Story Pole for Pony Walls
I next made a story pole for the risers, to reduce cutting errors. I took into account the difference in floor height across the first tread, which was only 1/8 inch out of level, then marked the framing elevation for every tread, after adjusting for the thickness of the finish flooring. I did the riser calculations several times to be sure; for me, moving too fast is a recipe for expensive mistakes.
I measured the studs for the individual pony walls directly from the story pole, and stacked and numbered them for quick assembly (10). I started installation from the top, with the tallest wall, nailing it to the floor and to the outer wall. I installed the support cleats for the backs of the treads as I went (11), constantly checking for level. I could have pre-attached the cleats, but I wasn't comfortable that they would come out as level as I wanted them to. And so it went - nail off a wall, position and attach the cleat, all the way to the bottom (12).
Once the walls were in place, I cut the treads from 3/4-inch subflooring (13), taking the dimensions from the original OSB pattern. I cut the ends square, knowing that the finish treads and plaster wall would smooth out the curve. We don't use temporary treads in this area, so these subtreads are well glued and stapled. I use plenty of adhesive and always take the time to push the tread into it, pull it back up to break the tension, then force it down again. This method creates an excellent bond with no squeaks. I also use high-quality, fresh adhesive, and let it sit in the sun to warm up beforehand. It has never made sense to me to cut corners at this stage, when callbacks for movement and squeaks are so expensive.
The next step was attaching 1/4-inch plywood to the inside wall (14). By using adhesive and staples and lots of pressure, we got the plywood to conform to the arc. I ran it long at the top, then clamped on a strip of 1/8-inch oak for scribing a fair curve about 4 inches above the tread nosings (15). Because the stair is elliptical, the slope of the curve changes from tread to tread, so I eyeballed the descending slope to get the best appearance as I cut and fine-tuned it with a belt sander.
Working to this line, I cut 2x6 studs, beveled at the top to match the slope, and glued and nailed them to the elliptical plate at about 6 inches on-center where the radius was tight and 8 inches on-center elsewhere (16). I figured the close spacing would make it easier for the plasterers to bend and nail the drywall. I blocked between these studs, using both glue and nails, and added solid blocking at top and bottom to provide anchor points for the metal railing (17).
Finally, the curved wall was topped with two layers of 1/4-inch OSB. I cut these wide and glued and screwed them down, then used a flush-cutting panel bit to trim them to the plywood wall on the inside of the curb. After sanding this cut smooth, I made a 5 1/2-inch scribe block from a scrap of wood and used it to mark a line on the stair's open side. I cut this line carefully with a jig saw, then finished up with the sander, again striving for a good visual flow (18). The plasterers will finish up and provide a smooth wall cap for the railing to attach to.
Mike Nathan is a builder in Hailey, Idaho.