Last September I began working on a 14-by-28-foot wing at a house in Orleans, Mass. The plan was to expand a small and awkward loft area into a full second floor and replace the existing, L-shaped stair with a double winder.
I don't build stairs as a specialty; over the course of 30 years, I've done maybe a couple of dozen. And until this project, I'd only built the kind of winders that come to a common point and aren't legal anymore. These days, a code-compliant winder can't measure less than 6 inches at its narrowest point. Depending on the available space, conceptually this can be kind of a head-scratcher.
Building designer Mark Farber drew the basic layout, fitting the stair between one wall and the loft's existing rim joist (see "Stair Plan," below). In addition to having the two-winder sections, the stair started and finished with common treads. The winders were also separated by two common treads at mid-flight, for a total of 12 treads and 13 risers.
Layout
When I built the stair, I followed Mark's plan. Only later did I really start to think about how to lay out a winder to maintain a 6-inch minimum on the narrow end of the tread. Mark told me he'd done it by first assuming that the number of risers needed to reach the second floor would be the same as for a straight stair; in this case, 13 would cover the total rise of 93 1/2 inches. He then drew a scaled plan, dividing the 90-degree turn into four 22.5-degree "treads," as shown in his stair plan. Using a compass, he struck an arc that crossed each of the six lines representing winder risers at a point that would ensure they met the 6-inch code minimum. For this four-tread/three-riser winder design, he found that a 31-inch-diameter circle did the trick. It also meets the requirement for a minimum tread depth of 10 inches at the "walk-line," which is measured 12 inches from the inner edge.
In a eureka moment, I realized that his method of laying out four treads at 22.5-degree intervals would serve for a stair of any width, and that there were only so many other ways to do it. The most common two would be a three-tread winder at 30-degree intervals or two treads at 45-degree intervals. But while the radius layout works for any width stairs, the actual width of a particular stairwell makes a difference in other ways: Each tread needs to be wide enough to meet code (36 inches), and because the stairs turn 90 degrees at both ends, the tread width affects the overall length of the stairwell. In this case, where the stairwell was constrained by existing walls on three sides, this overall length was critical and led to the next step in the layout process: figuring out how the turn radius affects the overall stairwell dimensions.
Lengthwise, the two common treads at each end take up 72 inches (36 + 36). Next I added 31 inches—the sum of each winder radius at the two ends (15 1/2 + 15 1/2)—for a subtotal of 103 inches (72 + 31 = 103). Subtracting that subtotal from the overall length of the stairwell (123 1/2 inches) leaves 20 1/2inches (123 1/2 – 103 = 20 1/2). Two 10 1/4-inch-deep common treads cover this distance perfectly. Widthwise, the stairwell was wide enough to accommodate a 10 1/4-inch common tread as the first step down from the top.
Modular Construction
Since there would be plenty of running up and down during demolition and construction, I decided to frame the rough staircase early in the game. 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.
To verify the plan dimensions, I began by marking out the stair's overall footprint directly on the floor. I checked it for square and then used a laser to plumb the corners up to the second-floor joists. I also confirmed there would be no need to adjust the total rise because of out-of-level framing. Next, I drew a full-size winder plan on 1/4-inch plywood. Mark's drawing indicated that each winder occupied a square 4 feet 3 1/2 inches on a side. I made a cut at 4 feet 4 inches (adding 1/2 inch to allow for drywall build-out on the wall) and squared off my template with a 4-inch-wide strip fastened to one of the long sides.
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. I used a site-made trammel stick to swing a 15 1/2-inch arc across the radial lines from their origin point. I checked to make sure this met the winder's 6-inch requirement by measuring the straight-line distance between radials at the arc's intersection. To verify the minimum allowable tread depth of no less than 10 inches at the walk-line, I swung another arc 12 inches beyond the first. Finally, to make the boxes stackable, I offset the back line of each tread on the template by 3/4 inch—the planned thickness of the adjacent tread's riser.
