I'm not a math whiz or a full-time custom stair builder, but I
still like to tackle the occasional curved staircase. I enjoy
the challenge of figuring out how to build it, and I like the
distinctive look that custom stairs give my custom homes.
As with straight staircases, all you really need to know to
build curved stairs is the rise and run. But a curved stair has
two runs: The inside stringer has a tighter radius and thus a
shorter run, while the outside stringer has a larger radius and
a longer run.
To keep things simple, I lay out curved stairs using only the
inside radius. I also do a full-scale layout — right
where the stair will be built — with a story pole and a
trammel arm. I frame a pair of curved stud walls to support the
treads. These walls also serve as forms for both the curved
stringers and the curved handrails, which I glue up on site
with bending rail.
Before starting, I always check with the local building
inspector about specific stair requirements; I want to make
sure that the rise and tread depth meet local code. On most
curved stairs, tread depth (not including the part of the tread
that overhangs the riser) is measured 12 inches in from the
inside radius of the stair, along the walk line.
Rise. I begin the vertical layout on
a 2x4 story pole, starting from the top and working down. To
get the minimum number of treads needed, I divide the total
rise between floors by the greatest rise per step allowed by
code (remembering to account for finish-floor materials and
thicknesses). Because this usually results in a fractional
value, I round up to the next whole number to get the actual
number of risers. For example, a stair with a 96-inch rise and
maximum 7-inch riser height will require at least 14
67/8-inch-high risers and 13 treads.
Run. I draw the horizontal tread
layout in full scale right on the floor (see Figure 1). First,
I use a trammel arm to draw the inside tight radius for the
curved portion of the stair (the stair shown here also has a
short straight run at the top). The length of this radius
varies depending on how much room is available for the
stairway; in this case, I used an 84-inch radius. Using the
same pivot point, I then swing another arc with a 96-inch
radius, which is where the measurement for tread depth along
the walk line will be taken.
Figure 1. The author uses a trammel arm to
draw three arcs on the floor representing the stair's inner and
outer radii and the walk line (top left). To lay out the
treads, he measures the available run along the walk line with
a digital scale (top right), divides the run into equal
code-compliant increments, and snaps lines from the pivot point
through those increments to the outer radius (bottom left).
Another mark 1 inch from each snapped line represents the face
of the 2x4 pony walls that will be built to support the treads
Finally, I swing an arc with a 126-inch radius, to create a
42-inch-wide step. I let all the lines run a couple of feet
beyond the point where I think the stair will start (see
After drawing arcs for the inner and outer faces of the
stair, the author divides a third arc — the walk line
— into equal increments. Lines snapped from the pivot
point through these increments represent each tread.
Measuring the walk line. Next I use a digital
measuring device called a Scale Master (Calculated Industries,
measure the total length of available run along the
96-inch-radius walk line. I then divide this run by the total
number of treads.
To meet code in my area, this dimension — the tread depth
along the walk line — has to be greater than or equal to
8 inches. If it is, I mark this increment along the 96-inch
radius, beginning at the stair's starting point; otherwise, I
need to move the stair's starting point so that there's more
available total run.
Marking out the treads is a simple matter of hooking a chalk
line over the screw at the pivot point and snapping lines
through these incremental marks to the 126-inch outside radius.
Each line marks the face of a riser; the space between the
lines is the tread size without any overhangs. Using this
snapped layout for reference, I can now make a full-size tread
template, remembering to account for the 1 1/4-inch-thick
laminated stringers on each side as well as the overhang.
Building the Stringers
At this point the story pole and floor layout contain all the
information I need to build the stairs. First I cut and fit an
MDF skirtboard for the short straight stair section. Then I
frame a pair of stepped curved walls to support the stair
treads; these walls — which also provide a form for the
curved stair stringers — are glued up from three layers
of 5/16-inch-thick bending plywood and a finish layer of
1/4-inch birch plywood.
Skirtboard. This stairway follows a
straight wall at the top, and has three straight and three
pie-shaped treads that fit into the skirtboard. I use the story
pole and a level to lay out the tread locations directly on the
wall, and then I tack the skirtboard in place and mark it
(Figure 2). I dado the stair treads into the skirtboard with a
router and a 1 1/8-inch pattern bit. In addition to fitting
tightly into the dado, each tread is supported by 2x2 cleats
glued and screwed to the skirtboard.
