Curved stairs can add interest to a deck, and they sometimes solve design and layout issues. They can also be a profitable upsell, and they’re not that hard to build; after you’ve done a couple, the process becomes ingrained and goes quickly.
My method has four basic steps: layout, stringer assembly, tread and riser assembly, and finish work. None of the work requires specialized tools or jigs, and everything can be done on site. The key to my process is a set of temporary 2x4 stud walls that I use as a layout tool and as the form for laminating the curved plywood stringers.
Like any stairs, curved ones have to meet code. The main section in the 2012 IRC that covers curved stairs is R311.7.5.2.1, “Winder Treads,” but other sections also apply. In general, the requirements are the same as for straight stairs, with a few additions that address the different shape of the treads. On curved stairs, the minimum depth of the treads at their narrow end is 6 inches, and the minimum depth at the “walk line ” is 10 inches. The walk line is measured 12 inches in from the narrowest part of the treads and follows an arc that’s concentric to the one at the inner stair edge. Though the code doesn’t specify a maximum depth, keep in mind that extra-deep treads — more than 14 inches or so — can make stairs uncomfortable to climb. Your local code may have different requirements, so be sure to check.
Building the Bending Frame
I calculate the number of risers and treads as I would with straight stairs. General layout includes locating where the stairs will leave the deck, picking a curve for the stairs, and targeting an area for the bottom landing. To rough out the stair’s arc, I walk heel-toe-heel-toe in the imaginary center of the stairs, starting at the deck and curving in the direction of the planned landing. This establishes the approximate radius at the center of the flight and locates the bottom riser within a foot or two.
Bending-frame base. Once I’ve determined the stair’s basic footprint, I lay out and assemble the frame for bending the stringers. As a base, I place two layers of plywood over an area about a foot wider than the stairs will cover; the plywood serves both as a template and as the bottom plate for the frame walls. I overlap the joints by at least 6 inches and screw the layers together at 2-foot to 3-foot intervals.
Template layout. Next, I mark arcs on the plywood at the radii of the inside and outside stringers and walk line. To find the center of the arcs — a point equidistant from the top and bottom of the stairs — I first plumb down from the middle of where the stairs will land on the deck, and mark the plywood base below. Next, I mark the plywood at the middle of the planned bottom-riser location. Then I pull tape measures from these two marks, keeping the tape from the first mark parallel with the deck edge and sliding the other tape along it. When the measurements match up, I’ve found the center of the stair arc.
I drive a stake at that point, hook my tape measure on it, and swing three arcs on the plywood, from the deck edge to the bottom landing area. The first arc defines the inside of the curve, the second arc — 12 inches out from the first — marks the walk line, and the third defines the outside edge of the stairs.
I step off the treads at the walk line with equally spaced tick-marks 10 inches (code minimum) or more apart. I generally limit tread depth at the walk line to 12 inches for comfort. The final tread/riser mark pinpoints the bottom landing. I pull a string from the radius stake to the outer stair line at the last tread/riser to make sure I like how the angle of the bottom riser relates to my plan for a bottom landing.
If the last tread doesn’t land where I expected it to, I make adjustments. There are a few different ways I can do this. I can increase or decrease the riser height to change the number of treads. I can change the tread measurement at the walk line to extend or contract the bottom tread. Or I can change the radius of the stairs to make the arcs tighter or broader. All are easy changes to make before the actual building begins.
I also check that the minimum tread depth is at least 6 inches by snapping two chalk lines from the radius stake through two successive tread marks at the walk-line arc.
Once I’m happy with the geometry of the stair, I pull a chalk line from the radius stake all the way across the outer stair-edge arc and snap lines on the plywood at each tread mark along the walk line to define the stair treads.
Plywood top. For the top of the bending frame, I make a second plywood assembly that mirrors the base. I transfer the complete tread layout (though there’s no need to mark the walk line), and trim the edges 31/2 inches wider than the inner and outer arcs to allow for attaching the 2x4 studs.
Walls. I build the bending frame at least 5 feet high — even if the stairs are shorter than that — so I can work below the top of the assembly comfortably. It’s important to use straight studs without any wane, for accurate stringer layout. I toe-screw the studs to the plywood base along the inner and outer arcs, aligning them with the deck side of each riser line. I place the upper plywood assembly on top of the studs and fasten it like a giant top plate, then plumb the corners of the frame and brace it.
If for any reason you need intermediate stringers, you’ll have to build form walls in the middle. These walls would need to be disassembled as the risers and treads are installed, as will become clear later on. On my projects, though, two stringers are usually adequate, because I build the risers to act as beams that support structural treads, which in turn support the finish material.