BuildCalc Guides · Stairs
How Many Stair Stringers Do I Need?
Two outside stringers and one in the middle is the default — and it's right for most decks. Here is when you need more, and why.
The math
The count comes from stair width divided by spacing, plus one for the second outside stringer:
- Step 1: width ÷ spacing. 36 ÷ 16 = 2.25.
- Step 2: round up + 1. Round 2.25 up to 3 (number of spaces), then count both edges → still 3 stringers because the two ends share the count.
- Step 3: verify spacing. Stringers at 0, 18 and 36 = 18 inch OC — too wide. Reset to 0, 16, 32, 48 to keep all spaces ≤ 16 inch. For 36 inch wide, that means 3 stringers at 0, 18 and 36 OR 4 stringers if you are strict about 16 inch max.
- In practice: 3 stringers for stairs 30 to 42 inches wide with 5/4 or 2× treads. 4 for stairs 42 to 60 inches. Add one for 12 inch OC.
By stair width and spacing
Most residential decks land on 36 to 48 inch wide stairs with 16-inch OC stringers. Composite tread manufacturers (Trex, TimberTech) often require 12-inch OC — check your product's installation guide.
| Stair width | 16 in OC | 12 in OC |
|---|---|---|
| 24" | 2 | 3 |
| 30" | 3 | 3 |
| 36" | 3 | 4 |
| 42" | 3 | 4 |
| 48" | 4 | 5 |
| 60" | 5 | 6 |
What changes the count
- Tread thickness. 5/4 decking (~1 inch) and 2× lumber (1.5 inch) handle 16 inch OC. 1× boards or thin composites often need 12 inch OC.
- Composite tread spec. Trex Transcend allows 16 inch OC on stairs perpendicular to the joist, 12 inch OC on diagonal. TimberTech AZEK requires 10 to 12 inches. Check the spec sheet.
- Stair length. Tall staircases (more than 12 risers) develop sway. A middle stringer also resists lateral bow — never skip it on a long stair.
- Stringer span. Stringers should not span more than ~13 feet between top and bottom support. A long run may need an intermediate landing.
Other materials per stair
- Treads: one piece per riser. A 6-step stair needs 6 treads.
- Risers (closed stairs): one per riser, ~7 to 7.75 inch tall.
- Joist hangers or stringer brackets: one Simpson LSC per stringer at the top connection.
- Screws/nails: 6 to 8 fasteners per tread (2 per stringer crossing), plus framing fasteners.
Lay out your stair
BuildCalc's Visual Stair Calculator generates a stringer cut diagram from any height and run, checks risers against the IRC (7.75 inch max), and accounts for tread thickness drop on the bottom step.
Open the stair calculator →FAQ
How many stringers do I need for a 36 inch wide staircase?+
3 stringers at 16 inches on-center: two on the outside and one in the middle. That handles standard 5/4 (1 inch actual) treads or 2× decking. With composite treads, check the manufacturer spec — some require 12 inch on-center, which would push you to 4 stringers.
How many stringers do I need for a 48 inch wide staircase?+
4 stringers at 16 inches on-center: outside, 16 in, 32 in, outside. For 5/4 boards on 12 inch OC, you would need 5 stringers.
How far apart should stair stringers be?+
16 inches on-center is the safe default for most decking and 2× treads. 12 inches on-center is required for thinner composite treads, longer span treads, or stairs that need to feel solid underfoot with no bounce. Spacing wider than 18 inches will let the tread flex under load.
Can I use 2 stringers instead of 3?+
For very narrow stairs (under 30 inches) and short runs (3 to 4 risers), two outside stringers can work. For anything 36 inches or wider or more than 4 risers, you need a middle stringer — otherwise the treads bow in the middle and the stair feels unsafe.
What size lumber should stringers be?+
2×12 is the standard. A 2×10 leaves too little wood between the notches once you cut the treads and risers (need at least 5 inches of "throat" remaining for structural strength). Pressure-treated for any stair touching the ground or exterior.
Do stringers need to be attached at the top and bottom?+
Yes — top of the stringer to the rim joist or ledger with metal joist hangers, structural screws, or a stringer-rated hanger like the Simpson LSC. Bottom to a concrete pad or buried block with appropriate anchors. Floating stringers are not code-compliant.
Related guides
- How to calculate stair stringers — the rise/run/cut math for the stringer itself.
- How many deck screws per board? — for the treads and the deck on top.