Every now and then I'm asked if I have any information on winding stairs. I've only built one, a project the owner finished himself. This text should probably be read after viewing the images below.
The half circle part of the winder was made around a wood half cylinder, basically some 3/4" plywood semi circles with 1x2s screwed to them. If I recall it was about eight feet long. I forget the run and the rise of the straight part of the stair, something like 8 in 12 or so, but the inside part of the winding 180 degree section was 8 in 8. The reason for the difference is that the place where you actually stepped on the treads was held to the same run and rise as you transitioned from the straight flight to the curved part. The area where you walked stayed Stayed 8 in 12 the whole length of the stair. I seem to recall the
treads being 36" long.
I started by drawing the 8 in 8 rise/run line on the form, which happened to be a 45 degree angle. I ripped an oak 1x12 into several 1/8th inch by 12 inch wide pieces, sort of like thick veneer. I next cut cheap 1/4" luan plywood into 12" wide strips. I cut the plywood
diagonally from the 4x8 sheets so it bent properly, it wanted to twist when I laid it on the form. I started the stringer lamination by first using a piece of the thin oak, then 8 pieces of the plywood, then the final oak. I ended up with a stringer a bit more than 2 inches thick with oak on two sides. I used 16" long strips of 1 by 2s to clamp the veneers to the form. I didn't clamp the sticks parallel to the form though, a bit more following the curve
of the form seemed to clamp the middle of the lamination better. (see picture) I had the joints staggered as far apart in the plywood as I could get them. I was laminating the straight section and the curved at the same time so the joint between the two could be made as one by alternating the laps. If you think about that joint a little you might see that the 8 in 12 straight section didn't come up at a 45 degree angle as did the curved so the joint was actually a dog leg.
After planing off the ragged edges of the stringer I veneered the edges in oak so the thing finally looked like a single piece of wood. The crippled section near the bottom of this staircase was built to follow a jog in the downstairs wall. You can also see where I'm getting started on the handrail.