When it comes to pattern drafting, hacks, and alterations, you always want to make sure you're working from the stitching lines and not from the cutting lines (which include seam allowance). Conversely, if you've drafted a pattern piece from scratch, you may find that you need to add seam allowance around the drafted pattern piece. Either way, you'll find it helpful to become comfortable with adding and removing seam allowance.
Removing seam allowance / Drawing stitching lines
What if you want to alter a sewing pattern piece like Figure A in the image below? (I can't imagine a particularly complex alteration to a pocket piece like this, but let's keep the shape simple for the purposes of this post!) If this piece includes seam allowance, you'll need to trace off a stitching line inside the pattern edge, like the red dotted line in Figure C. The distance between the pattern edge and your new line should match the seam allowance for the pattern piece in question. You would then draft your alterations based on the new line, not the outer edge, and draw new cutting lines as needed once the adjusted stitching lines have been drafted.
Adding seam allowance / Drawing cutting lines
Say Figure A in the image above is a pocket piece you've just drafted. Once you've finalised the shape, you'll want to add seam allowances so it can be used to cut out your fabric. This is easily done by using a clear ruler or tape measure to trace a new line outside the black line, like the red line in Figure B.
Shaped hems and facings
When you're adding a hem allowance to a pattern piece with a shaped edge, such as the skirt shape in Figure 1 below, you'll want to ensure that the hem allowance you're drafting in is shaped to match the pattern piece in question.
- Draw your cutting lines around the edges other than the hem edge (Figure 2).
- Fold your pattern paper backwards, at the hem line (Figure 3 / 4).
- Cut along the side seams, with the hem allowance section still folded flat behind (Figure 5).
- Unfold the hem allowance and shorten it to the desired depth of hem allowance (Figure 6).
Use this technique of folding back the drafting paper whenever you're drafting a shaped hem allowance or facing, such as for skirt hems, sleeve hems, and blouse button plackets.
How wide should the seam allowance be?
If you have drafted a pattern from scratch, it's really up to you how wide a seam allowance you should add. Some examples are:
- 1/4" (6mm) or 3/8" (1cm): these narrower seam allowances are particularly useful around curves like the neckline and armscye. It is easier to sew accurate curves with a smaller seam allowance. You could also use a 3/8" (1cm) seam allowance for the main seams, if you are certain of the fit and your fabric is stable and tightly woven.
- 1/2"(12mm) or 5/8" (15mm): these are typical seam allowances for the main body seams such as the side seam or shoulder seams.
- 1" (25mm): If you are making a toile, or you want to leave fabric in the seam allowances in case of future alterations, you might choose to have a wide seam allowance in the side/shoulder seams so that you have more wiggle room if you need to let out those seams in future.
- 1.5" (4cm): For hem allowances you might use this allowance (or an even wider hem allowance), to give the garment a nice amount of weight of fabric to support them hem.