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.
I drew a full-size winder plan on 1/4-inch plywood.
I tacked the panel to an area of floor where I could work freely around all sides.
From one corner of the template, I drew radial riser lines at 22.5-degree increments.
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.
I then cut the rough treads with a Festool track saw.
I set each tread in PL-400 Premium adhesive …
… kneeling on the box to eliminate gaps and secured it with finish nails.
I routed the tops flush with the sides using a straight pattern-bit.
To maintain an accurate overlap, I tacked temporary stops 3/4 inch in from the back edge of my first winder tread.
A temporary stop helps with stacking boxes.
Plywood cleats tie the back of each riser to the back of the box below it.
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.
After closing in the exterior wall, each box was fastened to studwall. The curved wall was lathed …
… and plastered.
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.
The welded steel railing was mostly site-assembled, a story for another day.