How Many Stringers Does a Staircase Need?
The count, by width and tread
| Tread material | Max stringer spacing | 36 in stair | 48 in stair |
|---|---|---|---|
| 2x lumber treads (1.5 in thick) | 16–18 in | 3 | 4 |
| 5/4 decking treads (1 in) | 12 in | 4 | 5 |
| Composite treads | 9–12 in, per manufacturer | 4–5 | 5–7 |
The arithmetic is the same one that spaces deck joists: stringers = ceiling( stair width ÷ max spacing ) + 1 — supports at both edges, then enough between them that no tread span exceeds its rating. A tread is just a very short deck board, and the spacing limit belongs to the tread, not the stringer: put composite treads on three stringers built for 2x lumber and every step becomes a diving board.
Why the middle stringer is not optional
A 36 in stair with only edge stringers asks each tread to span a full yard under a foot placed, naturally, dead centre. Solid 2x12 treads can technically carry it, but the bounce telegraphs through the whole flight and loosens fasteners over time — and stair feel matters, because people trust a stair with their balance. Three stringers cut the clear span roughly in half, and the middle one carries the traffic line where feet actually land.
Cut stringers must start as 2x12
A cut stringer loses its notches: with a 7.5 in rise and 10 in run sawn out of a 2x12, the remaining solid wood at the throat is only about 5 in deep — that 5 in strip is the beam carrying the flight. Start with anything smaller than a 2x12 and the throat goes below safe depth, which is why every stringer table and inspector expects 2x12 stock, ideally with the growth rings crowning up and knots avoided at the throat line.
Solid (housed) stringers — where treads sit in routed dadoes or on cleats between two uncut boards — keep their full depth and can work as a pair on narrow flights, which is the traditional basement-stair pattern. The moment you want the classic sawtooth profile, count on cut stringers and the table above.
Getting the count to hold up
- Hang the top properly. A stringer hanger or a ledger under the stringer seat — not screws through the back into rim board end grain.
- Land the bottom on something. A concrete pad or footing, with the stringer feet anchored so the flight cannot kick out. Grade-contact lumber where wood meets concrete.
- Match riser count to the plan. The stringer count sets the width; the stair calculator sets everything else — riser height, tread depth, stringer length and the angle — from your total rise.
Material note: each stringer for a typical flight is one 2x12 at 12 to 16 ft, so the third or fourth stringer is a 30 to 50 dollar decision on a structure people fall down when it is wrong. Round up.
Enter the total rise — floor to floor, not floor to ceiling — and get the riser count, the exact riser height, the total run, the stringer length and the angle, checked against the usual residential limits.
Frequently asked questions
Are two stringers ever enough?
Only on narrow utility stairs around 24 to 30 inches wide with thick solid treads, or with housed stringers where the treads sit in routed dadoes. For a standard 36 inch stair with cut stringers, three is the floor.
Do stair stringers have to be 2x12?
Cut stringers do — after the notches are sawn out, only about 5 inches of solid depth remains at the throat, and starting smaller leaves too little. Uncut housed stringers keep their full depth and have more latitude.
What spacing do composite stair treads need?
Most brands require support every 9 to 12 inches on stairs — tighter than the 16 inch deck spacing, because treads take concentrated foot strikes. The exact figure is in the manufacturer installation chart and it is a warranty condition.
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