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How long should stair stringers be?

Prefills a 42″ total rise (common low-to-mid deck) so tip-to-tip length and IRC flags show immediately. Toggle Standard vs Flush, then use Copy lumber / layout script before the lumber aisle.

Stair stringer layout

Emphasizes stringer length, run, and IRC R311.7.5 checks before you cut. Defaults follow common municipal reprints of IRC limits.

Also planning deck rise / tread focus? Deck stair calculator

Scenario templates

Calc mode

Total rise (deck to grade) (in)Max riser (in)IRC-style starting point: ≤ 7¾ in (verify local).Tread depth (in)IRC-style starting point: ≥ 10 in (verify local).

Stringer mount

Standard mount: stringer drops below deck so the first tread is one riser down — treads = risers − 1.

Tread type (affects depth hint)

Common PT board depth planning

Stringer summary

risers

7 in

actual riser (≤ 7¾ in target)

treads (risers − 1)

65.3 in

planning length tip-to-tip

Script is plain text for a yard call or lumber run — not a quote from YardBuild.

Interactive 3D stair (6 risers · 5 treads)

Drag to orbit

Updates when riser count, riser height, or tread depth change. Planning visual only — not a cut list or engineered stringer.

Stringer run: 50 in (4.17 ft); ≈ treads × tread depth · standard mount
Stringer length (planning): 65.3 in; √(run² + total rise²) — tip-to-tip ballpark, not a cut list
Stair angle: 40°
Tread depth used: 10 in; Meets common ≥10 in starting point

IRC R311.7.5 — common municipal reprint

Rule	Limit	Your layout
Max riser (R311.7.5.1)	≤ 7¾ in	7 in ✓
Min tread (R311.7.5.2)	≥ 10 in	10 in ✓
Uniformity	≤ ⅜ in within flight	Equal risers by construction ✓
Mount	Standard / Flush	standard

Based on IRC language as reprinted by municipal handouts (e.g. Placer County, City of Boise). Confirm the edition your AHJ adopted.

Compare mounts (same rise)

Based on commonly adopted IRC R311.7.5 language (max riser 7¾ in, min tread 10 in, uniformity within ⅜ in) as reprinted by municipal handouts. Local amendments, landings, handrails, guards, and nosings are separate — confirm with your building department before cutting stringers.

Worked example (42″ rise, Standard, 10″ tread)

Risers = ceil(42 ÷ 7.75) = 6; actual = 42÷6 = 7.0 in (≤ 7¾ ✓). Standard treads = 5; run = 50 in; length = √(50²+42²) ≈ 65.19 in (~5.43 ft). Buy script: “Tip-to-tip about 65 inches — an 8 ft 2×12 is enough if you lay out carefully.”

Full IRC table and mount compare: stair stringer calculator. Deck-rise framing focus: deck stair calculator.

FAQ

How long should stair stringers be?

Planning length ≈ √(stringer run² + total rise²), tip-to-tip. Run ≈ treads × tread depth (Standard: treads = risers − 1; Flush: treads = risers). Buy 2×12 stock longer than that length — this is not a finished cut list.

Do I need an 8 ft or 10 ft 2×12?

Compare tip-to-tip inches to store lengths (96″ / 120″). Example: ~65″ tip-to-tip fits an 8 ft board with waste room; ~100″ needs 10 ft or longer. Always confirm IRC-style riser/tread flags and your AHJ.

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