How to Cut Stair Stringers

1. A straight, crown-free 2×12 (or 2×10 for shallower runs) — sight down the edge and reject any board with a bow.
2. A framing square with stair gauges (the little brass buttons that clamp to the blade and tongue).
3. A circular saw plus a sharp handsaw to finish the inside corner of each notch.
4. A sharp pencil or a knife, a tape measure, and a scrap block the exact thickness of one tread.

Clamp one stair gauge on the tongue (the short arm) at your unit rise and the other on the blade (the long arm) at your unit run. Working in inches, that is roughly 7³⁄₁₆″ on the tongue and 11″ on the blade for the example flight. The gauges turn the square into a jig: every triangle you trace will now be the same.

Lay the square on the board near the bottom end with the tongue and blade pointing toward you, so the rise and run marks both touch the top edge of the stringer. Trace along both arms. That single V is the first step.

Slide the square up the edge until the heel of the run mark meets the toe of the step you just drew, and trace again. Repeat for all 15 treads of the example. Number each notch as you go — it is shockingly easy to lose count on a long board. When you reach the top, the last rise line marks where the stringer meets the framing.

Before you cut anything, check the total: the vertical distance from the bottom of the first riser line to the top of the last should equal your total rise, and the horizontal run should equal (N − 1) × unit run, or 13′-8¾″ for the example. If it is off by more than a pencil width, your gauge settings drifted — re-check rather than fudge it.

Here is the classic mistake. The first riser as drawn is a full unit rise. But the bottom tread will have a finished tread board sitting on it, and the top will not — so once flooring goes down, your bottom step ends up one tread-thickness too tall and the rest too short. To fix it, shorten the bottom of the stringer by the thickness of one finished tread. If your treads are 1″ thick, cut 1″ off the bottom plumb cut.

This "dropping the stringer" trims the first riser so that, after the treads are installed, every riser — bottom, middle, and top — is the equal 7³⁄₁₆″ the calculator promised. Skip it and you reintroduce exactly the riser variation the code forbids.

Cut each line with the circular saw, but stop the blade right at the inside corner where the rise and run meet — do not overcut. A circular blade is round, so running it to the corner on the visible face means it has already cut past the line on the back, which weakens the board. Finish each inside corner with a handsaw or jigsaw.

Mind the throat — the narrowest strip of solid wood left behind the deepest notch. Keep at least 3½″ of throat so the stringer does not crack under load; our throat formula is throat = board width − (rise × run) ÷ √(rise² + run²). If your numbers leave less than that, step up a board size (2×10 → 2×12) rather than cutting deeper.

Cut one stringer, then set it in place against the framing and lay a level across two or three notches. If the tread seats are level and the plumb cuts sit flush top and bottom, use that first stringer as the template and trace the others from it — never re-step them with the square, or small differences creep in between stringers.

A standard flight uses three stringers (two outers plus one center) for 36″ wide stairs; add a fourth if you go wider or use thinner tread stock. Fasten the top with joist hangers or a ledger and anchor the bottom to a solid base.

Everything above describes a cut (notched) stringer, the most common kind, where the steps are sawn into the board and the throat is what holds it together. The alternative is a solid (housed) stringer, used on finished interior stairs, where the treads and risers slot into routed grooves on the inside face of an uncut board. A solid stringer is stronger because nothing is removed from the load path, but it requires a router jig and precise housings, so most site-built and deck stairs use cut stringers.

Stringer count follows the tread material and the width. Three stringers — two outers and a center — suit a typical 36″ flight with standard tread stock. Go to four if the stairs are wider than about 3′-6″, if the treads are thin and flexible, or if code in your area requires intermediate support at a tighter spacing. The stair stringer calculator recommends a board size and flags when your throat is getting thin, which is the usual signal to step up from a 2×10 to a 2×12.

Failing inspection on riser variation almost always traces to one of three things: a bowed board, a stair gauge that slipped, or a forgotten bottom-riser drop. Re-sight the board, re-clamp the gauges, and confirm the drop before you blame the math. A second common failure is overcutting the inside corners with the circular saw, which weakens every notch at once — finish those corners by hand even though it is slower.

Run the exact rise, run, stringer length, throat, and recommended board size for your stairs through the stair stringer calculator, then come back to this layout. It also prints a full-scale template you can lay straight on the 2×12 and trace — which removes the framing-square step entirely if you would rather not step it off by hand. Once one stringer is cut and test-fit, every other stringer is just a trace of that first one.