How the risers and treads are worked out
Stairs are laid out from the total rise: the height from the top of the deck boards down to the landing or ground where the bottom step will sit. Divide it by the tallest riser you’ll allow, 7¾ in under the IRC, and round up to get the number of risers. Dividing the rise by that number makes every riser the same height. The deck is the top step, so there’s one tread fewer than there are risers.
Worked example: a deck 36 in above the ground
- Risers: 36 ÷ 7.75 = 4.65, rounded up to 5.
- Riser height: 36 ÷ 5 = 7.2 in, just over 7 3/16 in on a tape measure.
- Treads: 4. With a 10½ in run each, the stairs reach 42 in (3 ft 6 in) out from the deck.
- Angle: atan(7.2 ÷ 10.5) = 34.4°.
- Stringer: √(36² + 42²) = √3,060 = 55.3 in on the diagonal, about 4 ft 7⅜ in. Add 12 in for the end cuts and an 8 ft board covers it.
- Throat in a 2×12: √(7.2² + 10.5²) = 12.73, so 11.25 − 7.2 × 10.5 ÷ 12.73 = 11.25 − 5.94 = 5.31 in, above the 5 in JLC recommends.
- Stringers under 2× wood treads, 36 in wide: (36 − 1.5) ÷ 18 = 1.9, rounded up to 2 spaces, so 3 stringers 17¼ in apart on center.
Decks.com’s stair guide works the same way with 10½ in runs: three treads reach 31½ in from the deck.
Laying out a stringer with a framing square
Mark the riser height on one arm of a framing square and the run on the other, with stair gauges clamped at both marks if you have them. Lay the square on the board with both marks on its top edge and trace the riser and the run. Slide the square along so it starts where the last run ended, and repeat once for each tread. Decks.com’s layout guide marks a 10 in run and a 6⅜ in rise this way. Two notches then need adjusting:
- Drop the stringer by the tread thickness. Each tread sits on top of its notch. Cut every riser full height and the bottom step comes out one tread thickness too tall, and the top step one tread thickness too short. Cutting the bottom riser shorter by the tread’s thickness fixes both: Decks.com’s example cuts a 6⅜ in riser down to 5⅜ in for 1 in thick 5/4 decking.
- Shorten the top run for riser boards. If you’re fitting riser boards, Decks.com takes the board’s thickness off the back of the top run: a 10 in run is cut at 9 in with a 1 in riser board. With open risers, cut the full run.
How long a board to buy
Beyond the diagonal, a board needs room for the plumb cut where the stringer meets the deck and the level cut where it sits on the landing. The calculator adds 12 in for those and picks the shortest of 8, 10, 12, 14 and 16 ft that covers the total. The 12 in is a working allowance, not a rule.
Stringer size, spacing and throat
Treads span from stringer to stringer, so the tread material sets how far apart the stringers can be. For wood stairs, AWC’s DCA 6 asks for at least three cut stringers and, on stairs wider than 36 in, no more than 18 in between stringers. Composite and PVC treads have their own limits, set by each maker:
| Treads | Stringers at most | 36 in wide | 48 in wide |
|---|---|---|---|
| 2× wood (AWC DCA 6) | 18 in on center | 3 stringers | 4 stringers |
| Trex Signature, Transcend or Select 1×6; Transcend 1×4 | 12 in | 4 | 5 |
| Trex Enhance 1×6 | 9 in | 5 | 7 |
| Trex Transcend or Select 2×6 | 16 in | 4 | 4 |
| TimberTech composite | 9 in | 5 | 7 |
| TimberTech Advanced PVC | 12 in | 4 | 5 |
| Fiberon composite | 12 in | 4 | 5 |
The counts follow the formula above, with the outer stringers flush with the sides of the stairs. Trex’s guide gives the same 4 stringers for a 36 in stair at 12 in. Decks.com, which Trex publishes, never spaces stringers more than 16 in apart. Spacings are the makers’ own figures, linked below; brand names identify products only.
