Which code, and which section numbers
The 2024 IRC renumbered chapter 3, so the stair, handrail and guard rules all have new section numbers. Handrails, once part of the stairway section, became a section of their own. The limits on this page are from the 2024 edition.
| Topic | 2021 IRC | 2024 IRC |
|---|---|---|
| Stairways | R311.7 | R318.7 |
| Handrails | Part of R311.7 | R320 |
| Guards | R312 | R321 |
Many jurisdictions still enforce the 2018 or 2021 IRC, so a permit, plan review or inspection report may cite the older numbers. Local amendments apply on top of whichever edition is in force.
Your building department decides
The local building department has the final say on which code applies, how it has been amended, and whether a stair passes. Ask before you cut stringers.
The limits at a glance
| Item | 2024 IRC limit | Section |
|---|---|---|
| Riser height | 7¾ in at most | R318.7 |
| Tread depth, nosing to nosing | 10 in at least | R318.7 |
| Variation in one flight | ⅜ in at most, for risers and for treads | R318.7 |
| Nosing | ¾ to 1¼ in, not required on treads 11 in deep or more; edge radius 9/16 in at most | R318.7 |
| Open risers | No 4 in sphere through, where more than 30 in above grade | R318.7 |
| Width | 36 in at least; at and below the handrail, 31½ in with one handrail or 27 in with two | R318.7 |
| Headroom | 6 ft 8 in at least | R318.7 |
| Rise of one flight | 12 ft 7 in (151 in) at most between floors or landings | R318.7 |
| Landing depth | 36 in at least, in the direction of travel | R318.7 |
| Handrail | On at least one side of a flight with four or more risers; 34 to 38 in high | R320 |
| Guard | Where the walking surface is more than 30 in above grade; at least 36 in high, 34 in along stairs | R321 |
| Guard openings | No 4 in sphere through; 4⅜ in on the open sides of stairs; 6 in at the stair triangle | R321 |
Summarized from Jefferson County, Colorado’s pages on the 2024 IRC; section numbers from the ICC’s 2024 table of contents.
Risers, treads and nosings
A riser is the height of one step and a tread is the part you stand on. Risers can be no more than 7¾ in, and treads no less than 10 in deep, measured from the front edge of one tread to the front edge of the next. In one flight, the tallest riser may be no more than ⅜ in taller than the shortest, and the deepest tread no more than ⅜ in deeper than the shallowest.
That tolerance is why you divide the total rise, measured from the top of the decking down to where the stair will land, into equal risers:
Worked example: a deck 36 in above the ground
- 36 ÷ 7.75 = 4.65, rounded up to 5 risers.
- Each riser is 36 ÷ 5 = 7.2 in, under the 7¾ in limit.
- There are 5 − 1 = 4 treads, because the deck itself is the top step.
- At 10½ in per tread, the total run is 4 × 10.5 = 42 in.
Cutting full 7¾ in risers instead goes wrong: four of them make 31 in, which leaves a 5 in step at the bottom, 2¾ in shorter than the rest and far past the ⅜ in tolerance.
A nosing is the front edge of a tread that sticks out past the riser below. The code calls for ¾ to 1¼ in of it, though none is required where the treads are at least 11 in deep, and the edge can be rounded to a radius of no more than 9/16 in. Open risers, with no board between the treads, must not let a 4 in sphere through where they are more than 30 in above grade.
Width, headroom and landings
A stair must be at least 36 in wide. At and below the handrail, the clear width may narrow to 31½ in with a handrail on one side or 27 in with handrails on both, 4½ in less for each rail (36 − 31½ = 4½, and 36 − 27 = 9). The American Wood Council’s DCA 6 deck guide also calls for stairs at least 36 in wide.
Headroom must be at least 6 ft 8 in; outdoors, check it where a stair passes under an upper deck or a roof. A single flight may rise no more than 12 ft 7 in (151 in) between floors or landings, so a stair to a deck higher than that needs a landing partway down. Landings must be at least 36 in deep in the direction of travel.
