Deck stair code
A deck stair that meets code can still be uncomfortable, and a stair that feels right can fail inspection by half an inch. The code numbers and the comfort numbers are different sets, and both matter.
The dimensions
| Element | Limit | IRC reference (2021) |
|---|---|---|
| Maximum riser height | 7¾″ | R311.7.5.1 |
| Minimum tread depth | 10″ | R311.7.5.2 |
| Riser height variation | 3/8″ maximum between any two risers | R311.7.5.1 |
| Nosing projection | ¾″ to 1¼″ | R311.7.5.3 |
| Minimum stair width | 36″ above the handrail height | R311.7.1 |
| Minimum headroom | 6′8″ | R311.7.2 |
| Handrail height | 34″ to 38″ above the nosing | R311.7.8 |
| Maximum rise per flight | 12′ 7″ between landings | R311.7.3 |
Exterior stairs have one additional consideration most interior rules do not: the bottom riser is often shorter than the rest because the landing is grade, and that difference is what trips people. The 3/8″ variation limit applies to it too.
The riser variation rule is the one that fails
Not the maximum riser, not the minimum tread — the variation. A stair where the first riser is 6″ and the rest are 7½″ is a trip hazard that an inspector will measure, and it is extremely common on decks because the grade at the bottom is never exactly where the plan says.
The fix is arithmetic: divide the total rise by the number of risers and build to that number, then adjust the bottom riser last by cutting the stringer or adding a landing. Do not adjust the top — the deck surface is fixed.
Code is not comfort
Code allows a 7¾″ riser with a 10″ tread, which is a 37.8° stair. That stair is legal and it is steep. The rule of thumb carpenters use is 2R + T = 24 to 25 inches:
- 7½″ riser + 10″ tread = 25″ — the steep end of comfortable
- 7″ riser + 11″ tread = 25″ — comfortable
- 6½″ riser + 12″ tread = 25″ — a gentle garden stair
Below 24 the stair feels cramped; above 25.5 it feels like a ramp and people trip on the tread depth. A 7.2″ riser with an 11″ tread lands at 25.4, which is where most well-built deck stairs end up.
Handrails and guards are different things
On a stair you may need both:
- A handrail is graspable, 34–38″ above the nosing, and required on at least one side of a stair with four or more risers.
- A guard is the barrier that stops a fall, 36″ minimum, required where the stair is more than 30″ above grade.
A 2×6 on edge is a guard, not a handrail — it cannot be gripped. A graspable rail is 1¼″ to 2″ in cross-section with a profile your hand closes around. On a deck stair that is open on both sides, you often need a guard on each side with a graspable rail attached to it.
What the code does not settle
- It does not replace your adopted code edition. Numbers above are 2021 IRC; check the edition your jurisdiction enforces.
- It does not design the stringer. Notch depth, board size and stringer count are structural; see the stringer calculator.
- It does not cover interior stairs, which have different headroom and width rules.
- It does not cover accessibility ramps, which are governed separately and are much more prescriptive.
Questions people actually ask
What is the maximum riser height for deck stairs?
7¾ inches under the 2021 IRC, with no more than 3/8 inch variation between any two risers in the flight. Most comfortable deck stairs use 7 to 7½ inches.
What is the minimum tread depth for outdoor stairs?
10 inches under the IRC. Many builders use 11 inches because it lands the 2R+T comfort rule closer to 25.
Do deck stairs need a handrail on both sides?
A handrail is required on at least one side of a stair with four or more risers. Where the stair is open on both sides and more than 30 inches above grade, guards are required on both sides and each needs a graspable rail.
Can I use a 2x6 as a stair handrail?
No. A 2×6 on edge is a guard, not a graspable handrail. The rail must be 1¼ to 2 inches in cross-section so a hand can close around it.