Where the stair runs along the flat wall,
the author lays out an MDF skirtboard (top left), then uses a
router equipped with a pattern bit to cut dadoes for the treads
(top right). The 1 1/8-inch-diameter bit allows the tread to
slide in easily and almost perfectly matches the bullnose
profile cut on 1-inch-thick tread stock (above).
Framed walls. Taking the dimensions from the
story pole, I cut the studs and plates for each tread (Figure
3). I cut the top and bottom plates at a slight angle —
taken from the layout on the floor — so that the support
wall follows the stair's radius. I assemble the walls from the
bottom up, checking for plumb as I go and adding temporary
bracing as needed.
Figure 3. Along the curved layout lines,
the author frames a pair of stepped walls to help support the
treads (top); wall heights are taken from the story-pole
layout. Each curved stringer is laminated from three layers of
5/16-inch-thick bending plywood, trimmed in place with a router
equipped with a flush trim bit (top left). A final lamination
of 1/4-inch birch plywood gives the stringers a smooth,
paint-ready surface (top right).
The stringers are laminated from 16-inch-wide rips of
5/16-inch-thick bending plywood (the type that rolls up into a
4-foot-tall — not 8-foot-tall — cylinder). Even
though most of the stair's strength comes from the framed
walls, I glue and staple each of the three layers together and
stagger the joints by at least 2 feet.
A fourth layer of 1/4-inch birch plywood gives the stringers a
smooth surface that's ready for paint. When fastening it in
place, I keep the staples close to the bottom edge and near the
stair cutouts so they'll be covered by moldings.
I cut out the stair-tread openings with a flush trim router
bit, a process that takes only a few minutes but makes a real
mess. The last few inches at the top and bottom — where
the router base bumps up against a floor or wall — have
to be trimmed with a handsaw.
Bending the Rail
Bending rail looks like an ordinary handrail that's been sliced
up into several indexed laminations. When glued back together
and sanded smooth, the laminations usually disappear, and the
rail holds the shape it's been formed into.
On a straight stairway, I typically assemble the railing right
on top of the treads; if everything fits there, it will fit
when I lift the railing up into place. I glue up curved
railings the same way. To make sure the handrails match the
curve of the stairs, I form the bending rail right on the
stringers (Figure 4).
Figure 4. In preparation for glue-up, the
author lays out the handrail's centerline, notches the
temporary treads (to keep them from interfering with handrail
positioning), and installs adjustable clamps (top left). After
spreading yellow glue on the individual laminations (top
right), he binds the bending rail with packing tape and places
the assembly in the clamps, which can be quickly tightened with
an impact wrench (bottom).
Rail glue-up. Besides taking a lot of manpower,
gluing up a long handrail usually takes about all the clamps I
own; I use a combination of metal L brackets cut from angle
iron and special clamps from R&R Clamp (920/863-2987,
www.rrclamp.com), which I
like because they tighten quickly with an impact wrench.
Once we've spread the glue and wrapped the laminations together
with packing tape, we wrestle the rail into place. We start in
the middle — one person bending, the other two adjusting
the clamps — and work toward each end, using damp rags to
wipe off as much of the glue squeeze-out as possible.
Because a laminated rail will spring back slightly when the
clamps come off, I actually overbend it in the middle by about
1/2 inch. Bending rail tends to twist a little on each end, so
I tweak the ends a bit beyond square with pipe clamps; it's
hard to attach a handrail fitting to a twisted rail.
Glued-up rails need to stay clamped for at least 12 hours.
Before unclamping, I make some indexing marks so that later I
can put the rails in exactly the same place. It always takes a
little work with a sharp chisel, block plane, and sandpaper to
clean up the rails and prepare them for fittings.
Making the stair treads takes about as much time as building
the walls and stringers, and requires at least six heavy-duty
clamps and a 13-inch-wide planer.