Handrails and guards
A handrail is the rail you hold. One is required on at least one side of every flight with four or more risers, 34 to 38 in high (R320 in the 2024 IRC). The five-riser stair in the example needs one; a three-riser stair doesn’t, by this rule.
A guard is the barrier along an open edge that stops a fall. Guards are required where a walking surface is more than 30 in above the grade at any point within 36 in horizontally of its edge (R321; R312 in the 2021 IRC). A guard must be at least 36 in high, or 34 in along stairs, and 34 to 38 in where it also serves as the handrail. The deck in the example is 36 in up, so it needs guards.
Openings in a guard must not let a 4 in sphere through, with two exceptions: 4⅜ in on the open sides of stairs, and 6 in at the triangle formed by the riser, the tread and the guard’s bottom rail.
Guard heights vary
States and cities amend these rules, and guard height is one they change: California’s residential code requires guards 42 in high. Check the height with your building department before you buy posts and balusters.
Stringers and the throat
The code limits the steps; the American Wood Council’s DCA 6 guide covers the stringers that carry them. It calls for stringers of at least 2×12 lumber and at least three of them when only cut stringers are used. On stairs wider than 36 in that combine cut and solid stringers, it spaces them no more than 18 in on center.
Cutting the notches for treads and risers takes wood out of each stringer. The uncut band between the inside corners of the notches and the bottom edge of the board is the throat, and the Journal of Light Construction says to leave at least 5 in of it. InterNACHI shows how to check it on a 2×12, which is 11¼ in deep:
With InterNACHI’s 7¼ in risers and 10½ in runs, the diagonal is √(7.25² + 10.5²) = 12.76 in, each notch reaches 7.25 × 10.5 ÷ 12.76 = 5.97 in into the board, and the throat is 11.25 − 5.97 = 5.28 in, just over 5 in. The worked example’s 7.2 in risers leave 5.31 in. Taller risers or deeper runs cut deeper, so check the throat whenever you change either.
The stair stringer calculator works out the risers, treads, stringer length and count, gives the layout marks and runs the throat check.
Stringer spacing for composite treads
Composite and PVC tread makers call for closer stringers than DCA 6’s 18 in for wide wood stairs:
| Treads | Stringers at most | For a 36 in stair |
|---|---|---|
| Trex Signature, Transcend or Select 1×6 | 12 in on center | 4 stringers |
| Trex Enhance 1×6 | 9 in on center | 5 stringers |
| Trex 2×6 | 16 in on center | 4 stringers |
| TimberTech composite | 9 in on center | 5 stringers |
| TimberTech Advanced PVC | 12 in on center | 4 stringers |
| Fiberon, residential stairs | 12 in on center | 4 stringers |
| Wood stairs built to AWC DCA 6 | 18 in on center | 3 stringers |
The last column counts the spaces across the stair, with a stringer at each side:
At 12 in, a 36 in stair has 36 ÷ 12 = 3 spaces and four stringers, the number Trex’s guide gives for a 36 in stair. At 16 in, 36 ÷ 16 = 2.25 rounds up to 3 spaces, so four again; at 9 in, 4 spaces and five stringers.
Trex also sets the tread itself: at least 11 in deep, overhanging the riser below by no more than ¾ in. Decks.com’s stair guide shows how that fits: two 5½ in boards with a ¼ in gap make an 11¼ in tread, and a ¾ in nosing leaves a 10½ in run, the run in the worked example.
Measured nosing to nosing, as the code measures it, that tread is 10½ in deep. That’s under 11 in, so the code requires a nosing of ¾ to 1¼ in, while Trex allows no more than ¾ in. On that stair the nosing has to be exactly ¾ in.
For the deck surface itself, the deck board calculator counts the rows, board lengths and butt joints.
Sources for the numbers on this page
- International Code Council — 2024 International Residential Code: table of contents (section numbers, PDF) · checked October 3, 2026
- 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
- 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
- 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
Corrections and updates are logged on the sources page.