In choosing the material for my sub-treads, I considered the likelihood of weather exposure during construction. That ruled out plywood, with its tendency to swell and delaminate. I also wanted to make sure that the finished assembly would ultimately walk without the squeaks that typically result from shrinkage and warping. I decided to use AdvanTech flooring, which I've seen perform flawlessly in other subfloor applications. For the vertical sides of the boxes, I used 3/4-inch UL plywood because it has no significant voids in its laminations.
I ripped the UL into 7 3/16-inch-wide strips, the common riser height for this stair. As it turned out, Mark hadn't expected me to make actual radiused tread ends, and they could easily have been flat facets. But sticking to the plan and referencing the template, 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. Chopped to riser height, three pieces bonded together with yellow wood glue formed "curved" end pieces. I glued up the required eight ends in short order—low-tech masking tape provided sufficient clamping power—and let the glue dry overnight.
To cut and assemble the box pieces, I tack-nailed temporary stops along the front and rear offset lines on the winder template. The stops made it easy to accurately mark the cut lines in place without measuring, and provided a direct visual of the end-cut angles. They also held the boxes to a tight uniformity, which would be vital to final stair assembly. I used PL-Premium adhesive for all joints. To avoid splitting the plywood laminations, I used 2 1/4-inch 18-gauge finish nails and trusted the adhesive to provide a strong, permanent bond. 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 the AdvanTech and traced the outline with a Sharpie. I used the body of the pen to offset the line and allow for a small overhang, then cut the rough treads from the sheet. I set each tread in adhesive—kneeling on the box to eliminate gaps—and secured it with finish nails. It took one long day to make the eight winder boxes. The following day, with the adhesive dried, I routed the tops flush with the sides using a straight pattern-bit. I was ready to start stacking.
Stacking the Stair
For the starting tread, I'd already framed and leveled a regular, rectangular box in place, using cleats, screws, and adhesive to anchor it. I was working from a finished first-floor surface, so to account for the finish tread thickness, I reduced the riser height of this box by 3/4 inch. I also made it deep enough to provide full support for the next box up. To maintain an accurate overlap, I tacked temporary stops 3/4 inch in from the back edge of my first winder tread. I ran a bead of adhesive along this edge, then stacked the next box on top and aligned them end for end.
I assembled the boxes in place, clamping a temporary support leg to each one. The clamps allowed me to check and adjust each tread for level as I went along. Once I had all four boxes of the lower winder stacked, I added plywood cleats that tied the back of each riser to the back of the box below it. Again, I used plenty of glue and screws. With the lower winder section assembled, I called it a day and let the glue dry.
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.
I assembled the upper winder just like the lower one, and finished up with the final, common tread at the upper landing. With the stair completely assembled, I shimmed and screwed each individual box from below to the studs and blocking in the exterior wall.
On the stair's open side, I supported the treads on 2x3 studs, following the curves of the two winders. The back side of the curved wall section is finished with wire lath and plaster. 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.
I installed painted pine risers and, as a detail, let their ends project about 1/8inch beyond the finish wall plane. Using the same 1x8 southern yellow pine that was installed on the upstairs floor, I cut tapered lengths, three pieces per tread, and joined them with wood glue and biscuits. I'd considered using a taper-jig on the table saw to cut these pieces, but rejected that idea in favor of using my Festool track saw. This quickly produced accurate, glue-ready edges and also made short work of cutting the glued-up tread blanks to their scribed outlines.
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. I cut a curved profile into the ends using a router on a site-made plywood trammel base, which let me swing a suitable arc. Later, I used a round-over bit to create the nosing.
I installed the treads with PL-Premium, locking the back edge in under the elevated base of the finish risers. I face-nailed at the front of the treads and filled the heads with wood dough. The treads and upper floor are finished with three coats of a clear waterborne urethane.
The welded steel railing was mostly site-assembled, a story for another day. The stair itself walked like a rock-solid, monolithic unit the day the glue dried. In all, it took seven days to complete and cost around $5,700 without the railing.
