Laying out with a framing square
A stringer is a board with a row of identical notches, and a framing square draws each one. Hold the square on the board so that the riser height on its narrow arm, the tongue, and the tread depth on its wide arm, the blade, both meet the edge. Clamp a stair gauge at each of the two figures and the square falls into the same place every time.
Worked example. A total rise of 108 in: 14 risers, 13 treads of 10 in.
- On the tongue: the rise, 7 11/16 in (7.714 in exactly)
- On the blade: the run, 10 in
- From one mark to the next along the edge: 12.63 in
- First riser, with treads 1 in thick: 6 11/16 in
- Throat on a 2 × 12: 5.14 in
- Board to buy: 16 ft
Stepping the square along the board adds a little error at each step, and thirteen little errors make a top riser that is off. The calculator gives every mark as a distance from the first one, worked out from exact figures and rounded last: mark them all with a tape hooked once, then draw each notch at its mark. The table under the drawing lists them.
Dropping the stringer
The notches are cut for the treads to sit on, and the treads have a thickness. Once they are on, the first step is too tall by that thickness and the last one too short, unless the bottom of the stringer is cut shorter by the same amount. That is the drop: the first riser is 6 11/16 in where the others are 7 11/16 in, and every step you climb is the same.
If the floor at the bottom will get tile or boards later, the drop changes: measure the rise to the finished floors.
Throat depth and stock size
The throat is what is left of the board under the deepest point of a notch. It is the part that carries the stairs. Each notch cuts into the board by the rise times the run divided by the distance between marks; here that leaves 5.14 in of a 2 × 12, which is 11.25 in wide, and 3.14 in of a 2 × 10 (9.25 in). Those widths are the real ones, from Archtoolbox’s table.
The calculator shows the throat and does not judge it. How much is enough depends on the span of the stringer, its wood and your code: ask your building department before you cut a deep notch into a narrow board.
How many stringers?
That is yours to type, and the board count follows it. The field opens empty: we found no published figure for how many stringers a stair of a given width takes, so we do not offer one. Stairs are carried by at least two, and how far apart they may stand depends on the treads: a maker of composite or of thin treads prints a maximum span, and a local code may set one.
Cutting tips
A circular saw’s blade is round, so a cut that stops at the corner on top has not reached it underneath. Stop each cut at the line and finish the corner with a handsaw: a cut that runs past the corner takes away throat.
Cut one stringer, check it against the stairs’ place, and trace the others from it. Crown up: a board with a bow carries best with the bow toward the treads.
Questions about stringers
How long should a stair stringer be?
As long as the line from the foot of the stairs to the floor above: the square root of the rise squared plus the run squared. For 108 in of rise and 130 in of run, 169 in, so a 16 ft board.
Stairs with a landing have no such line: each flight is carried by stringers of its own, from the floor to the landing and from the landing up. Enter the riser the landing comes after, and the calculator lays out both. With a landing after riser 7, each flight of the example takes 6 notches and a board of 8 ft.
2×10 or 2×12?
The notches are the same; the throat is not. On the example a 2 × 12 leaves 5.14 in and a 2 × 10 leaves 3.14 in.
How far apart should stringers be?
As far as your treads span and your code allows, and no further. Thick lumber treads span more than deck boards. The tread maker’s instructions give the figure.
How many notches does a stringer have?
One for each tread: 13 on the example, for 14 risers. Fixed flush with the floor above, the stringer carries one more.
Sources
- Stairbuilders and Manufacturers Association, Visual Interpretation of the International Residential Code, 2021: Stair Building Code. IRC 2021 as quoted there. R311.7.1: stairways not less than 36 in in clear width. R311.7.2: headroom not less than 6 ft 8 in, measured from the sloped line adjoining the tread nosings or from the floor of a landing. R311.7.5.1: riser height not more than 7 3/4 in, the greatest within a flight not more than 3/8 in over the smallest. R311.7.5.2: tread depth not less than 10 in. R311.7.5.3: a nosing of 3/4 to 1 1/4 in. R311.7.6: a landing as wide as the flight it serves and, where the stairway has a straight run, not less than 36 in in the direction of travel. R311.7.8.1: handrail height 34 to 38 in. Read on .
- Archtoolbox, Lumber Dimensions: Nominal vs Actual. Surfaced softwood lumber: a 2 x 10 is 1 1/2 x 9 1/4 in and a 2 x 12 is 1 1/2 x 11 1/4 in; a 4 x 4 is 3 1/2 x 3 1/2 in and a 6 x 6 is 5 1/2 x 5 1/2 in; a 1 x 4 is 3/4 x 3 1/2 in and a 1 x 6 is 3/4 x 5 1/2 in. Standard lengths run from 6 to 24 ft in steps of 2 ft. Read on .
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