Stair stringers for a 30 in high deck
A 30 inch high deck needs 4 risers of 7-1/2 inches each and 3 treads to cut — the deck is the top step. With 10½ inch treads the stairs reach 31-1/2 inches (2 ft 7-1/2 in) out from the deck, and each stringer is about 43-1/2 inches (3 ft 7-1/2 in) long.
How to work it out
- 30 in ÷ 7.75 in max = 3.87 → round up to 4 risers, each 30 in ÷ 4 = 7-1/2 in
- 3 treads × 10-1/2 in = 31-1/2 in run · √(30² + 31.5²) = 43-1/2 in along the slope
- Comfort check: 2 × 7-1/2 in + 10-1/2 in = 25.5 in (24–26 in is comfortable) · stair angle 35.5 degrees
- Throat: 11-1/4 in board − 6.1 in notch depth = 5-1/8 in
- Tread board: 10-1/2 in tread + 3/4 in nosing + 0 in riser board = 11-1/4 in
- Top level cut: 7-1/2 in riser + 1 in tread = 8-1/2 in below the deck surface
Thirty inches is a common height for a deck over gently sloping ground, and it splits into four equal risers of exactly 7½ in — no fractions to chase. Measure from the deck surface to where the bottom step will really land.
Adjust for your project
More options: Max riser · Spacing · Stringer board …
- Steps to cut
- 3 steps to cut in each stringer + the deck is the top step
- Total run (how far the stairs reach out)
- 31-1/2 in (2 ft 7-1/2 in)
- Stringer length (along the slope)
- 43-1/2 in (3 ft 7-1/2 in)
- Stringers needed
- 4 stringers
- Stringer stock to buy
- 4 × 2×12 × 8 ft
- Tread board depth to buy (tread + nosing + riser board)
- 11-1/4 in
- Drop the stringer (trim off the bottom)
- 1 in
- Top level cut, measured down from the top surface
- 8-1/2 in
- Wood left below the notches (throat)
- 5-1/8 in
| Item | What to buy | Notes |
|---|---|---|
| Stringers | 4 × 2×12 × 8 ft | Pressure-treated and rated for ground contact (check the end tag) — the bottoms sit near the ground. |
| Treads | 6 pieces 36 in long (2 per step) → buy 2 × 5/4×6 × 10 ft | 2 boards 5½ in wide with ¼ in gaps cover 11-1/4 in of the 11-1/4 in each step needs; 3 pieces per 10 ft board. |
| Hardware | Sloped joist hangers or a hanger board at the top (DCA 6 Fig. 31); screws or nails rated for treated lumber (hot-dip galvanized or stainless) | Plus a handrail on at least one side — four or more risers (2021 IRC R311.7.8). |
Where to mark each step (height above the bottom)
| Step | Height above the bottom | Guard on open sides? |
|---|---|---|
| Tread 1 | 7-1/2 in | — |
| Tread 2 | 15 in | — |
| Tread 3 | 22-1/2 in | — |
| Deck surface | 30 in | — |
One step fewer or more
| Risers | Each step | Treads | Tread depth | Total run | Check |
|---|---|---|---|---|---|
| 3 risers | 10 in | 2 | 10-1/2 in | 21 in | Risers too tall |
| 4 risers (shown) | 7-1/2 in | 3 | 10-1/2 in | 31-1/2 in | OK |
| 5 risers | 6 in | 4 | 10-1/2 in | 42 in | OK |
- The 4 stringers come from the 16 in default spacing — the joist spacing 5/4 wood decking needs on a deck (2021 IRC Table R507.7). DCA 6 allows cut stringers up to 18 in apart — 3 for this width — if your tread boards are rated for that span (check the tread maker).
- What the bottom sits on: DCA 6 doesn’t let deck stringers bear on a pad or patio that isn’t founded below frost depth — set them on posts or footings. See what the bottom of the stringers sits on.
- Leave a landing at the bottom at least 36 in deep and as wide as the stair (IRC R311.7.6); the deck is the top landing.
- Trim 1 in off the bottom of each stringer (one tread thickness) so the first step comes out the same 7-1/2 in as the rest.
