It didn’t take me long to rule out cut stringers; I can still barely imagine what they’d look like, let alone what I’d cut them from. Instead, I decided to make uniform tread “boxes” and stack them in succession.
With a little trial-and-error I ripped 2-by scrap lumber into roughly 2-inch trapezoids with a 3 1/2-degree bevel on two opposing edges.
I glued up the required eight ends in short order—low-tech masking tape provided sufficient clamping power.
To cut and assemble the box pieces, I tack-nailed temporary stops along the front and rear offset lines on the winder template.
With the four basic sides of a box completed, I then added intermediate bracing at 18-inch intervals to ensure a stiff tread surface.
Next, I applied the sub-treads. I flipped each completed box upside-down on to 3/4-inch AdvanTech. I traced the tread's outline with a Sharpie, using the pen's body to create an offset.
To maintain an accurate overlap, I tacked temporary stops 3/4 inch in from the back edge of my first winder tread.
After installing the starter tread—a regular, rectangular box which I leveled—I stacked the four boxes of the lower winder. The two common treads at mid-flight were conventional cut stringers, notched to bear on the winder below. I extended the top tread to support the winder tread above.
Lastly, the four boxes of the upper winder were installed; each box supported by temporary support legs on the wall side and 2x3 studs on the stair's open side.
The exterior wall was removed to accommodate four, two-story high engineered posts needed for bracing around a new, stepped window array. The full-height second floor was added later.
I finished the front side of this wall with vertical 1x3 T&G fir, which knuckled around the curves as if custom-made for the job.
In the area under the stairs, we added lighting and a built-in desktop to create a small “found space” office.
To make thicker 1-inch tread nosing, I biscuit-joined 5/4 stock to the front edge of each tread and added breadboard nosing to the ends.