A winder stair turns without a flat landing by using pie-shaped treads — narrow at the inside of the turn, wide at the outside. They save floor area, which is why they are common in loft conversions and tight stairwells, but their geometry is where inexperienced builders get caught.
Because each winder tread is a wedge, its depth changes with how far out you measure. Codes define a walking line at a fixed offset from the narrow side — 12.0 inches under the IRC — and require the tread depth along that line to be at least 10.0 inches. The depth at any radius equals that radius times the angle the tread sweeps, in radians.
The most common winder failure is a narrow end that pinches below the minimum — 6.0 inches under the IRC. A tight inner newel radius or too many treads crammed into the turn shrinks that narrow point. The fixes are to widen the inner radius, reduce the turn angle, or use fewer winder treads.
Start from the turn angle (often 90 degrees) and a tread count, then check both the narrow end and the walking line. If either falls short, adjust one variable at a time. The winder calculator shows all three depths live and flags each against your selected code.
When the narrow end of a pie tread falls below the 6-inch minimum, you have three levers. Increase the inner newel radius so the narrow edge starts further from the center. Reduce the turn angle each tread sweeps by using more winder treads across the turn (three 30-degree treads instead of two 45-degree ones). Or widen the stair so the whole wedge grows. Usually a combination of a fatter newel and more treads solves it.
A winder turns within the run, saving the floor area a landing would consume — the reason they appear in lofts and tight stairwells. The cost is geometry: pie treads are harder to climb safely (your foot meets a different depth depending on where you step) and harder to build and inspect than a simple landing. If you have the floor space, a landing-based quarter- or half-turn is the more forgiving choice; if you don't, winders are how you make the turn fit.
A 90-degree turn is most often made with three winder treads sweeping 30 degrees each, which keeps the walking-line depth reasonable. Two 45-degree treads save a step but sweep so much angle that the walking-line depth gets shallow and the narrow end pinches. More treads across the turn almost always gives a safer, more code-compliant winder.
Check narrow-end and walking-line depth automatically in the winder calculator.
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