DCA 6 also calls for stringers of at least 2×12, and for at least three of them where only cut stringers carry the treads. Every notch takes wood out of the top edge of the board, so a cut stringer carries the stairs on the uncut strip below the notches: the throat. InterNACHI’s example of 7¼ in risers and 10½ in runs on a 2×12 leaves “approximately 5 inches”; the formula gives 5.28 in. JLC recommends at least 5 in.
A 2×10 is 2 in shallower than a 2×12 (9¼ in against 11¼ in), so the same notches leave only 3.28 in. Taller risers and longer runs cut deeper, too: 7¾ in risers with 11 in runs leave 4.91 in, even in a 2×12.
Code limits for deck stairs
Deck stairs on houses are built to the International Residential Code (IRC) in most of the US. The 2024 edition renumbered chapter 3: stairs moved from R311.7 to R318.7, handrails to R320, and guards from R312 to R321. These are the limits the calculator checks or reminds you of, in our own words:
| Rule | Limit | 2024 IRC | 2021 and earlier |
|---|---|---|---|
| Riser height | 7¾ in at most | R318.7 | R311.7 |
| Run, nosing to nosing | 10 in at least | R318.7 | R311.7 |
| Nosing | ¾ to 1¼ in, unless treads are 11 in deep or more | R318.7 | R311.7 |
| Variation in a flight | ⅜ in at most between the tallest and shortest riser, or the deepest and shallowest tread | R318.7 | R311.7 |
| Width | 36 in at least | R318.7 | R311.7 |
| Headroom | 6 ft 8 in at least | R318.7 | R311.7 |
| Rise of one flight | 12 ft 7 in (151 in) at most between landings | R318.7 | R311.7 |
| Landings | 36 in deep at least, in the direction of travel | R318.7 | R311.7 |
| Handrail | On flights with 4 or more risers, 34 to 38 in high | R318.7, R320 | R311.7.8 |
| Guards | Where a walking surface is more than 30 in above grade: 36 in high, 34 in along stairs | R321 | R312 |
| Open risers | More than 30 in above grade, no gap a 4 in sphere can pass through | R318.7 | R311.7 |
Which code applies
Many places still build to the 2018 or 2021 IRC, where the old section numbers apply, and states and cities amend some limits: California’s residential code raises guards to 42 in. Your local building department has the final say.
The ⅜ in rule is the one sloping ground breaks. If you measure the rise straight down from the deck and the ground falls away from it, the bottom step lands lower than you measured and comes out taller than the rest. Measure at the spot where the bottom step will sit, or build a level landing pad there first; the concrete slab calculator works out the concrete for a pad. The deck stair code guide goes through each rule in more detail.
Landings, handrails and guards
A single flight can rise at most 12 ft 7 in (151 in) between landings. At 7¾ in a riser that’s 20 risers: 151 ÷ 7.75 = 19.5, rounded up to 20 risers of 7.55 in. A higher deck needs a landing partway down, at least as wide as the stairs and 36 in deep in the direction of travel; JLC’s article also puts one at the top and bottom of each flight.
Stairs with 4 or more risers need a handrail 34 to 38 in high. At 7¾ in a riser, that’s any rise over 23¼ in (3 × 7.75); stairs of up to 3 risers need none under the IRC.
Guards are required where a walking surface is more than 30 in above the ground: at least 36 in high on the deck, and 34 in along the stairs. In the 36 in example the deck needs guards, but the highest tread is 4 × 7.2 = 28.8 in up, so on level ground the stairs don’t. Open risers are allowed, but where they’re more than 30 in above the ground the gaps between treads must not pass a 4 in sphere. A 7.2 in riser under a 1 in tread leaves a 6.2 in gap, so tall stairs with open risers need riser boards or another way to close the gaps.
Questions people ask
How many steps do I need for a 36 in or 48 in deck?
