To build deck stairs, you measure the total rise from the deck surface to the landing below, divide it into equal risers no taller than 7-3/4”, and give each tread at least 10” of depth. Then you cut stringers, space them for your tread material, and set the bottom on a solid landing. The math is simple. Most failed inspections come from small things: uneven risers, a short bottom step, or stringers spaced too far apart for composite treads.
This guide walks through the layout the way a builder does it on site. The numbers come from the International Residential Code (IRC) and the American Wood Council’s deck guide. Your local building department may amend them, so check before you cut.
The code numbers that control stair layout
Most US jurisdictions build their deck rules on the IRC. These are the stair limits you will design around:
| Item | Typical IRC requirement | Source |
|---|---|---|
| Riser height | 7-3/4” maximum | IRC R311.7.5.1 |
| Riser variation | Tallest no more than 3/8” over the shortest in a flight | IRC R311.7.5.1 |
| Tread depth | 10” minimum, nosing to nosing, with 3/8” maximum variation | IRC R311.7.5.2 (Placer County handout) |
| Nosing | 3/4” to 1-1/4” projection, not required when treads are 11” or deeper | IRC R311.7.5.3 |
| Stair width | 36” minimum clear width | IRC R311.7.1 (Placer County handout) |
| Landing | Required at top and bottom, at least 36” deep in the direction of travel | IRC R311.7.6 (Placer County handout) |
| Handrail | Required on at least one side with 4 or more risers, 34” to 38” high | AWC DCA 6, p. 22 |
The 3/8” variation rule is the one that catches people. It applies to the bottom step too. If your landing pad ends up an inch higher than planned, the bottom riser comes out short and the whole flight fails.
Step 1: Measure the total rise correctly
Total rise is the vertical distance from the top of the finished deck surface to the top of the finished landing where the bottom tread will land. It is not the height of the deck at the house wall. On sloping yards the ground can drop several inches between the rim joist and the spot where the stairs end.
The reliable way to measure it:
- Lay a straight board or level off the deck surface, extending out past where the bottom step will land. Most stairs project 3 to 6 ft from the deck.
- Level it, then measure straight down to grade or to the planned top of the concrete pad.
- If you will pour a pad or set pavers, measure to the finished top of that surface, not to the dirt.
The run of the stairs depends on the rise, and the rise depends on where the stairs end. So you may need to estimate, measure, and adjust once.
Step 2: Work out risers and treads
Divide the total rise by 7-3/4” and round up. That gives the minimum number of risers. Then divide the total rise by that number to get the actual riser height.
Worked example, 30” total rise:
- 30 ÷ 7.75 = 3.87, so round up to 4 risers.
- 30 ÷ 4 = 7-1/2” per riser.
- Treads are always one fewer than risers, because the deck surface acts as the top “step.” So 4 risers means 3 treads.
- With treads 11” deep, total run = 3 × 11 = 33”.
Many builders prefer a slightly lower riser, around 7” to 7-1/2”. If you add a fifth riser to the example (30 ÷ 5 = 6”) you get a gentler stair, but it also needs a fourth tread and an extra 11” of run. Both meet code; the choice depends on your yard.
Four risers also triggers the handrail requirement in our example. Plan for it now rather than discovering it at inspection. Our deck railing height and baluster spacing guide covers the handrail and guard details.
The deck stair calculator runs this math for you. It also flags a riser count that would push you over 7-3/4” or a run that would leave treads under 10”.
Step 3: Pick tread boards and size the tread
Deck stairs usually use the same boards as the deck surface. Two 5-1/2” boards with a small gap give a tread just over 11” deep. That clears the 10” minimum and the 11” nosing exception. That is why two boards per tread is the default on most decks.
A few material notes:
- Wood treads. The American Wood Council’s DCA 6 guide (Figure 29) allows 2x or 5/4 treads on cut stringers spaced no more than 18” apart. Each tread gets two 8d threaded nails or two #8 screws at least 2-1/2” long at every stringer.
- Composite treads. Composite boards are less stiff than wood, so manufacturers require closer stringers. Fiberon’s install guide calls for 12” on center for solid boards and 9” for scalloped boards. TimberTech’s guide lists 9” to 10” for most of its lines. Always use the chart for the exact product you buy.
- Gaps. Fiberon specifies a 1/8” gap between tread and riser and 3/16” between tread boards. Water has to drain off a stair just like it does off the deck.
When you count boards, remember the stairs. The deck board calculator can add stair tread boards to your main decking order, so you buy one batch from the same production run and the color matches.
