Winder Tread Layout Math: The 6-Inch Walk-Line Rule

A winder is the wedge-shaped tread that turns a stair through 90 or 180 degrees where a landing would normally sit. Used well, winders save real floor space in tight L-shaped and U-shaped stairs. Used badly, they create treads too narrow at the inside edge, and an inspector will fail the stair. The code controls winders with just three numbers, and every one of them is a measurement problem: where each measurement is taken, and the layout math that satisfies all three.

What the Code Actually Requires

The residential rule for winders is IRC R311.7.5.2.1, and it sets three checks:

1. Six inches at the narrow end. Each winder must be at least 6 inches deep at its narrowest point.
2. Ten inches at the walk line. Each winder must also be at least 10 inches deep at the walk line, measured a horizontal distance of 12 inches from the side where the treads are narrowest (the inside of the turn).
3. Uniformity at the walk line. Within any flight, the greatest winder depth at the 12-inch walk line cannot exceed the smallest by more than 3/8 inch. The 3/8-inch riser uniformity rule in R311.7.5.1 applies to winders too.

There is no code-mandated number of winders, no mandated angle per winder, and no minimum stair width. All of those come out of the layout math once you respect the 6-inch and 10-inch minimums. If your stair's geometry is not settled yet, run it through our stair calculator first, so the unit run and riser height of the main flights are fixed before you lay out the turn.

Why the Walk Line Sits 12 Inches From the Narrow Edge

On a winder the tread is a wedge, so usable depth changes across the width, and nobody steps at the inside corner where the tread may be only 6 inches deep. The code picks one representative spot to measure: 12 inches in from the narrow side. That is the walk line, where tread depth must reach the 10-inch minimum that also applies to straight treads in R311.7.5.2.

Note that the job-site phrase "6-inch walk-line rule" is slightly off. The 6 inches is not measured at the walk line. The 6 inches is the absolute minimum at the narrow end of the wedge, and the 10 inches is the minimum at the walk line. Mixing those two up is a common reason winder layouts fail inspection.

Laying Out a 90-Degree Winder Turn, Step by Step

Take a standard layout: a 36-inch-wide stair making a 90-degree turn with three winders, so each winder fans out 30 degrees.

Step 1. Fix the riser and unit run of the whole flight. Winders are part of the flight, so their risers follow the same rules: maximum 7-3/4 inches, and no two risers in the flight may differ by more than 3/8 inch. Lock the unit run before touching the turn; use a stair tread calculator when sizing nosings and boards.

Step 2. Do not fan the winders from a sharp point. Draw three winders meeting at one inside corner and check it: at 30 degrees each, the walk line at 12 inches radius gives each winder an arc of (30/360) x 2 x pi x 12, about 6.3 inches of depth. That fails the 10-inch minimum, so a sharp point corner is mathematically impossible on a 90-degree turn.

Step 3. Size the newel square from the 6-inch rule, then check the walk line. Instead of a point, the inside corner becomes a small square newel, and each winder is a circle segment centered on it. With each winder spanning 30 degrees, the inner-edge chord is 2 x R x sin(15 degrees), where R is the radius from the fan center. Setting that chord to 6 inches gives R = 6 / (2 x 0.2588), about 11.6 inches, so the newel square needs roughly a 12-inch radius. The walk line radius is then R + 12 = about 23.6 inches, giving each winder a walk-line depth of 2 x 23.6 x sin(15 degrees) = about 12.2 inches, which clears the 10-inch minimum with room to spare.

Step 4. Draw the turn full scale and confirm uniformity. With the fan center fixed, draw three 30-degree sectors, extend the division lines to the full stair width, and transfer the drawing to your tread stock. Because all three winders are identical sectors, the 3/8-inch uniformity rule passes automatically; if you mix winder angles, measure each at the walk line and keep the largest and smallest within 3/8 inch of each other.

A Worked Number Example

Stair width: 36 inches. Turn: 90 degrees with 3 winders, 30 degrees each. Unit run on the straight flights: 10 inches.

• Inner corner radius R from the 6-inch rule: 6 / (2 x sin 15 deg) = 11.6 inches.
• Walk line radius: 11.6 + 12 = 23.6 inches.
• Winder depth at the walk line: 2 x 23.6 x sin 15 deg = 12.2 inches. Required: 10 inches. Pass.
• Inner depth: 6 inches. Required: 6 inches. Pass.

Try the same math on a 30-inch-wide stair: R is still 11.6 inches, so the walk line depth is still 12.2 inches. It passes the code, but only 6.4 inches of stair width (30 minus 23.6) lies outside the walk line, so the walking path feels cramped. This is the real reason most carpenters keep winder stairs at least 36 inches wide even though the code never says so.

Common Layout Mistakes That Fail Inspection

Measuring depth at the wrong line. Inspectors measure at the 12-inch walk line, not at the outer edge where the tread looks generous. Check your numbers at the walk line from the start.

Shrinking the inner corner to save space. Every inch stolen from the newel square comes off the inner depth. Below 6 inches at any point, the stair fails, and no site adjustment fixes a layout that was wrong on paper.

Mixing winder sizes. Tapering winders so the turn "looks" smoother often breaks the 3/8-inch uniformity check. Identical sectors are easier to cut and automatically compliant.

Forgetting the riser rules. Winders inherit the flight's riser requirements. A winder riser 1/2 inch taller than its neighbors fails even when every tread depth is perfect. See our guide on laying out and cutting a stair stringer for the framing-square technique that keeps risers consistent.

Skipping the full-scale drawing. Angle errors compound across three wedges, so draw the turn full scale on plywood or paper before cutting a tread.

Frequently Asked Questions

Can I use two winders for a 90-degree turn?

Yes, if the math works. Two winders mean 45 degrees each: 6 / (2 x sin 22.5 deg) gives about 7.8 inches of inner radius, and the walk line depth comes out to about 14.6 inches. Many builders prefer two winders because each is wider and easier to cut.

Do winders need the same riser height as the rest of the stair?

Yes. Winders are part of the flight, so every riser, winder or straight, must be within 3/8 inch of every other riser, with a 7-3/4 inch maximum. Set the riser count from the total rise first, then divide the turn's rise evenly among the winders.

What is the minimum stair width for a winder turn?

The IRC sets no minimum width for the stair itself, only the 6-inch and 10-inch depth minimums. Practically, 36 inches is the sensible minimum, because the walk line sits at roughly 23 to 24 inches radius on a typical layout and you need walking width outside it.

Do winder treads need nosings?

The nosing rules apply to the whole flight: a 3/4 to 1-1/4 inch projection is required where tread depth is less than 11 inches, and nosings in the flight cannot vary by more than 3/8 inch. Apply the nosing at the walk line and keep it consistent across every tread.

Can winders replace a landing in an existing stair?

Structurally, yes, and that is their main appeal: three winders occupy roughly the same footprint as the turn itself, while an L-shaped landing needs a full square of stair width. But a landing is easier to build and more comfortable to walk. Choose winders when the floor plan truly has no room for a landing.

How do winders differ from spiral stair treads?

Both are wedge-shaped, but spiral stairs have their own code section (R311.7.10.1): 26-inch minimum clear width, 7-1/2 inch minimum tread at a walk line 12 inches from the narrow side, and a 9-1/2 inch maximum riser. Do not mix the two rule sets.

A winder turn is one of the few places where a pencil, a protractor, and five minutes of trigonometry replace hours of on-site guesswork. Respect the 6 inches at the narrow end, verify the 10 inches at the walk line, keep every winder within 3/8 inch of its neighbors, and draw the turn full scale before you cut.