Stair Stringer Calculator

Length = √(totalRise² + totalRun²) — pure right-triangle hypotenuse. For 42″ rise + 55″ run (5 treads × 11″): √(42² + 55²) = √(1,764 + 3,025) = √4,789 ≈ 69.2″ or 5′-9.2″. Buy 8′ stock and trim — gives 6.8″ of cut waste, plenty for the top + bottom miter and seat cuts. DeckMath auto-rounds up to the next stock length (8/10/12/14/16 ft).

For cut/notched stringers (16″ o.c. max spacing per AWC DCA-6): ceil(42/16) + 1 = 4 stringers. For solid/cleated stringers (24″ o.c. max): ceil(42/24) + 1 = 3 stringers. Cut stringers need tighter spacing because notching removes ~40% of the cross-section. Wider stairs scale proportionally — 48″ cut needs 5 stringers, 60″ cut needs 5, 72″ cut needs 6.

Throat depth = remaining lumber web BELOW the deepest notch cut into a cut/notched stringer. AWC DCA-6 (the IRC's referenced deck guide) requires throat ≥ 5 inches. A 2×12 (11.25″ deep) with a 7″ riser notch leaves 4.25″ — FAILS. The throat carries the entire structural load (the notched portion has zero strength), so insufficient throat = stringer buckles under load. Fixes: bigger lumber, smaller riser, or switch to solid-stringer style (no notch).

Cut/notched: easier to lay out (framing square + carpenter pencil + circular saw), traditional look, no exposed hardware. Drawbacks: notching removes 40-50% of lumber strength, requires careful throat math, more stringers needed (16″ o.c. max). Solid w/ cleats: full lumber strength retained, fewer stringers (24″ o.c. ok), exposed metal cleats visible from below. More material-efficient overall but more hardware. Modern code-conscious DIY → solid. Traditional aesthetic → cut.

Hung: stringer top face-mounts to the deck rim with a Simpson LSCZ stringer hanger (1 per stringer, 8 × SDS screws each, ~$14.50/bracket). Stringer bottom doesn't extend above deck — cleaner finish, allows drainage gap. Rested: stringer is notched at the top so it sits ON the deck rim, with the deck edge tucking into the cutout. No bracket needed but stringer must be 1.5″ deeper than the notch, and the visible stringer top breaks the clean deck-edge line. Most modern contractors use hung-style.

θ = atan(rise / run). For 7″ rise + 11″ run (IRC-comfort default): atan(7/11) = 32.5° from horizontal. IRC-recommended comfort band is 30-38°. Steeper than 38° (rise/run > 0.78) feels like a ladder; shallower than 30° (rise/run < 0.58) wastes horizontal space. Most code-compliant deck stairs land at 32-36°.

Yes — IRC 2021 R317.1 requires PT for any wood within 6″ of grade. Stringer bottoms sit on the landing pad, often soaked by rain runoff, so PT (UC4A ground-contact rated) is mandatory not optional. PT standard (SYP #2, 25-yr rot warranty) costs $3.20-$4.10/lf. PT premium (Yellawood / MicroPro / Wolmanized AC2) adds 28% for a lifetime warranty + cleaner chemistry that doesn't corrode standard galvanized fasteners as fast — worth it on a part that gets soaked.

Sometimes — but the math has to work. 2×10 (9.25″ depth) with a 7″ riser notch leaves a 2.25″ throat — FAILS AWC DCA-6's 5″ minimum. 2×10 only passes when riser height is ≤ 4.25″ (rare on residential decks) OR when style = solid (no notch, full 9.25″ retained). For 99% of deck stairs, use 2×12 cut OR 2×10 solid. DeckMath flags the failing combo and recommends the cheapest passing alternative.

Hung stringers: 8 Simpson SDS 1/4″ × 1-1/2″ structural screws per LSCZ bracket (per Simpson's allowable-load chart) — bracket attaches to deck rim. Plus a general kit (~10 deck screws, framing nails) per stringer for tread + riser attachment. Rested stringers: no bracket, but each stringer needs 2 × 6″ structural screws toe-screwed through the notched seat into the deck rim, plus the general kit. Bottom of stringer attaches to landing pad with a steel angle + 1/2″ × 4″ wedge anchors (1-2 anchors per stringer).