How to True Dart Roofs Correctly: Convex vs Concave Math
Why your fabric develops a missing notch or puckers when folding darts, and how algorithmic parametric reflection eliminates this error completely.
The Problem with Straight Seam Allowances on Darts
When drafting a pattern by hand or in basic vector drawing software, sewists frequently draw a straight cutline connecting the two ends of a seam across a dart opening.
However, when the dart legs are brought together and pressed (typically toward Center Front on waist darts, or downwards toward the waist on side bust darts), the fabric folds underneath. If the seam allowance was drawn straight across the open dart, the folded fabric edge falls short of the seamline, leaving an unsightly triangular notch or indentation on your cut garment edge.
The seam allowance peak is mirror-reflected across the dart folding axis.
The Mathematical Solution: Folding Reflection
In classical tailor drafting, the cutter temporarily folds the paper pattern closed along the dart lines and traces across the seam with a spiked tracing wheel. When the dart is reopened, a distinct pointed "roof" (or crest) is revealed.
In the sloper.fit CAD engine, this physical tracing process is solved mathematically using vector transformation matrices:
- Identify the active fold leg (e.g. upper bust dart leg vs lower bust dart leg).
- Rotate the adjoining seam segment around the dart apex by the exact dart intake angle θ.
- Intersect the rotated seamline with the dart centerline to locate the exact crest coordinates.
- Construct the outer boundary polygon connecting the seam edges to the peak.
Try It Live on Free Bodice Slopers
Inspect the trued dart roofs on our free Winifred Aldrich Bodice Sloper blocks (graded from UK 6 to UK 18).