Curved stairs add interest to a deck, solve design issues, differentiate your work, and wow clients. They can also be a profitable upsell, and they’re not that hard to build; after you’ve done one or two, the process becomes ingrained and goes quickly.
My method of building curved stairs breaks down into four basic steps: layout, stringer assembly, tread and riser assembly, and finish work. None require specialized tools or jigs and all can be done on site. Key to my process is a set of temporary curved stud walls that I build to use as a layout tool, a form for laminating plywood for the curved stringers, and an assembly template.
If you’re working on a sloping grade, the rough staircase can be built elsewhere on the site or even inside, using the same process, and then carried into position.
Like any other stairs, curved ones have to meet code. The main section in the 2012 IRC that covers curved stairs is R3188.8.131.52.1, 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 “walkline” is 10 inches. The walkline 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.
Lay Out and Build a 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 and the location of the bottom riser within a foot or two.
Bending-frame base. Next, I lay out and assemble the frame for bending the stringers. For the base, which will serve as a template and as a bottom plate for the frame walls, I lay two layers of plywood sheets over an area about a foot wider than the stairs will cover. The joints should overlap at least 6 inches and be screwed together at 2- to 3-foot intervals.
Template layout. Mark arcs on the plywood at the radii of the inside and outside stringers and walkline (Figure 1). To find the center of the arcs — a point equidistant from the top and bottom of the stairs — first plumb down from the middle of where the stairs will land on the deck, and mark the plywood base below. Next, mark the plywood at the middle of the planned bottom-riser location. Then 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, you’ve found the center of the stair arc.
Figure 1. Lay out the inner and outer arcs of the stair on a temporary floor made from a double layer of plywood.
Drive a stake at that point. Hook a tape measure to 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 walkline, and the third defines the outside edge of the stairs.
Step off the treads at the walkline with equally spaced tick-marks 10 inches (code minimum) or more apart (Figure 2). I generally limit tread depth at the walkline to 12 inches for comfort. The final tread/riser mark pinpoints the bottom landing. Pull a string from the radius stake to the outer stair line at the last tread/riser to see if you like how the angle of the bottom riser relates to your plans for a bottom landing.
Figure 2. After laying out the inner and outer radii and the radius of the walkline, mark the tread locations on the latter at 10-inch-minimum intervals.
If the last tread doesn’t land where you expected it to, you can make adjustments by increasing or decreasing the riser height to change the number of treads; by changing the tread measurement at the walkline to extend or contract the bottom tread; or by changing the radius of the stairs to make the arcs tighter or broader. These are all easy changes to make before you start building.
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 walkline arc. The distance between the points where the chalk lines cross the inner arc is the dimension of the narrow end of the treads (Figure 3).
Figure 3. Snap chalk lines between the center point and two adjacent walkline tread marks to outline a tread. Verify that the minimum tread width is at least 6 inches.
Once you’re happy with the geometry of the stair, 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 walkline to define the stair treads (Figure 4).
Figure 4. Outline every tread on the stair by snapping lines between the center and the walkline marks.
Plywood top. For the top of the bending frame, make a second plywood assembly that mirrors the base — complete with the stair layout, though there’s no need to mark the walkline arc. Trim the edges 3 1/2 inches wide of the inner and outer arcs of the stair, to allow for attaching 2x4 stud walls (Figure 5).
Figure 5. Make a two-layer plywood mirror of the stair layout, with its edges cut 3 1/2 inches wide of the inner and outer radii of the stair.
Walls. I build the bending-frame walls 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. Toe-screw the studs to the plywood base along the inner and outer arcs, aligning them with the deck side of each tread/riser line (Figure 6). Place and fasten the upper plywood assembly atop the studs, with the studs aligned exactly as they are on the base (Figure 7). Then plumb up the corners of the frame and brace it (Figure 8).
Figure 6. Toe-screw straight, wane-free studs to the base at the riser locations. Each stud should be behind its riser line.
