How Many Stringers Do I Need for Deck Stairs?
The number of stringers depends on two things: how wide the stairs are and how far your tread material can span between stringers.
The rule
The American Wood Council’s deck guide (DCA 6), which many jurisdictions accept for deck construction, limits cut 2x12 stringers to 18” on center for typical wood treads. Some tread materials need closer spacing.
Stringers = (stair width ÷ spacing), rounded up, + 1
Stringer count by width
| Stair width | 18” spacing (wood treads) | 12” spacing | 9” spacing |
|---|---|---|---|
| 36” | 3 | 4 | 5 |
| 42” | 4 | 5 | 6 |
| 48” | 4 | 5 | 7 |
| 60” | 5 | 6 | 8 |
| 72” | 5 | 7 | 9 |
Composite and PVC treads
Composite and PVC deck boards are more flexible than wood, and most manufacturers require closer stringer spacing on stairs than on the deck itself, often 9”–12” on center. Spacing also tends to be tighter for commercial use. Check the stair section of your decking’s installation guide before you buy stringers. Too few stringers can void the warranty and make treads bounce.
Wood tread thickness
- 2x6 or 2x12 treads (1½” thick): comfortably span 18” on center.
- 5/4 deck boards (about 1” thick): usually fine at 16”–18” in pressure-treated pine. Some builders go to 12” for a stiffer feel, especially with softer species like cedar.
When stringers need more support
Cut stringers lose strength at every notch. Long flights may need a mid-span support such as a post and beam under the stringers. DCA 6 lists maximum spans for cut stringers by species. If your stringer is long (a total rise of 5 ft or more is a good point to check), look up the span limit for your lumber.
Solid (uncut) stringers with tread cleats or hangers carry more than cut stringers. They’re common on wide or long stairs.
Fastening
- Use two structural screws or nails per tread board at each stringer.
- Hang stringers at the top from stringer connectors or a hanger board on the rim, never by toenailing alone.
- Rest the bottoms on a level concrete pad or footing.
Get the full materials list
The stair stringer calculator works out the stringer count for your width, the 2x12 length to buy, how many tread boards you need, and the screw count. To lay out and cut them, see how to cut stair stringers.