I glue up 5/4-inch-thick stock for 1-inch-thick finished
treads, making the glue-up wide enough to cut two treads at
once and adding several inches of length to cover any snipe
from the planer (Figure 5). The exact size of the blank and
angle of cut comes from the tread template I made during the
layout stage, with enough added for the treads with mitered
Figure 5. The author glues up blanks from
5/4-inch stock, making them wide enough to cut two wedge-shaped
treads with a tapering jig and long enough to allow for planer
snipe. To make it easier to cut the mitered returns,
2-inch-wide nosings are fastened to the treads with pocket
screws and glue.
Since the stringers are curved, each end of the tread has a
slight angle — determined, again, by the full-scale
layout. I add a 2-inch-wide bullnose edging to each tread with
pocket screws and glue. The miter cuts for the bullnose returns
are slightly less than 90 degrees at the inside radius and
slightly more than 90 degrees at the outside radius. When
applying the edge nosings, I'm careful to locate the pocket
screws so that they will be out of the way of the dowel screws
for the balusters.
We use trim-head screws and lots of construction adhesive to
attach treads, and make shims out of poplar to level them.
Starting from the top and working from the inside, I use pocket
screws and yellow glue to join each riser to the tread above
it. This makes a really strong tread and eliminates another
Starting step. The radiused starting
step is wide enough to accommodate the starting newels and
their volutes and balusters (Figure 6). I use a 3/4-inch-thick
subtread on the starting step to provide a form for the curved
riser. A matching nailer on the floor serves as a form for the
bottom of the riser and also helps anchor the starting
Figure 6. At the first step, a radiused
subtread and matching nailer provide a form for the curved
riser. Here, the author uses the laser to mark the position of
the starting newel and help locate the center of the starting
To make sure that lengths and angles are right, I measure, cut,
and dry-fit the stair parts with the curved rails sitting on
top of the treads (Figure 7). I use two pitch blocks for the
angles — one for the inner radius and one for the outer
radius — and a laser to align the rails with the starting
Figure 7. Measuring and fitting components
is easier with the rail clamped firmly in place on the treads
(top); after the newels have been installed and the rail lifted
into position, a laser is handy for transferring the baluster
layout to the underside of the handrail
This stair has a pair of starting volutes; in order for them to
be level with one another at the bottom step, the rail height
along the inside radius — which has the steeper pitch
— needs to be about 4 inches lower than that along the
outside radius. To my eye, this solution looks better than
out-of-level volutes, and the rail heights still meet
Once I'm satisfied the assembled handrails are accurately
positioned on the treads, I lay out the balusters — two
per tread along the inside radius and three per tread along the
outside radius. After we've installed the newel posts and
raised the handrails into position (with a couple of temporary
supports to keep the centers of long rails at the right
height), I use the laser to transfer the baluster layout to the
bottom of the handrails.
We use an adjustable jig called a Bore Buster (L.J. Smith Stair
www.ljsmith.net) to hold
the drill at the correct angle when we bore the baluster holes
in the bottom of the handrail (Figure 8). To get a really
strong balustrade, we fasten the balusters to the treads with
special double-threaded dowel screws (also available from L.J.
Smith, along with a special driver bit for installing
Figure 8. The author bores the balustrade
holes with the help of a jig (top). A simple site-built box
that fits snugly over the balusters allows him to screw each
one tightly to the tread without damage (bottom).
Labor and Cost
The first time we built a curved stair, framing and finishing
it took two men two full weeks. With our second job, we were
about two days faster. Today, building a curved staircase takes
us about 40 hours longer than building a comparable straight
Two days are spent just gluing up and building the custom
treads (on a straight run of stairs, we use manufactured tread
stock); for this project, we used about 160 board feet of
5/4-inch-thick red oak lumber. We also used a lot of plywood,
including three sheets of bending plywood, two sheets of
1/4-inch birch plywood, and three sheets of 3/4-inch thick
birch plywood (for the risers).
At $14 per lineal foot and up, bending rail isn't cheap —
though it's not as expensive as it once was. Sometimes you can
find a manufacturer who supplies it in 8-foot lengths, but on
this project we used two 16-foot-long sections. We also went
through plenty of glue, including a gallon or two of Titebond
and eight to 10 tubes of construction adhesive.
Gary Striegleris a builder in