             - ½" (1.3cm)                               - ⅜" (1cm) ; trimmed during overlock  Side seams, shoulder seams
  - ⅜" (1cm)                            - slightly more than ¼" (8mm) ; trimmed during overlock Armscye seam
  - ¼" (6mm)               - ¼" ; unfinished raw edge Curved edges: necklines and sleeveless armscyes (faced, or bound and turned under) 


FRENCH SEAM


 SEAM TYPEWIDTH TO CUT FINISHED WIDTH APPLICATION 
 **French seam             - ⅝ (1.6cm) - ⅜" ; 1st pass ¼", 2nd pass ⅜" Side seams, shoulder seams. armscye

French seams as done by custom dressmakers and tailors are narrower. The cut width starts at ½". After the 1st pass (at ¼"), the raw edge is trimmed down by ⅛", turned and pressed. The 2nd pass can then be sewn at ¼" from the turned edge, making the finished width at ¼". In RTW, there is no need to trim for a finished width of ⅜".

A narrow French Seam


FLAT FELLED SEAM


 SEAM TYPE WIDTH TO CUT FINISHED WIDTH APPLICATION 
 ***Flat Felled seam                      - ⅝",¼" corresponding opposite seam  - ⅜" ; 1st pass staggered placement ¼", 2nd pass tucked under ⅜" Men's shirt: side seams, shoulder seams. armscyes

Flat felled seams are usually made with a special flat felling foot that folds and tucks the wider edge under the narrower one. The finished widths can vary on different types of garments especially if they are triple top-stitched, like on denim jeans. Here is a graphic on the steps.

 
A flat felling foot

The list is by no means comprehensive simply because there is no one rule to follow. Everything is merely a recommendation, although standards exist in RTW production that the home sewer need not adhere to, but can choose to follow because they really are quite efficient. Dare to experiment and find that sweet spot which works for you. Remember though, narrow seam finishes means less bulk along your seam lines which gives your garment a nice and smooth finish.

If you'd like me to add other seam types to the list above, please let me know in the comments. 

As always, happy sewing!


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