Deck stair rules (IRC) in plain English
Most deck inspection failures happen at the stairs. These are the IRC numbers and the deck-guide practices that apply, in one place.
The numbers
| Rule | Requirement | Reference |
|---|---|---|
| Riser height | 7¾″ maximum | IRC R311.7.5.1 |
| Tread depth | 10″ minimum, nosing to nosing | IRC R311.7.5.2 |
| Uniformity | Risers and treads within ⅜″ of each other in a flight | IRC R311.7.5 |
| Nosing | ¾″ to 1¼″ where treads are under 11″ deep | IRC R311.7.5.3 |
| Width | 36″ minimum clear | IRC R311.7.1 |
| Headroom | 6′-8″ minimum | IRC R311.7.2 |
| Handrail | Required with 4 or more risers; 34–38″ high; graspable | IRC R311.7.8 |
| Guards | Open sides more than 30″ above grade; 34″ minimum on stairs | IRC R312.1 |
| Guard openings on stairs | 4⅜″ sphere; 6″ at the riser/tread/bottom-rail triangle | IRC R312.1.3 |
| Cut stringer spacing | 18″ o.c. maximum (closer if the tread product requires it) | AWC DCA 6 |
| Cut stringer throat | 5″ minimum of solid wood below the notches | AWC DCA 6 |
Some states and cities amend these. For example, a few allow 8¼″ risers. Your local code wins.
Uniform risers start with an exact measurement
Measure from the top of the decking to the finished landing surface where the stair will end, not to the ground at the deck edge. Divide by 7¾″ and round up to get the number of risers, then divide the rise evenly. The stair calculator does this and draws the stringer.
The top riser is measured to the deck surface and the bottom riser to the landing, so a stringer cut from the layout must be trimmed at the bottom by one tread thickness (“dropping the stringer”). Otherwise the bottom step comes out taller than the rest.
Stringers
- Cut from 2x12s. After notching, a 2x10 rarely leaves the 5″ throat.
- Space them at 18″ o.c. or less, and 12″ or less for most composite treads. Check the tread manufacturer.
- Cut stringers have limited span. DCA 6 limits them to about 6 ft horizontally (southern pine 2x12 about 7 ft). Longer stairs need intermediate support or a landing.
- Attach the top with hangers or a hanger board rated for the load, and sit the bottom on concrete or compacted gravel, not on soil.
Landings
The IRC requires a landing at the top and bottom of each stair, at least as wide as the stair and 36″ deep in the direction of travel. For a deck stair, the bottom landing is often a concrete pad or pavers on compacted base.
Lighting and graspability
Exterior stairs need a light that illuminates them. A 2x6 flat on top of the guard is not a graspable handrail. Add a round or shaped handrail within the size limits, returned at the ends.