Divide the rise by 7¾ in and round up. A 36 in rise needs 5 risers of 7.2 in and 4 treads, because the deck itself is the top step. A 48 in rise needs 7 risers: 48 ÷ 7.75 = 6.2, rounded up to 7, each 6.86 in (6⅞ in) tall, with 6 treads. With 10½ in runs, those stairs reach 42 in and 63 in out from the deck.
How far apart should stair stringers be for composite treads?
As far apart as the tread’s maker allows, which is closer than for wood. TimberTech allows 9 in on center under its composite treads and 12 in under Advanced PVC. Trex allows 12 in for Signature, Transcend and Select 1×6 treads, 9 in for Enhance and 16 in for Transcend or Select 2×6 treads. Fiberon allows 12 in. For wood stairs, AWC’s DCA 6 asks for at least three cut stringers, and no more than 18 in between stringers on stairs wider than 36 in. At 9 in on center, a 36 in wide stair takes 5 stringers.
How deep should deck stair treads be?
The IRC asks for a run of at least 10 in, measured nosing to nosing, and a nosing of ¾ to 1¼ in unless the treads are at least 11 in deep. Two 5½ in deck boards with a ¼ in gap make an 11¼ in tread; less a ¾ in nosing, that’s the 10½ in run Decks.com uses. Trex treads must be at least 11 in deep and overhang the riser by no more than ¾ in.
Do deck stairs need a handrail?
Under the IRC, any flight with 4 or more risers needs a handrail 34 to 38 in high. With 7¾ in risers, that’s any rise over 23¼ in. Separately, the open sides of a deck or stair more than 30 in above the ground need guards.
How long should a stair stringer be?
Start from the diagonal: the square root of the rise squared plus the total run squared. A 36 in rise with a 42 in run gives √3,060 = 55.3 in, about 4 ft 7⅜ in. Add room for the cuts at the top and bottom (the calculator adds 12 in) and buy the next length up: an 8 ft board here.
What is the maximum rise for deck stairs?
Each riser can be at most 7¾ in, and one flight can rise at most 12 ft 7 in (151 in) between landings. A deck higher than that needs a landing at least 36 in deep in the direction of travel, which splits the stairs into two flights.
Sources for the numbers on this page
- Jefferson County, Colorado — Stairways: residential requirements under the 2024 IRC · checked October 3, 2026
- Jefferson County, Colorado — Guards: residential requirements under the 2024 IRC · checked October 3, 2026
- Jefferson County, Colorado — Handrails: residential requirements under the 2024 IRC · checked October 3, 2026
- Jefferson County, Colorado — Landings: residential requirements under the 2024 IRC · checked October 3, 2026
- California Building Standards Commission, via Codebook — 2025 California Residential Code, section R318.7: stairways (2024 IRC text, unamended) · checked October 3, 2026
- California Building Standards Commission, via Codebook — 2025 California Residential Code, section R321: guards (California raises the height to 42 in) · checked October 3, 2026
- International Code Council — 2024 International Residential Code: table of contents (section numbers, PDF) · checked October 3, 2026
- American Wood Council — DCA 6: Prescriptive residential wood deck construction guide (2015 IRC edition) · checked October 3, 2026
- Journal of Light Construction, via Village of Somers, Wisconsin — Building code-compliant deck stairs (stringer throat, PDF) · checked October 3, 2026
- InterNACHI — Inspecting stair stringers · checked October 3, 2026
- Decks.com — How to build deck stairs and steps · checked October 3, 2026
- Decks.com — How to lay out deck stair stringers · checked October 3, 2026
- Trex — Decking installation guide, 2026 edition (gaps, spans, fastening, stairs) · checked October 3, 2026
- TimberTech — Frequently asked questions (joist and stair stringer spacing) · checked October 3, 2026
- Fiberon — PE decking fastening instructions, FIB-0354-LIT 1/22 (distributor copy, PDF) · checked October 3, 2026
- Southern Forest Products Association — Southern Pine standard sizes (nominal and actual dimensions) · checked October 3, 2026
Corrections and updates are logged on the sources page.