Stair Stringer Layout That Passes Inspection (2021 IRC)
The short answer
A stair stringer passes a typical residential inspection when every riser is 7¾ inches or less and every tread is at least 10 inches deep. Within one flight, the tallest riser can’t beat the shortest by more than 3/8 inch, and the same goes for the deepest and shallowest tread. Those are the 2021 International Residential Code (IRC) limits in section R311.7. Most stringers that fail don’t fail on those big numbers. They fail on the 3/8 inch tolerance, usually at the top or bottom step, because the rise was measured to the wrong surface.
This guide covers the rules, a worked deck stair, the layout itself, and the mistakes inspectors catch most.
The rules your stringer has to meet
These are the 2021 IRC numbers, as reproduced with the International Code Council’s permission in the Stairway Manufacturers’ Association visual guide. Your city or county can amend them, so check which edition and amendments it uses.
| What | 2021 IRC limit | Section |
|---|---|---|
| Riser height | 7¾ in maximum, measured between the leading edges of the treads | R311.7.5.1 |
| Tread depth | 10 in minimum, nosing to nosing | R311.7.5.2 |
| Variation within a flight | Largest riser or tread no more than 3/8 in over the smallest | R311.7.5.1, R311.7.5.2 |
| Open risers | No 4 in sphere through the opening where it is more than 30 in above the floor or grade below | R311.7.5.1 |
| Nosing | ¾ to 1¼ in projection, not required when treads are 11 in or deeper | R311.7.5.3 |
| Headroom | 6 ft 8 in minimum, measured from the sloped line along the nosings | R311.7.2 |
| Width | 36 in clear above the handrail | R311.7.1 |
| One flight | 12 ft 7 in rise maximum between floors or landings | R311.7.3 |
| Landings | A floor or landing at top and bottom, at least as wide as the stair and 36 in deep in the direction of travel | R311.7.6 |
| Handrail | One side, on any flight with 4 or more risers, 34 to 38 in above the nosing line | R311.7.8, R311.7.8.1 |
| Guard | Where the open side is more than 30 in above the grade below; 34 in minimum on stairs | R312.1.1, R312.1.2 |
For deck stairs, most inspectors also lean on the American Wood Council’s DCA 6 deck guide, because the IRC has no stringer span table. DCA 6 calls for:
- 2×12 stringers at minimum.
- At least 5 inches of solid wood left below the notches (the “throat”).
- A maximum horizontal span of 6 ft 0 in for a cut stringer and 13 ft 3 in for a solid one. Past that, add a 4×4 support post on a footing or an intermediate landing.
- At least three cut stringers on a 36 inch stair, and no more than 18 inches on center on wider stairs.
- Footings at least 12 inches below undisturbed ground or below the frost line, whichever is deeper. Stringers must not bear on a concrete pad that isn’t founded that deep.
Worked example: a deck stair with a 40 inch rise
A deck surface sits 40 inches above the spot where the bottom of the stair will land. There are 52½ inches of yard in front of it for the stair, and the stair will be 48 inches wide.
1. Risers. Divide the rise by the maximum: 40 ÷ 7.75 = 5.16. Round up to 6 risers. Each one is 40 ÷ 6 = 6.667 inches, which is 6 11/16 inches on a tape measure. Five risers would be 8 inches each and fail the 7¾ inch limit.
2. Treads. There is always one fewer tread than risers, because the last step up is onto the deck. So 6 risers means 5 treads. 52½ ÷ 5 = 10½ inches each. That clears the 10 inch minimum. It is under 11 inches, so each tread needs a ¾ to 1¼ inch nosing.
3. Stringer length. √(40² + 52.5²) = 66.0 inches, or 5 ft 6 in. Add about a foot for the top and bottom cuts and an 8 ft 2×12 is enough.
4. Throat. Each notch cuts into the board diagonally. The depth it removes is riser × tread ÷ √(riser² + tread²) = 6.667 × 10.5 ÷ 12.44 = 5.63 inches. A 2×12 is 11¼ inches deep, so 5.62 inches of solid wood remain. That passes the DCA 6 minimum of 5 inches.
5. Span and count. The 52½ inch horizontal span is inside the 72 inch (6 ft) limit, so no mid-span post is needed. At 48 inches wide and 18 inches maximum spacing, you need 4 stringers (about 16 inches apart).
