How wide do deck stairs have to be and how far apart can stringers go?

Width, and what it is measured between

860 mm (34 in) is the minimum for a flight serving a house. It is measured between the parts that stick into the stair, so a handrail projecting into the run counts against it, as does a heavy guard post set inside the stringer.

Most deck stairs get built at 3 ft or 4 ft, because those are stock widths and a 4 ft stair takes a person and a laundry basket at the same time. Once you pass 1.1 m (43 5/16 in) you owe a handrail on both sides, which is a reason a lot of deck stairs stop at 42 in.

Why the stringer is always a 2x12

A stringer starts as a plank and loses material at every notch. What the code cares about is what is left. At least 90 mm (3 1/2 in) of effective depth behind the cut, and at least 235 mm (9 1/4 in) of overall depth measured perpendicular to the bottom of the stringer.

Work it through with a 7 in rise and a 10 1/2 in run on a 2x10, which is 9 1/4 in deep. The diagonal depth left behind the notch comes out under 3 1/2 in, so it fails. On a 2x12, which is 11 1/4 in deep, it passes. That is the whole reason deck stair stringers are 2x12, and it is worth knowing so you do not accept a 2x10 stringer because it looked fine on the plan.

Stringer spacing

• Up to 900 mm (35 7/16 in) apart as a plain open riser stair. A 4 ft wide stair with stringers at each end and one in the middle sits at 24 in, which is comfortable.
• Up to 1.2 m (47 3/16 in) apart where riser boards support the front of the treads. The riser board turns two stringers and a tread into a box, which is stiffer than it looks.
• Composite and PVC treads are a separate matter. They deflect more than lumber and their manufacturer usually calls for stringers closer together, often 12 in for a diagonal or 16 in for a square lay. That instruction governs, not the lumber number.

Top and bottom

Stringers have to be well supported at both ends. At the top, a proper stringer hanger or connector is more robust than nails through the rim, and inspectors look for one. At the bottom, the stringer is either on a concrete pad or on treated material, because bare wood in soil rots at the cut end first.

Exterior stair parts are isolated from direct ground contact unless they are treated with a preservative. That is the same rule that governs the deck posts, and it is covered in the decay resistance answer.

Four things to check on a stair you did not build

1. Stand on the middle of a tread halfway down. If it moves, the stringers are too far apart or a tread fastener has let go.
2. Look at the bottom of the stringers. Dark, soft wood at the cut end means it has been sitting in soil.
3. Check the top connection. Nails through a rim board into the end grain of a stringer is the most common weak point after the ledger.
4. Measure the top and bottom risers. If either is out by more than about 10 mm (7/16 in) from the rest, the stringer was laid out without deducting the tread.