On our current rough frame project in Orinda California, this custom residential house has a round tower with circular stairs. The inside and outside stair stringers are freestanding so you can walk under the stairs.
The first time I saw a set of freestanding stairs being built was in 1982, by Glen Watkins. This was on a tract of homes in the San Jose California area. Glen wouldn't let anyone in the house he was working on until he was done building the stairs. He didn't want anyone stealing his trade secrets.
The second time I saw a set of freestanding stairs being built was in 1992, by Tom, can't remember his last name, and no it's not Tom White, Glen's Watkins nephew. This was on high-end tract homes in the Black Hawk area in Danville California. The set he was working on was elliptical. I was more interested in how he laid out the ellipse than his method of building the freestanding stairs.
The third time I saw a set of set of freestanding stairs being built was about 5 years ago by Tim Uhler in an article he wrote for JCL magazine based on Will Holladay's method. Good article to get you going on building a set of freestanding stairs. Tim's stairs had a freestanding stair stringer on both sides of the stairs and he used 1/4" CDX or 1/4" ACX plywood for the laminated stair stringers. He said he had about 1/4" deflection with someone standing on the stairs. 1/4" deflection is acceptable, but I was concerned about the deflection I would have on the set of stairs I was building with 18 stair treads.
Billy Dillon told me to used George R. diCristina's dropped riser method on page 81 in his book "A Simplified Guide to Custom Stairbuilding and Tangent Handrailing". The dropped riser method acts as a cantilevered joist system and supports the inside stringer to minimize deflection in the stairs. Our dropped riser is 15 1/2" tall. George diCristina gives an example in his book using a 1" thick outside stringer attached to the wall and a 1 1/2" inside freestanding stringer supported by the dropped risers. I thought the 1 1/2" inside stair stringer was too narrow. However, when we were building our set of stairs my son Erik said we didn't need to add anymore 1/4" layers of oak plywood to the stair stringer after installing 6 layers, but he said the stairs was already extremely stiff with the 6 layers of oak plywood at 1 1/2" thick. So, George diCristina is probably correct about only needing a 1 1/2" thick inside stair string using the dropped riser method.
I used my StairTangent app to layout the circular stairs on the subfloor. The app makes it pretty easy and quick to layout your stair tread with the correct 10" width at the 12" line of travel.
For the outside stringer against the wall, I used a laser to mark off plumb lines on the stair stringer, from the lines you snapped on the subfloor for the stair tread location. Then from the top of the subfloor on the second-floor mark off the stair riser height at each of these plumb lines. These marks are the top of the stair string attached to the wall. Use 0.131 by 2 1/2' gun nails to attach the CDX 5 ply to the wall. About 4 nails per stud. On the last layer, 3 total, use 0.131 x 3 1/4" gun nails to attach the last layer of plywood to the wall Make sure you get lots of glue, TiteBond III, on each layer your installing, too much glue is the correct amount.
The StairTangent app also gives you the stair housing angle that you will use to cut the head and foot cuts of the laminated stair stringer.
I didn't take any pictures of this next step of laying out the stair stringer riser and treads on the stringer, but here's how I laid it out. First, lay out the riser height at the top of the stairs. My riser height was 7 1/2" and I'm using 1 1/8" treads, so I measured down 8 5/8" from the top of the second floor subfloor to the top of the first riser. I cut a stud using the dimension from the bottom of first floor subfloor to this line I just made. Then I cut the rest of the riser studs incrementing the dimension by the riser height of 7 1/2". Cut the studs all the way down to the last riser stud of 6 3/8". Use your riser height stud to mark off the riser height on the stair stringer attached to the wall. Place the riser stud next to the stringer and use a 16" level to mark off the bottom of the treads at the stringer. Cut the stringer with a SkilSaw and finish off the cut with a multi-Tool.
For the first tread and riser. Attach a dropped riser against the stair joist head out beam. Then attach the next dropped riser, 15 1/2" tall, on our stairs. We glued behind the location of the dropped riser using LockTite PL subfloor glue, then Erik attached it to the wall with about 3 gun nails. Then he added about 4 -- 3 1/8" screws to the side of the dropped riser. Then use your riser height stud for the other end of the dropped riser. Nail- screw it together after you check it for level and nail the bottom of the riser stud to the subfloor on your arc line. Which should be 3 1/2", 14 layers of 1/4", back from your original stair radius line. Repeat until your down to the last couple of stair treads and determine if you need to cut the dropped riser down.
For the stair tread, we laid a piece of plywood on the snapped arc lines on the subfloor and scribed the pattern for the tread. Both ends of the tread have a radius cut. We also used a laser on the arc line at each tread to measure the length of each riser, theoretically, they should all be the same length. We also used the laser to check the location of the end of the tread-riser location and adjusted the two as necessary.
Next, mark off level lines from the bottom of the outside stringer to the riser height studs for the location of the bottom of your inside laminated stringer.
Glue the back side of the first piece of 1/4" oak and use staples, 1 1/2", to attach it to the stairs.
We used a flush cut trim router bit on the first piece 1/4" oak plywood we installed. Then decided we could wait and flush cut the plywood when we had about six layers on. That didn't work that well. Four layers were also too much. 3 layers of the 1/4" oak plywood seemed to be the best way to flush cut the plywood.
Because this stringer was going through the window we stapled a piece of 1/4" oak plywood to the outside stringer at the top edge only and clamped the bottom edge until the glue dried. There's only a 1 1/2" gap, between the stair stringer and glass in the window.
We installed 3x4 blocks at the top of the riser on the inside stringer to screw the stair tread too and to screw the 1/4" laminated stringers too. On the outside stringer, we install 3x4's with 3 -- 6" SDS screws attached to the outside wall. Two 3 1/2" SDS screws through the dropped riser into the 3x4 block and three screws above the tread through the riser into the 3x4 block.
Even though we stapled the 1/4" oak plywood as we installed each layer, 14 in total, we installed 4 3/4" screws in the last layers into the riser height studs that we installed. We also install screws about 3" O.C. into the bottom of the stringer and some into the 3x4 block at the top of each riser.
I had Erik stand on the middle of the stairs and there was zero deflection. This set of stairs feels stiffer than a lot of the straight set of stairs I've built.