What are the tread depth rules for winder (pie-shaped) stairs?
Winder treads must be at least 10 in deep at the walkline and never less than 6 in at the narrow end (IRC R311.7.5.2.1).
Winder treads must be at least 10 inches (254 mm) deep measured along the walkline, which is located 12 inches in from the narrower side of the tread.
At no point within the required stair width may a winder tread be less than 6 inches (152 mm) deep. This keeps the inside of a turn from narrowing to an unsafe sliver of a step.
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