Stringers for 30 in deck steps, by stringer spacing
| Max stringer spacing | Stringers needed | Stringer stock to buy |
|---|---|---|
| 12 in | 4 stringers | 4 × 2×12 × 8 ft |
| 16 in | 4 stringers | 4 × 2×12 × 8 ft |
| 18 in | 3 stringers | 3 × 2×12 × 8 ft |
What to know about steps for a 30 in deck
Three risers would be 10 in each, far over the 7¾ in maximum in IRC R311.7.5.1 (2021), so the flight takes four. You cut three notches in each stringer; the deck itself is the fourth step. With 10½ in treads, 2 × riser + tread is 25½ in, inside the 24–26 in range most people find comfortable, and the stair angle is about 35½°.
30 in is also the exact height where guards start. IRC R312.1.1 asks for a guard where a walking surface is more than 30 in above the grade within 36 in of its edge. A deck at exactly 30 in over level ground is not over that line, but ground that is an inch lower, or that slopes away within 36 in of the edge, puts it over — measure the low spot, not the spot under the stair. The top tread sits 22½ in up, so the stair itself needs no guard on level ground. Four risers does bring in a handrail on at least one side (R311.7.8).
The shopping list: each stringer is about 43½ in along the slope, so with a foot allowed for the top and bottom cuts the stock line lists one 8 ft 2×12 per stringer, pressure-treated for ground contact. At the 16 in default spacing a 36 in wide stair takes four stringers; DCA 6 allows up to 18 in, which drops it to three if your tread boards are rated for that span. The treads are six 36 in pieces of 5/4×6 — two boards per step with a ¼ in gap make the 11¼ in each step needs — and they come three to a 10 ft board, so two boards cover all three steps.
A little lower, How many steps for a 2 ft deck also has four risers, but only 6 in tall. Six inches higher, How many steps for a 3 ft deck adds a fifth riser and a fourth tread.
Different size? Use the full Stair Stringer Calculator.
What you’ll need
- Framing squareBudget price
The tool for laying out stair stringers and rafters: rise on one leg, run on the other.
See options on Amazon (opens in a new tab) - Stair gaugesBudget price
Small brass or steel buttons that clamp to a framing square so every rise and run you mark is identical.
See options on Amazon (opens in a new tab) - Circular saw (7¼ in)Mid-range price
The most versatile first power saw: crosscuts framing lumber, rips boards and, with a straightedge, breaks down plywood sheets.
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Frequently asked questions
How many steps for a 30 in deck with a 7 in maximum riser?
5 risers of 6 inches and 4 treads, reaching 42 inches (3 ft 6 in) out at 10½ inch treads — gentler steps, but one more tread and another 10½ inches of yard.
How long is the stringer for 30 in deck steps with 11 in treads?
About 44-5/8 inches (3 ft 8-5/8 in), with 33 inches (2 ft 9 in) of run. 2 × riser + tread rises to 26 in, the top of the comfortable range, and at 11 inches the IRC no longer requires a nosing.
Other common sizes
- How many steps for a 3 ft deck — 5 risers
- How many steps for a 2 ft deck — 4 risers
- How many steps for a 4 ft deck — 7 risers
- Stair stringers for an 8 ft rise — 13 risers
- Stair stringers for a 2.7 m rise — 15 risers
Sources
- International Code Council, 2021 International Residential Code, Chapter 3 Building Planning (opens in a new tab) — R311.7.1 width (36 in), R311.7.2 headroom (6 ft 8 in), R311.7.3 vertical rise of a flight (12 ft 7 in), R311.7.5.1 risers (7¾ in; open risers more than 30 in up must not pass a 4 in sphere), R311.7.5.2 treads (10 in), R311.7.5.3 nosings (¾–1¼ in unless the tread is 11 in or deeper), R311.7.6 landings (36 in deep), R311.7.8 handrails, R312.1 guards.
- American Wood Council, DCA 6 Prescriptive Residential Wood Deck Construction Guide (2015 IRC edition) (opens in a new tab) — Stair requirements, Figures 28, 29, 31 and 34: 2×12 minimum stringers; cut stringers span no more than 6 ft 0 in and keep at least 5 in below the notches; at least three cut stringers at no more than 18 in on center; sloped hangers or a hanger board at the top; an intermediate landing where the total rise exceeds 12 ft; stringers bear on footings below frost depth, not on pads or patios that aren’t founded that deep.