6. Rails. Six risers is more than four, so a handrail is required. The open side of the stair is more than 30 inches above grade near the top, so it also needs a guard at least 34 inches high from the nosing line. With open risers, the gaps near the top are more than 30 inches above grade and must not let a 4 inch sphere through. Close in the risers or add a riser board or rail there.
| Result | Value | Check |
|---|---|---|
| Risers | 6 at 6 11/16 in | Pass (7¾ max) |
| Treads | 5 at 10½ in | Pass (10 min), nosing needed |
| Stair angle | 32.4° | — |
| Stringer | 66.0 in, cut from an 8 ft 2×12 | — |
| Throat | 5.62 in | Pass (5 min) |
| Horizontal span | 52½ in | Pass (72 max) |
| Stringers | 4 for 48 in wide | 18 in max spacing |
The stair stringer calculator runs the same steps and checks each one against the code as you type. Entering 40 in rise, 52.5 in run and 48 in width reproduces this table.
Laying it out on the board
- Clamp stair gauges on a framing square. Put one at the riser height on the tongue (the short leg), here 6 11/16 inches. Put the other at the tread depth on the blade (the long leg), here 10½ inches.
- Sight down the 2×12 and put the crown up. That is the slight bow along its length. Start a few inches in from the end.
- Mark each notch. Hold the gauges against the top edge, trace the riser and tread, and slide the square along so the next notch starts where the last tread line ends. Mark all 6 risers and 5 treads.
- Mark the bottom drop. Cut the bottom of the stringer down by one tread thickness. Without this, the first riser comes out taller than the rest once the treads go on. Measure your actual tread stock; don’t assume.
- Mark the top for how it attaches. The top cut depends on whether the stringer hangs from a hanger on the rim joist or sits below the deck surface. Work out where the top tread line lands relative to the deck surface before cutting.
- Cut, then test. Finish the inside corners of the notches with a handsaw rather than overcutting with a circular saw, which eats into the throat. Set the first stringer in place, check every riser with a level, then use it as the pattern for the rest.
Common mistakes that fail inspection
Measuring the rise to the wrong surface. The rise is from finished surface to finished surface. On a deck, that means from the deck boards down to the finished landing, not to the dirt. A paver or concrete landing added later makes the bottom step shorter. Indoors, ¾ inch of hardwood added at the top makes the top riser ¾ inch taller than the rest. That alone is twice the 3/8 inch allowance.
Measuring the rise straight down from the deck. The bottom tread lands out in the yard, 4 to 5 feet away. If the ground slopes away, the real rise is bigger than what you measured at the rim. Run a straightedge and level out to the landing spot and measure there. If the ground falls 2 inches over that distance, this example’s rise is 42 inches: still 6 risers, but each is 7 inches, not 6 11/16.
Forgetting the bottom drop. Skip it and the first riser is taller than the others by the tread thickness. With 1 inch treads, that is well over 3/8 inch.
Overcut notches. A circular saw run past the line on the inside corner cuts into the throat. On a stringer that barely clears 5 inches, that can be the failing cut.
Spanning too far. Long cut stringers bounce, which is why DCA 6 limits them to 6 ft of horizontal span. Add a post on a footing or a landing.
Landing on a thin pad. A stringer resting on an ordinary patio slab can move with frost. DCA 6 wants the bearing below 12 inches or the frost line.
Uneven treads from a twisted board. One stringer cut from a crooked 2×12 throws every tread out of level. Pick straight stock and pattern-cut from one tested stringer.
When to call the building department or a pro
Many jurisdictions require a permit for a new stair, and a deck stair is normally reviewed with the deck. Call before you buy lumber if:
- You don’t know which IRC edition your jurisdiction uses or what it amended. States and cities adopt codes on their own schedules, and some add local deck rules on top of DCA 6.
- The stair rises more than 12 ft 7 in in one flight (the IRC needs a landing), or more than 12 ft total on a deck (DCA 6 requires an intermediate landing).
- The stair turns, uses winders, or is a spiral. These have their own rules not covered here.
- The deck itself is old, or the ledger and rim the stair hangs from show rot or rust.
An inspector would rather answer a phone call than fail a finished stair. Ask what they check at framing, and whether they want to see the stringers before the treads go on.