Figure 7. Place the upper template, or layout mirror, on top of the studs and fasten the studs to it the same way they were fastened to the lower layout.
Figure 8. Once all the studs are fastened, brace the frame plumb.
If you need or want multiple stringers, you can build intermediate walls to form middle stringers. These walls would need to be disassembled as the risers and treads are installed, as will become clear later on. Usually, though, two stringers are adequate, because I build the risers to act as beams that support structural treads, which in turn support the finish material.
Laminate, Mark, and Cut the Stringers
The stable frame makes it easy to lay up the stringer layers, and its studs double as layout guides for the risers. I generally rip 16-inch-wide strips of 1/2-inch pressure-treated plywood for the stringer layers and cut enough for four plies with joints offset by 2 feet. Sixteen inches may seem excessive, but it’s necessary since the tread cuts are pretty deep at the outer stringer.
The face of each stud represents a riser position. To establish the line representing the top of the stringer, you’ll need to mark out each tread level on the corresponding stud. Because you’ll be working outside on ground that’s probably not level, establish a level reference line on the inner and outer frame walls using a laser level.
Mark out a story pole for the riser heights and use it to mark the tread elevations on the face of each stud (Figure 9). Make sure to allow for the rough and finish tread thicknesses. I figure 1 1/2 inches for a structural 2-by tread, 1/2 inch for spacer strips, and 1 inch or 1 1/4 inches for the finish tread material, which is usually decking.
Figure 9. Use a story pole with the tread elevations marked on it to lay out the risers and treads on the frame.
The upper and lower plywood pieces will need to be trimmed before assembly to match the deck frame and the landing surface. I make cardboard templates of the angles where the stringers will meet the top and bottom landings to use as a guide. Align the top edge of the first layer of stringer plywood to the tread marks on the studs and tack-screw it in place. If needed, use clamps to draw and hold the layer tight to the studs while the screws are driven. Since the tack screws will have to come out later, drive them in at an angle to the studs through the top and bottom edges.
Apply a generous squiggle of exterior-grade construction adhesive to the first layer, then screw on the second layer with 1-inch-long galvanized or stainless screws. Offset joints in laminations by at least 2 feet. Glue and screw the third and fourth layers on using successively longer screws driven from both sides. Install screws for the first, second, and third layers about 8 inches apart along the top and bottom edges (Figure 10). After the fourth layer is installed, drive long screws from both outside faces of each stringer in a 4-inch to 6-inch grid pattern. Tip: To make it less likely you’ll cut through screws when making the riser cuts, don’t drive any screws within an inch of the face of the stud.
Figure 10. Align the stringer laminations with the tread marks on the frame, then screw and glue them together.
To lay out the riser and tread cuts on the stringers, hold a straightedge against the faces of matching studs on the inside and outside frame walls and mark across the tops of the stringers. The point where the line crosses the edge of the stringer is the outside corner of the riser and tread cut lines. Draw a plumb line down from the mark for the riser cut and a level line for the tread cut, only as far as the intersection with the tread or riser lines for the adjacent steps below and above (Figure 11).
Figure 11. Mark the risers on the stringer by plumbing down.
It’s best to wait a day or two for the adhesive to cure before cutting the stringers. Set the bevel on a circular saw to match the slight angle the risers will make where they’ll meet the stringers. The tread cuts will be flat — no bevel. So you don’t overcut at the inside corner, use a reciprocating or hand saw to finish out the cut (Figure 12).
Figure 12. Notch the stringers with a circular saw, but don’t overcut the corners. Finish the cuts with a reciprocating saw.
Structural Treads and Risers Hold the Assembly Together
Structural treads of pressure-treated 2-by combine with structural risers made from two layers of pressure-treated plywood to tie the stringers together and provide a solid base for finishes. Make a tread pattern from cardboard or thin plywood using the arc lines and tread chalk lines on the frame base as a guide (Figure 13).
