Guides/Laying out stringers
From a total rise to a cut stringer — and the one step, dropping the stringer, that decides whether your first and last risers come out equal.
Nearly every stair problem is the same problem: one riser that is not the same as the others. People climb by rhythm rather than by looking, so a step that is half an inch out is what catches a toe. Getting the layout right is mostly about refusing to absorb a leftover measurement into the top or bottom step.
Finished floor to finished floor — not subfloor. If the upper floor is getting ¾″ hardwood and the lower is getting tile on mortar, both belong in the number now. A stair laid out on subfloor heights and then floored over is a stair with two wrong risers at the ends.
Measure plumb, at more than one point if the floors are not dead level, and use the largest figure.
Divide the total rise by your maximum riser height and round up. That gives the riser count. Then divide the total rise by that count for the actual riser height — equal by construction.
Rounding up is the whole trick. Round down and every riser exceeds the limit. The stair stringer calculator does this and shows the 2R+T check alongside it.
Tread count is one fewer than the riser count, and total run is that times the tread depth. Fourteen treads at 10″ needs 140 inches of floor — and it needs headroom above it for the whole length.
If the run does not fit, something has to give: a deeper stair well, a landing, a winder, or a switchback. What does not give is riser equality.
Use a 2×12 and a framing square with stair gauges — the small clamps that fix to the square's edges. Set one at your riser height on the tongue and one at your tread depth on the blade.
The gauges are what make this repeatable. Marking freehand from a tape accumulates error, and by step twelve it shows.
Everywhere on the stair, the tread sits on top of a cut. At the bottom, the stringer sits on the floor, and there is no tread underneath it. So unless you take a tread thickness off the bottom, the first riser ends up one tread thicker than every other riser.
Trim the bottom of the stringer by exactly the tread material's thickness before you set it. On 1″ treads, that is 1 inch off the foot. Do this and the first and last risers match everything between them.
After cutting, measure the narrowest remaining width of the stringer, perpendicular to its edge. Keep at least 5 inches of continuous uncut material behind the notches. Many codes require it, and a stringer cut thinner than that is marginal whatever the code says.
This is why stringers are 2×12s. Cut the same notches into a 2×10 and there is not enough left.
Cut one stringer, set it in place, and check it against the actual opening — first riser, last riser, and that the top lands where you think it does. Then use it as the template for the rest.
A wrong stringer is a wasted 2×12. Three wrong stringers is a wasted afternoon.
Trimming the bottom of the stringer by one tread thickness before you set it. Without it the first riser ends up a full tread thicker than every other riser, because the treads sit on top of the cuts everywhere except at the floor. It is the single most common stair mistake.
Set stair gauges on the square at your riser height on the tongue and your tread depth on the blade, then walk the square down the stringer, marking both edges each time and sliding it so the square picks up where the last mark ended. The gauges are what keep every step identical.
A 2x12 is standard, because after the notches are cut you still need enough continuous material behind them. Keep at least 5 inches of uncut stringer behind the notches; many codes require it, and less than that is structurally marginal regardless.
Check it against the job. Building codes, span tables and product specifications vary by jurisdiction and manufacturer. Your approved drawings and your local code govern, not this page.