Measuring the deck height
Measure from the top of the decking straight down to the ground where the bottom step will land, not at the house. Ground usually falls away from a deck, so the rise at the landing spot can be inches more than at the ledger. A level or a straight board laid out from the deck to the landing point does the job.
Deck stair stringer rules (DCA 6)
The American Wood Council's deck guide sets these rules for outdoor wood stairs[1]:
| Item | Requirement |
|---|---|
| Stringer size | 2x12 minimum |
| Cut stringers | at least 5 in of throat; horizontal span at most 6 ft |
| Solid stringers | span at most 13 ft 3 in; treads on 2x4 ledgers |
| Stringer spacing | cut stringers at most 18 in apart; solid stringers up to 36 in with ledgers |
| Stair width | at least 36 in |
| Riser and tread | riser at most 7-3/4 in; tread at least 10 in |
| Landing | required when the total rise exceeds 12 ft; at least 36 in deep |
| Lighting | a light at the top landing, switched from inside the house |
Treads on cut stringers bear on every stringer, which is why the spacing limit follows the tread's ability to span. With the defaults (48 in wide, 18 in max) the stair takes 4 stringers.
Handrails and guards on deck stairs
- A handrail is required on stairs with four or more risers, 34 to 38 in above the nosings[2].
- A guard is required on the open side of a stair more than 30 in above the ground, at least 34 in high, with openings that stop a 4-3/8 in sphere; the triangle under the bottom rail must stop a 6 in sphere[2].
- The 2024 IRC moved these rules to R320 and R321. It also says an exterior stair to grade of three or fewer risers serving a deck, porch or patio, if it is not the required exit, needs a bottom landing at least 36 in wide[3].
Footings and the bottom landing
DCA 6 sets stair footings at least 12 in below grade or below the frost line, whichever is deeper[1]. A paver pad or concrete landing at least as wide as the stair gives the stringers a level, dry bearing. Cut the stringers with the stair stringer calculator for full layout marks, check the overall stair in the stair calculator, and order the frame from the deck calculator.
Frequently asked questions
What does a deck stair stringer calculator need?
The deck height to grade, the tread boards you will use and the stair width. It returns the risers, the run, the 2x12 stringer length and how many stringers fit at 18 in spacing.
How many steps do I need for a 3 foot deck?
A 36 in rise needs 5 risers of 7-3/16″ and one fewer tread.
How far apart should deck stair stringers be?
No more than 18 in for cut stringers under DCA 6. Solid stringers with tread ledgers can be up to 36 in apart.
Do deck stairs need a handrail?
Yes, once the stair has four or more risers. The rail goes 34 to 38 in above the nosings.
What size lumber for deck stair stringers?
2x12, with at least 5 in left below the notches on cut stringers.