Deck Stairs: How Many Steps, and What the Code Requires

Deck stairs are the part of a deck that fails inspection most often, and almost always for the same three reasons: the risers are not uniform, the landing at the bottom is missing or too shallow, and the footing at grade was never poured to the right depth. None of those is a hard calculation. They are all covered plainly in IRC 2021 Chapter 3, and the numbers are worth knowing before you buy lumber rather than after.

How many steps for a deck?

The step count comes from the total rise — the vertical distance from the top of the deck surface to the finished grade where the stairs land — divided by the maximum riser the code allows. In the base IRC that maximum is 7¾ inches, so the arithmetic is:

1. Measure the total rise. From the top of the decking down to the grade the bottom step sits on. Not to the top of the stringer, not to the bottom of the rim — to the surface people will walk on.
2. Divide by the riser limit. risers = total rise ÷ 7.75, then round up. A rise of 105 inches gives 105 ÷ 7.75 = 13.5, so 14 risers.
3. Divide the rise by the number of risers. 105 ÷ 14 = 7.50 inches per riser. Every riser in the run gets that number — you do not round each one.
4. You have one fewer tread than risers. The top "riser" is the step up onto the deck surface, so 14 risers means 13 treads plus the deck itself. Getting this wrong is the single most common cause of a stringer that comes out one step short.

The riser you end up with (7.50 inches here) is a design choice inside the limit, and most people aim for something between 7 and 7½ inches because it is a comfortable stride. Shallow risers with deep treads feel better than tall risers with shallow treads, even when both are legal.

Riser and tread limits — IRC 2021 R311.7.5.1 and R311.7.5.2

R311.7.5.1 caps the riser at 7¾ inches and requires that the greatest riser in a flight is no more than 3/8 inch taller than the smallest one. Open risers must not pass a 4 inch sphere. R311.7.5.2 sets the tread at a minimum of 10 inches, measured nosing to nosing, with the same 3/8 inch maximum variation between treads.

The variation rule matters more than the maximum. A flight of stairs with 7″ risers throughout passes; a flight with thirteen 7″ risers and one 7½″ riser can fail, because the odd step is the one people trip on. On a stringer you cut yourself, the only way to guarantee uniform risers is to divide the total rise evenly before you lay out the notches — which is exactly what the stair stringer calculator does, and what the stair math guide walks through by hand.

R311.7.5.3 governs the nosing: a projection of between ¾ inch and 1¼ inch past the riser face, with a radius no more than 9/16 inch or a bevel no more than ½ inch. A nosing is not required at all where the tread is at least 11 inches deep.

Note what that table is not: it is not a claim about which code your jurisdiction has adopted. It is four examples of amendments that genuinely exist, to make the point that the riser figure is a jurisdiction-by-jurisdiction number and not a universal one. Look up your own state's adoption before you commit to a layout.

Width, headroom and how far one flight may rise

Three rules sit above the geometry and are easy to forget on a deck because the stairs are attached to something else. R311.7.1 requires a clear width of at least 36 inches, which may narrow to 31½ inches at and below handrail height where there is a handrail on one side, or 27 inches where there are handrails on both. R311.7.2 requires 6 feet 8 inches of headroom measured vertically from the sloped plane of the nosings — the header or deck rim over the bottom of the stairs is the usual place this gets tight.

R311.7.3 limits a single flight to 12 feet 7 inches of vertical rise. That is the rule that forces a landing on a tall deck: once the total rise passes 12′ 7″, you are no longer building one flight, you are building two, and the intermediate landing has to meet the landing requirements below.

The American Wood Council's prescriptive deck guide (DCA 6) is stricter still: it requires an intermediate landing once the total height of a stairway exceeds 12 feet. So on a tall deck, build to the 12-foot figure and you satisfy both documents — a flight designed to 12′6″ of rise is legal under R311.7.3 and caught by DCA 6.