Step 4: Cut and space the stringers
Stringers carry the stairs. Two types are common on decks:
- Cut (notched) stringers. A 2x12 with the step pattern cut into the top edge. DCA 6 requires stringers to be at least 2x12. It also requires at least 5” of solid wood left below the notches (the “throat”), and it limits a cut stringer to a 6’-0” horizontal span.
- Solid stringers. An uncut 2x12 with treads supported on cleats or ledgers. DCA 6 allows these to span up to 13’-3” and to sit up to 36” apart with suitably sized treads (DCA 6, Figures 28 and 29).
If a cut stringer would span more than 6 ft, DCA 6 allows a 4x4 post on its own footing partway down, or an intermediate landing. Once a stair rises more than 12 ft total, it requires an intermediate landing.
How many stringers you need
Divide the stair width by the allowed spacing, round up, and add one:
| Stair width | Wood treads (18” max) | Solid composite (12” o.c.) | Scalloped composite (9” o.c.) |
|---|---|---|---|
| 36” | 3 | 4 | 5 |
| 48” | 4 | 5 | 7 |
| 60” | 5 | 6 | 8 |
DCA 6 also says a stair built only with cut stringers needs at least three, even at the 36” minimum width.
Laying out a cut stringer
Use a framing square with stair gauges (small clamps) set at your riser and tread dimensions. In the example that is 7-1/2” and 11”. Mark each step along the top edge of the 2x12, then make two adjustments:
- Drop the bottom. Cut the bottom of the stringer shorter by the thickness of one tread board. Otherwise the first step comes out taller than the rest by that amount. This is the most common layout mistake.
- Account for the top connection. Decide whether the top tread sits one riser below the deck surface, or whether the stringer hangs from the rim with the deck acting as the top step. Lay out the top cut to match.
For a 30” rise and 33” run, the stringer’s sloped length is about 45” (the square root of 30² + 33²). Add the extra length for the top and bottom cuts, and a 6 ft 2x12 is plenty. Cut one stringer, test-fit it, then use it as the pattern for the rest.
Step 5: Attach the top and support the bottom
At the top, DCA 6 shows stringers hung from the rim or outside joist with sloped joist hangers rated for at least 625 lb vertical capacity. Toe-nailing stringers to the rim is not an acceptable substitute.
At the bottom, the stringers need firm bearing that will not settle or heave:
- Per DCA 6, footings for stair posts must sit at least 12” below undisturbed ground or below the frost line, whichever is deeper.
- DCA 6 also says stringers should not bear on concrete pads or patios unless the pad itself is founded below that depth. A thin pad poured on grade can heave in a freeze and rack the stairs.
- Pressure-treated stringers within a few inches of soil may need ground-contact treatment. The pressure-treated lumber guide explains the UC4A rule.
If your area has deep frost, the deck footings and frost depth guide covers how deep to go.
Guards, handrails and lighting
Stairs are a walking surface, so the guard rules apply to them as well. The IRC requires guards on open sides of walking surfaces, including stairs, that are more than 30” above grade within 36” horizontally (IRC R312.1.1, Simpson Strong-Tie code reference). Stair guard openings get a slightly larger 4-3/8” sphere limit, and the triangle under the bottom rail gets 6”.
DCA 6 also calls for a light at the top landing that illuminates every step, switched from inside the house or controlled by a motion or timer switch.
Before you cut, confirm every number with your building department. Codes are adopted and amended locally, and inspectors interpret details differently. This guide explains common practice and is not engineering advice.
Frequently asked questions
How many steps do I need for a 30 inch deck?
With a 7-3/4” maximum riser, a 30” rise needs 4 risers of 7-1/2” each and 3 treads. With 11” treads that is a 33” total run. You could use 5 risers of 6” for a gentler stair, which adds one tread and about 11” of run.
Can deck stairs land directly on grass or gravel?
The bottom of the stringers needs stable support that will not settle or heave. DCA 6 calls for posts on footings below the frost line or at least 12” below undisturbed ground. A landing of compacted gravel or pavers often serves as the walking surface at the bottom, but ask your inspector what they accept for stringer support.
How far apart should stair stringers be for composite treads?
Closer than for wood. Fiberon publishes 12” on center for solid boards and 9” for scalloped boards, and TimberTech lists 9” to 10” for most lines. Wood 2x or 5/4 treads can span up to 18” under DCA 6. Use the chart for your exact product, because spans differ between lines from the same brand.
Do I need a handrail on deck stairs with three steps?
Under the IRC, a handrail is required once a stair has four or more risers. Three treads usually means four risers, so most three-step deck stairs do need one. Count risers, not treads, and confirm with your local building department, since some jurisdictions amend this rule.