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.

### Layout

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. 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.

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.

To lay out the treads, he measures the available run along the walk line with a digital scale.

He divides the run into equal code-compliant increments, and snaps lines from the pivot point through those increments to the outer radius.

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 illustration, below).

• 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, 800/854-8075, www.calculated.com) to 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.