How a stair stringer is laid out + cut

Hook a framing square at the unit-rise dimension (e.g., 7-1/2″) on the tongue and the unit-run dimension (e.g., 10″) on the body. Place the square on the wide edge of a 2×12 with the marks touching the lumber edge — this is the first riser + tread pair. Mark along both legs of the square, then slide the square down the board so the heel sits on the previous riser line. Repeat for every step. Most deck stairs need 3-5 notches per stringer.

2×12 dimensional PT pine or PT southern yellow pine — never 2×10 or smaller. IRC R311.7.5 requires at least 5″ of solid wood remaining beneath the deepest notch. A 2×12 (11.25″ actual) leaves 5.25″ when cut for 7-1/2″ riser × 10″ tread; a 2×10 (9.25″) leaves only 3.25″, below code minimum. Some jurisdictions allow LVL or PSL engineered lumber as an alternate — confirm with your AHJ before cutting.

Use a circular saw to cut each notch's riser and tread, but stop the cut at the inside corner — do not cut through the diagonal line. Continuing past the corner over-cuts the stringer and weakens it (visible saw kerf below the corner is a code failure). Finish each corner with a hand saw or jigsaw. Smooth the corner with a chisel if needed. The inside corner of each notch is the highest-stress point on the stringer.

Minimum 3 for a standard 36″-wide flight under residential live load: outer-left, outer-right, plus one center stringer. Add a 4th stringer for stairs over 42″ wide or stair runs over 12 risers. Spacing should not exceed 16″ O.C. between stringer centers per IRC R311.7. Premium decks sometimes use 4 stringers even at 36″ width for reduced bounce.

Notched (the animation): triangular cuts removed from the top edge — treads + risers bear directly on the cut surfaces. Stronger, code-default approach. Cleated: full-width 2×12 with metal angle brackets bolted to support each tread — no notches cut into the board. Used for engineered or laminated stringers, glass treads, or accessibility ramps. Notched is faster, cheaper, and more common; cleated is required for some glass-tread designs.

Because the top tread (the deck surface itself) covers the top of the stringer. If the stringer top aligned exactly with the deck top, the first riser would be too tall by one tread thickness (~1″). Standard practice: trim the top-most tread notch shallower by the tread thickness — typically 1-1/4″ for 5/4 PT or 1.25″ composite. The animation shows this trimmed-top geometry once the stringer is released (phase 3).