It’s easy to take stairs for granted. But the truth is that our safe, sure transit from one level to another depends on centuries of carpentry craftsmanship and building code regulations that govern all kinds of details—from the width of treads and the height of each step, to the distance between balusters and the installation of handrails.
There are many different types of stairs. But here we’ll be focusing on the details that pertain to wood-framed stairways–the interior stairways that connect levels, and the exterior stairways used on decks or installed against the outside of a house wall. Once you know your way around a wood-framed stair, you’ll be able to apply that same knowledge to other types, like precast concrete stairs, and outdoor stairs made from brick or landscape ties.
As shown in the drawing, stair construction always starts by measuring the total rise and total run of the stair–the vertical (rise) and horizontal (run) distances that the stair must span. The carpenter’s next task is to divide the total rise into individual risers that are no more than 7 ¾” high. Most carpenters start by dividing the total rise by 7” and then increasing or decreasing the riser height slightly to arrive at a final riser height (called unit rise). The building code does allow for a fudge factor: Riser height can vary by as much as 3/8”. Once your riser height is established, figure that you’ll have a 10” unit run for every tread on the stairway. Treads should be 11” wide, but there’s typically a 1” overhang (called a nosing) that extends beyond each riser.
The framing members that support a stairway are called stringers. A standard residential stairway usually has two outer stringers and a center stringer. While the center stringer will always be a “cut stringer” based on the unit rise and unit run of the stair, outer stringers can be cut or “housed.” A housed stringer has routed slots that allow treads and risers to be wedged into place. This type of stairway is typically fabricated by a millwork company that specializes in stairways.
Instead of building a stairway on site, a contractor can provide the stair company with rise and run dimensions, along with other details, and the company will simply deliver the stairway for installation. Wood-framed outdoor stairways are typically made from pressure-treated lumber. Sometimes builders leave the risers off an outdoor stairway to make it easier to clear snow and leaves from the treads.
Framing a stairway isn’t as difficult as it looks. Once you’ve calculated your unit rise and unit run, you can use a framing square to mark out the 2x12 boards from which most stringers are cut. With each stringer supported on sawhorses, make the angled cuts with your circular saw, stopping each cut when the blade reaches the 90° corner where rise meets run. You’ll have to complete these cutouts with a hand saw. Also remember to make a 1 ½” x 3 ½” notch at the base of each stringer, so that it can fit over a 2x4 thrust block fastened to the floor. Make sure to anchor each stringer securely to the upper floor framing. To do so you may need to install extra blocking, metal framing connectors, or both.
With the stringers in place, you’re set to install risers and treads. You’ll find different precut treads and risers at any building supply store. Oak treads are usually favored in upstairs living areas. Less-expensive pine treads are fine for basement stairs.
Trimming a Stair for Safety and Style
Once the stringers, treads and risers have been installed on a stairway, it’s time to complete the job with trim work. This means adding a balustrade that consists of a handrail, balusters, one or more newel posts, and (in some cases) skirtboards. A skirtboard serves a decorative function, running diagonally up the stairway to create a pleasing transition between the steps and the wall.
The baseboard molding does the same transitional work between the floor and the wall. But the balustrade is a safety feature, providing a handrail to grip while using the stairs, and individual balusters (spaced no more than 4” apart) that also prevent accidents.
Of course, these safety elements are also useful to express style and craftsmanship, with many variations possible.