Winder Stairs Design and Dimensions

IRC R311.7.5.2.1 requires every winder tread to have a tread depth of at least 10 inches measured at the walkline, which is an arc 12 inches from the narrow end of the tread (IRC R202). The tread cannot be narrower than 6 inches at its narrowest point. The most common failure for winder stairs is the walkline measurement: builders measure at the center of the tread instead of 12 inches from the inside curve. Every winder in a set must meet both the 10-inch walkline depth and 6-inch minimum width. The walkline is defined as an arc concentric to the stairway's inside edge at a radius equal to half the stair width; for stairs narrower than 30 inches, the walkline is simply a line 12 inches from the inside edge.

A 90-degree turn uses either two winders (each turning 45 degrees) or three winders (each turning 30 degrees). Three-winder turns are more comfortable because the tread depth changes gradually; a 2-winder turn has a sharper transition and often fails the walkline test on the middle tread.

Three-Winder Layout

Divide the 90-degree turn into three equal 30-degree segments. Each winder's walkline depth must be at least 10 inches. The total rise through the turn section must be uniform with the rest of the flight, every riser within 3/8 inch of each other. Mark the stringer notches from the walkline measurement, not from the outer edge. Use the stair stringer layout marks from a calculator set to 'winder mode' to avoid cutting the wrong notch depth.

Two-Winder Layout

Each winder turns 45 degrees. The narrow end of the first and last winder will be below 6 inches if the stair width is less than 36 inches. The walkline depth must still hold at 10 inches. Two-winder turns are common in existing homes but rarely pass code for new construction. If you must use a 2-winder turn, test the walkline depth on a full-scale mockup before cutting stringers.

For a 3-winder turn in a 36-inch-wide stair, the inside radius is 12 inches. The walkline is at 24 inches from the center (12-inch radius + 12-inch offset). The tread depth at the walkline is the chord length of the 30-degree arc at that radius. A 30-degree arc on a 24-inch radius gives a chord length of approximately 12.4 inches, which meets the 10-inch minimum. At the narrow end (12-inch radius), the chord is 6.2 inches, above the 6-inch minimum. For a 30-inch-wide stair, the inside radius is smaller, and the narrow end drops below 6 inches unless you increase the stair width. Always check the winder tread layout both at the walkline and at the narrow end before cutting materials.

A landing provides a flat platform for turning; a winder uses tapered treads. IRC R311.7.5.2.1 allows winders, but a landing is required at the top and bottom of every stair flight. A winder cannot replace a required landing. Use winders when floor space is tight, they save roughly 3 feet of run compared to a landing turn.For L-shaped stairs with winders, the turn section must not have more than three winders.

When Winders Fail the Space Check

If the stair is under 30 inches wide, the narrow end of a winder will be less than or equal to 6 inches. You must widen the stair or use a landing.The smallest stair width for winders is 36 inches in practice, though the IRC only sets a 6-inch narrow-end minimum.

The calculator's winder mode automatically enforces the IRC R311.7.5.2.1 rules. Enter your total rise, total run, and stair width. Select 'winder' and choose 2-winder or 3-winder turn. The calculator outputs the tread depth at the walkline and at the narrow end for each winder, plus the riser height for the entire flight. Verify that every riser is within 3/8 inch of the others, the calculator enforces this, but double-check if you adjust any dimension manually. The winder mode also calculates the stringer notch layout for the turn section. Do not rely on the calculator for local code amendments: some jurisdictions require a 10-inch walkline depth on a 9-inch tread width (the IRC allows 10 inches on 10-inch treads). Check your local stair code before cutting.

Winder stairs suit a builder or homeowner who needs a 90-degree turn in a space where a landing would require a 36-inch-long platform they do not have. They work for basement stairs, attic conversions, and tight floor plans where every linear foot counts. They do not suit anyone building stairs for commercial occupancy (IBC 1011 applies, which is stricter), anyone whose stair width is under 30 inches, or anyone who cannot verify the walkline depth on a mockup before cutting. The thing that most often goes wrong: a builder assumes the center of the winder is the measurement point. It is not. Measure 12 inches from the narrow end, every time.