Size Deck-Stair Rise and Run From the Boards You'll Use

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.