Stringers: length, depth, and when you need a landing

A stair stringer is a sloped beam, and its length is pure right-triangle geometry:

stringer length = √(total rise² + total run²)

where the total run is the number of treads times the tread depth. A flight with a 105 inch rise and a 120 inch run gives √(105² + 120²) = √(11025 + 14400) = √25425 = 159.5 inches, or about 13 feet 3½ inches of stringer — before you add anything for the bearing at each end. That number is the one you buy lumber against, and it is what the stringer calculator returns along with the notch layout.

Cut stringers are weaker than solid ones because the notches remove material along the depth of the beam. The American Wood Council's Prescriptive Residential Wood Deck Construction Guide (DCA 6) analyses cut stringers at an effective depth of 5.1 inches, which is derived from the standard 7¾:10 rise-to-run ratio, and tabulates solid stringer spans only up to 13 feet 3 inches of horizontal run. Past that span the guide expects a mid-span support or a landing — which lines up neatly with the 12′7″ rise limit in R311.7.3, since a run that long puts you into landing territory anyway.

On how many stringers a given width needs: the IRC does not prescribe a stringer spacing. Treat the manufacturer's instructions for your tread material as governing — which is what 110.3(B) requires regardless — and remember that a guard post landing on the outside of a stair has to be anchored into framing, not into a tread.

How many stringers does a deck stair need?

This is the question that gets the most contradictory answers, because the IRC genuinely does not set a stringer count or spacing — the tread manufacturer's instructions govern, and 110.3(B) makes them binding. In the gap sits the American Wood Council's Prescriptive Residential Wood Deck Construction Guide (DCA 6), which is what a lot of inspectors and deck builders work from. Its stair provisions are specific enough to answer the question directly:

• If only cut (notched) stringers are used, a minimum of three are required — at any width, so the treads are not cantilevered off two outer stringers.
• For a stair wider than 36 inches, cut and solid stringers may be combined, spaced at a maximum of 18 inches on center. That is the number to build to — 18 inches on center, not "a stringer at each end".
• Every stringer is a minimum 2×12 nominal, cut or solid. The notches are what reduce capacity, not the lumber size.

Run that on the two widths people actually build. A 36-inch stair: 36 ÷ 18 = 2 spaces → three stringers, which is both the code minimum and the practical answer. A 48-inch stair: 48 ÷ 18 = 2.67, round up to 3 spaces → four stringers. A guarded stair is a different animal from a bare one, because the guard post carries bending load: DCA 6 wants that post bolted through the stringer with a bearing block, not lagged to a tread.

Two more DCA 6 numbers decide the layout, and both are commonly misstated. Span: a solid 2×12 stringer is limited to a horizontal span of 13 feet 3 inches, measured along the run rather than up the slope, and a cut or notched stringer to roughly 6 feet. Past that, the guide expects a 4×4 mid-span post — notched and bolted to the stringer with two ½-inch through-bolts, centered on a 12-inch diameter or 10-inch square, 6-inch thick footing — or an intermediate landing. Tread stock: DCA 6 Table 6 sets the minimum at 2×4 or 5/4 for a cut stringer, and for a solid stringer 2×8 with Southern Pine, 2×8 or 3×4 with DF-L, hem-fir or SPF, and 2×10 or 3×4 with redwood, the cedars and the pines — all based on a 300 lb concentrated load at L/288.

Landings — R311.7.6

A landing is required at the top and bottom of every stairway. The requirement that gets missed is the depth: at least 36 inches measured in the direction of travel, and at least as wide as the flight. A 24 inch pad at the bottom of the stairs looks like a landing and is not one. Landings also have to be within 2% slope under R311.7.7, which relaxes to 5% only where the code requires the landing to drain.

Practically, the bottom landing is where the deck stairs meet grade, so it is also where the frost rules below decide how much concrete you are pouring.

Handrails — R311.7.8 — and guards — R312.1

A handrail is required on at least one side of any flight with 4 or more risers, at a height of 34 to 38 inches, measured vertically from the sloped plane adjoining the tread nosings. The handrail is about grip; the guard is about not falling, and they are different requirements that happen to sit on the same posts.