Figure 13. Make a template for the structural treads from cardboard or thin plywood.
The wide end of the treads will usually be wider than a 2x12, so it will need to be laminated. I cut blanks an inch or two longer than needed and rip the taper. Then I glue and pocket-screw the fall-off from the taper cut to the blank. A final rip trims the excess off the filler. The tread ends are slightly curved but can usually be cut with a circular saw.
The risers stabilize the stringers and support the treads, so use a minimum of 1/2-inch plywood. Rip the riser stock the same width as the riser height. The length equals the outside dimension of the stairs, but rather than accounting for the slight bevel, I cut the risers square and 1/8 inch short. The width of the bottom and top riser boards will be different from the rest.
The inner riser layer is attached to the stringers first, using screws and construction adhesive. Then the tread below is installed, and the inner riser is screwed to its back. The inner risers also bear the front edge of the treads above (Figure 14). Run a bead of construction adhesive along the top of the riser before installing the tread. Use screws and construction adhesive to secure the tread to the stringers. The second riser layer laps the front of the upper tread, and is screwed and glued to it. This helps to keep the treads from warping (Figure 15).
Figure 14. Laminate the risers from two layers of plywood to create a notch that will support the front of the structural tread.
Figure 15. Offsetting the riser layers allows them to support both the front and rear of the treads. In this configuration, the risers act like beams to transfer the loads from the treads to the inner and outer stringers, eliminating the need for middle stringers on most stairs.
Eventually the top of the bending frame will interfere with tread installation. At that point, just install the risers; wait until the frame is dismantled to install the treads.
Install the Finishes
Wait a day or two to let the riser and tread adhesive cure before removing the frame. The rough staircase emerges from the frame ready for intermediate support posts and the finishes — skirts, risers, treads, and railings — though sometimes you’ll find a screw tip poking through a stringer face that needs to be filed or ground off flush. On large staircases, the outside stringer needs to be supported before the studs are removed, which you can do by cutting and fastening a couple of 2x4s and setting them on blocks until permanent posts and footings are placed (Figure 16). Finally, install the top treads and risers (Figure 17).
Figure 16. Support the stringers with posts placed on footings.
Figure 17. Once the frame is removed and the stringers supported, finish installing the last treads and risers.
Curved stair stringers need intermediate supports, which I extend upward to double as guardrail posts. I space posts along the arcs at equal intervals — between 4 feet and 6 feet apart — and set them on footings (Figure 18).
Figure 18. Intermediate guardrail posts can simply be extensions of support posts.
It usually takes as long to apply the finishes as it does to build the curved stair frame — and may take longer, depending on the details. Skirt boards can be mitered into the risers or closed. Closed skirt boards require less work and look better over time. Clamp, glue, and screw synthetic or rot-resistant-wood skirts alongside the stringers, shimming where needed to maintain a fair curve (Figure 19).
Figure 19. Use fascia material for the finished skirt boards, and decking for the treads.
Water that gets beneath the treads needs a way to drain, so bore a few 3/4-inch holes through the structural treads. Lay 1/2-inch or thicker wood strips over the rough treads as spacers for the finish tread decking. I rip the finish decking strips on a taper and ease the edges. This looks much better than having a couple of straight decking strips and a wedge-shaped closing piece meeting the riser at the back of the tread. This approach won’t work with capped decking, however, because easing the edges of the tapered pieces would expose the capped decking’s substrate. Space the finish riser boards 1/4 inch from the structural risers for drainage.
Bending and laminating curved rails is an article in itself. Done correctly, a curved rail needs the same helical shape as the stringers. One workaround is to cut rails out of wide 2-by stock to match the curve of the stair, but this only works if the intermediate posts are less than 4 feet apart. Rails done this way will have a slightly odd slope as the rail turns.
Mike Guertin is a builder in East Greenwich, R.I., and a regular presenter at DeckExpo.