Stair Stringer Layout

The Problem

Stair math is where deck stairs go wrong. The total drop has to divide into equal rises, the run has to match the treads you're actually installing, and a guess that's off by half an inch per step compounds into stairs that feel wrong and may not pass inspection.

The Technique

1. Measure the total rise: level out from the top of the deck framing to the point where the stairs will land, and measure down. In the source video that's 38-1/2 inches.
2. Divide by a trial number of rises: 38-1/2 divided by 5 comes to about 7-3/4 inches per step. If the result comes out steeper than you want, add a rise and divide again; shallower, take one away.
3. Let the tread material set the run. Two 5-1/2 inch deck boards make an 11-inch tread; with a 1-inch nosing overhang on each step, that leaves a 10-inch run. No ripping boards, and the treads come out of stock material.
4. Lay out the first step on 2x12 stringer stock — 2x12 leaves enough meat behind the notches for a deck stringer — measuring 7-3/4 inches up from a corner and 10 inches along for the run, and carry that rise-and-run pair down the board for each step.

The Problem

Stairs are the job where close enough isn't. Every riser has to be the same height — if one step is a half-inch off, people trip on it, and inspectors fail it. The layout also hides a trap that catches nearly every first-timer: cut the stringer straight off your rise measurement and the bottom step ends up a tread's thickness taller than all the others.

Watch the Full Walkthrough First

If this is your first set of stairs, watch both parts before cutting anything. Training Hands Academy goes through the terminology, the math, and the tools in order, and does it more clearly than most — clear enough that you finish feeling like you understand the reasoning rather than following steps on faith. Part two picks up where the first leaves off:

• Part 1: https://www.youtube.com/watch?v=UFIqiwtaAYA
• Part 2: https://www.youtube.com/watch?v=jx7-uJ_eVoE

What follows is the short version — enough to know what you're walking into, and to check your own work against afterward.

The Math

1. Measure the total rise: finished floor to finished floor, straight up.
2. Divide the total rise by roughly 7" to get a starting number of risers, then round to a whole number.
3. Divide the total rise by that whole number. That's your exact riser height, and every riser gets it. Codes generally cap risers around 7¾" and allow no more than about ⅜" variation across the flight.
4. You always have one fewer tread than risers. Pick a run per tread — 10" minimum is typical — and multiply by the number of treads for your total run.

The Layout

5. Clamp stair gauges to a framing square at your riser height on the tongue and your run on the blade.
6. Set the square on a 2×12 and step it along the board, tracing the rise and run at each position so the notches march up the stringer.
7. Drop the stringer. Trim the bottom by the thickness of one tread. Without this, the finished first step is a tread thicker than all the others — the single most common stair mistake.
8. Cut the notches with a circular saw, stopping short of each inside corner, and finish the corners with a handsaw or jigsaw. Overcutting past the corner weakens the stringer even though it looks fine.