Deck Stair Code Requirements (IRC 2021)
Most US jurisdictions build their residential stair rules on the International Residential Code (IRC). This checklist uses the 2021 IRC, Section R311.7 for stairs and R312 for guards. Many cities and counties adopt the code with amendments, so treat this as the baseline and confirm with your building department before you build.
Quick reference
| Requirement | 2021 IRC | Section |
|---|---|---|
| Maximum riser height | 7¾” | R311.7.5.1 |
| Minimum tread depth | 10” | R311.7.5.2 |
| Riser and tread uniformity | Largest within ⅜” of smallest | R311.7.5.1, R311.7.5.2 |
| Minimum stair width | 36” | R311.7.1 |
| Minimum headroom | 6’ 8” | R311.7.2 |
| Maximum rise per flight | 151” (12’ 7”) | R311.7.3 |
| Handrail required | 4 or more risers | R311.7.8 |
| Handrail height | 34”–38” above the nosings | R311.7.8.1 |
Risers
Risers can be no taller than 7¾”, and within one flight the tallest may exceed the shortest by no more than ⅜”. That uniformity rule is why the bottom of the stringer has to be “dropped” by one tread thickness. See how to cut stair stringers.
Open risers (no riser boards) are allowed, but on stairs with a total rise over 30”, the gap between treads must not let a 4” sphere pass through. With 7”+ risers and 1” treads that gap is usually too big, so tall open-riser stairs often need a riser board or a horizontal strip in each gap.
Treads and nosings
Treads must be at least 10” deep, measured from the front of one tread to the front of the next. Where the tread depth is less than 11” and the risers are solid, the code requires a nosing of ¾” to 1¼” projecting past the riser below. Many builders simply make the run 11” or more, or let the deck boards overhang about 1”.
Width and headroom
The clear width must be at least 36” above the handrail height. Headroom of 6’ 8” is measured vertically from the sloped line of the nosings, which matters when stairs run under a second-floor deck or roof.
Handrails
A handrail is required on at least one side when the stair has four or more risers.
- Height: 34”–38” measured vertically from the line of the tread nosings
- Must be continuous for the full length of the flight
- Must be graspable: a round rail typically 1¼”–2” in diameter, or a shape the code defines as equivalent. A flat 2x6 cap on top of a guard is not a graspable handrail.
Guards on open sides
If an open side of the stair is more than 30” above the ground below, it needs a guard. On stairs the guard can be as low as 34” measured from the nosing line, but the openings must block a 4⅜” sphere. The triangle formed by the tread, riser and bottom rail can’t pass a 6” sphere.
Landings
There must be a landing at the top and bottom of each flight, at least as wide as the stair and 36” deep in the direction of travel. On a deck the deck surface is the top landing. At the bottom, plan a firm, level surface such as a concrete pad or pavers rather than bare soil.
Long flights
A single flight can rise at most 151”. Anything taller needs an intermediate landing. Long cut stringers may also need mid-span support. Check the span limits in the American Wood Council’s Prescriptive Residential Wood Deck Construction Guide (DCA 6), which many jurisdictions accept for deck construction.
Lighting
The IRC requires exterior stairs to have a light source at the top landing. Many owners add step lights as well.
Check your layout
The stair stringer calculator checks riser height, tread depth, width, flight rise and stringer throat against these rules as you type, and flags when a handrail is required.