Stair construction can intimidate even the most seasoned car­penters, and I'm often asked to give impromptu job-site les­sons in stair design and construction. I tell people that there are lots of ways to build stairs, but regardless of the method you choose, every set of stairs requires the same basic approach to get from one level to another safely, comfortably, and legally.

Stair math

The size and proportions of the staircase are determined by the design of the building and the openings in the floors. My first task is figur­ing out the details, and I always start with the building code, which gives me the parameters for the riser height, the tread depth, and the allowable ratio between them. Code also dictates other restrictions such as headroom, stair width, railing requirements, and landing dimensions.

Stair mathematics begins with one verified dimension: the total rise, or the distance from one finished floor to the next. I make sure to include the thickness of floor finishes in my calculations. Floors often are out of level, so I never take a simple vertical measurement from one floor to the next. Instead, I use a long level to measure the exact distance from the floor where the stairs start to the next floor where they end.

When I've figured the total floor- to-floor dimension, a simple set of calculations gives me the individual riser height. A construction calculator that works in feet and inches is great for calculating stairs, although you can do it longhand or with a regular calculator. For this stair, I started by dividing the total rise, 102 1/8 in., by 7 (approximate riser height), which gave me just over 14, which should be the number of risers. I then divided 102 1/8 in. by 14, for an individual riser height of 7 5/16 in.

After I have the number of risers and their exact dimension, I figure the run, or depth, of the treads with this formula: (2 x rise) + (1 x run) = 25 (plus or minus 1). For this stair, 25 minus 14 5/8 in. (2 x rise) gave me a tread depth of 11 1/8 in. I try to limit the run dimension to around 10 in. so that I can use standard 11 1/4. tread stock. In this case, I chose 11 1/8 in. for the run.

The number of treads is one fewer than the number of risers. So I multiply the number of treads (13) by the run dimension to get the  overall run of the stair (in this case, 131 5/8 in.). I take a quick measurement to make sure that the stair is long enough to reach the ends of the floor openings, but short enough to leave plenty of headroom. If I find a problem, I may need to go back and recalculate the stair with one more or one fewer riser until I get comfortable stairs that fit the space.

Engineered stringers don't shrink

A few years back, I read a Fine Homebuilding article by a carpenter who laminated strips of M-in. plywood for stringer stock. I thought it was a great idea. Then it occurred to me that an LVL (laminated-veneer lumber) stringer would be even better, and that's what I've been using ever since. Laminated-strand lumber (LSL) is fine, too, and I'm tempted to try pressure-treated parallel-strand lumber (PSL) for exterior stairs sometime. Meanwhile, there is nothing wrong with kiln-dried 2x12s for most applications, and in a pinch, I've even used regular "green" lumber.

Regardless of the stock I choose, I always check for crown. More often than not, even engineered lumber isn't perfectly straight.

I set the stringer stock on sawhorses and set up a framing square to mark the cuts. Instead of stair gauges, I use two sets of long-nose locking pliers, which provide a longer, more accurate bearing surface on the stock's edge. I mark one stringer with standard rise-and-run cuts from one end to the other; then I count risers and make modifications to the top and bottom ends of the stringer to allow for the size of the stair opening as well as for the thick­nesses of the flooring and treads.

Double-check before cutting

Because LVL stock is expensive and I may not have extra pieces, I check my work in several ways before I do much cutting. First, I mark the anticipated landing points of the stringer in the stair opening and measure the distance between them. Checking these dimensions on the stringer before cutting helps to prevent a mistake such as one too few steps and allows me to make adjustments.

When I've done the initial checks, I cut only the ends of the first stringer for a test fit. If the lower end of the stair lands on a framed floor instead of a slab, I notch the bottom edge of the stringer to wrap around the edge of  the framing. I try to make the horizontal part of the notch at least 4 in. long for proper bearing. The notch helps to keep the stringer in place during installation, and it provides a better point of attachment.

I test the stringer on both sides and check the fit and headroom of the ends. I also make sure that the tread cuts will be level. When I'm satisfied with the fit, I mark the studs along the bottom edge stringer for the 2x4 spacers. The spacers create a gap between the stringers and the wall studs for finish and trim, which makes the treads easier to fasten later.

Two cutting strategies

Next I cut the steps from my test stringer, which becomes a pattern for the remaining stringers. Finished interior stairs up to 3 ft. wide need only two stringers. Wider flights, porch stairs, or open-riser stairs may need three or more stringers. If the stringers are supported by walls over most of their length, I cut though the notches with a circular saw. If a stringer is unsupported, though, I finish the notch cuts with a handsaw, a jigsaw, or a recip­rocating saw for maximum strength.

While I have the pattern stringer on the horses, I trace its lower edge on the two 2x4 spacers and cut them to length. To install the stairs, I first nail the 2x4 spacers to the walls along the marks I made when I test-fit the stringers. Then I nail the outside stringers to the spacers. The center stringer goes in last, and a level ensures that the riser and tread cuts are aligned perfectly.

A short part of the lower section of this stair was open on one side, so I laid out a short string­er from my pattern, leaving the top end long. After the main stringers were nailed in, I set a 4-ft. level on the tread cuts, aligned the level with the tread line on the short stringer, and scribed the top edge against the wall framing.

With all the stringers secured, the final step is putting in temporary treads made of scrap 2x8. I make these treads about an inch shorter than the full width of the stringers so that they don't get in the way of drywall and trim installation.

I attach the treads with as few nails as possible, usually one nail per stringer location. On some projects, though, where people need to be comfortable on the stairs, I install bigger 2x10 treads. Sometimes I even add temporary 1/2-in. plywood risers to provide another level of strength, stability, and visual comfort.

I've been on plenty of jobs where everyone on site climbed ladders from floor to floor until the finished stairs showed up with the trim crew. Whenever possible, I try to build the stairs to each floor while the rest of the crew is still setting the joists. By the time the joists are ready for plywood, we all can walk up and down the stairs.