R312.1.1 requires a guard on the open sides of a walking surface — including stairs and landings — where the drop is more than 30 inches measured vertically to the grade below at any point within 36 inches horizontally of the open edge. R312.1.2 sets the guard at a minimum of 36 inches, but allows 34 inches on the open sides of stairs and 34–38 inches where the guard also serves as the handrail. R312.1.3 limits openings so a 4 inch sphere cannot pass — with two exceptions worth knowing: the triangular opening formed by the riser, tread and bottom rail at the open side of a stair, where the limit is a 6 inch sphere, and guards on the open side of stairs, where it is 4⅜ inches.

The 30 inch trigger catches people on low decks. A deck two feet above grade needs no guard; the same deck at 32 inches does, and so does the open side of the stairs running down from it.

Footings at the bottom of the stairs

The stair footing rules are the most useful thing in this article, because they are more permissive than most people assume. R507.3.2 requires deck footings to bear at least 12 inches below the undisturbed ground surface, and R507.3.3 requires frost protection where the deck is attached to a frost-protected structure — which normally means extending the footing below the local frost line in Table R301.2(1).

But R403.1.4.1 carves out the bottom of the stairs explicitly: the footing at the grade-level termination of stairs or ramps attached to decks or landings needs only to be at least 12 inches below the undisturbed ground surface — whether or not the deck itself is attached to a dwelling. So on a house-attached deck in a 42 inch frost-depth jurisdiction, the deck posts may need to go down 42 inches, but the pad under the bottom stringer generally does not.

The other exception in the same section is worth knowing for free-standing work: a deck that is not supported by a dwelling need not have its footings extend below the frost line at all. That is narrow — it means the deck must be structurally independent of the house — and it is not a licence to skip the 12 inch minimum.

One honest caveat, because it trips people up: DCA 6 is stricter here than the IRC. The guide requires stair footings to bear at least 12 inches below the undisturbed ground surface or below the frost line, whichever is deeper, and it says stringers shall not bear on a new or existing concrete pad or patio that is not founded below that depth. So the IRC exception above is real, but if your inspector is working from DCA 6 — which is common — you pour to the frost line. Either way the stringer itself bears on the post through a 2×4 bearing block, not directly on concrete.

Worked example — a deck 105 inches above grade

• Risers. 105 ÷ 7.75 = 13.55 → 14 risers.
• Riser height. 105 ÷ 14 = 7.50 inches, uniform throughout, within the 3/8 inch variation rule.
• Treads. 14 risers − 1 = 13 treads.
• Run. 13 treads × 10 inches = 130 inches of run. The flight is under the 12′7″ rise limit (105 inches), so no intermediate landing is required — but the bottom landing still needs 36 inches of depth in the direction of travel, so the overall footprint is longer than the run alone.
• Stringer. √(105² + 130²) = √(11025 + 16900) = √27925 = 167.1 inches, about 13 feet 11 inches. That is inside DCA 6's 13′3″ solid-stringer span limit only if you count the horizontal run rather than the sloped length — the run here is 130 inches (10′10″), which is inside it.
• Footing. Concrete pad under the bottom stringer, at least 12 inches below undisturbed grade (R403.1.4.1).
• Guard and handrail. 105 inches is far more than 30, so guards are required on the open sides and a handrail on at least one side of the flight.

Common mistakes on deck stairs

• Dividing the rise by the wrong thing. Cutting the stringer from a per-step riser instead of the uniform riser the total rise produces. Lay out the first notch last, not first.
• Forgetting there is one fewer tread than riser. The step onto the deck is a riser, not a tread. This alone accounts for a lot of short stringers.
• A bottom landing under 36 inches in the direction of travel. The landing depth is measured the way you walk, not across the stairs, and it is required at both ends.
• Guard rule read from the deck only. The 30 inch trigger applies to the stairs and landings too, and the open side of a flight needs a guard just as the deck edge does.
• Over-notching a cut stringer. Every notch removes beam depth. DCA 6 analyses cut stringers at 5.1 inches effective depth; a deep notch can leave you well under that.
• Skipping the footing because "it is only stairs". The bottom stringer needs bearing on something that will not settle differently from the rest of the run, 12 inches below undisturbed grade at minimum.
• Assuming one national riser number. 7¾″ is the base IRC 2021 maximum; several states allow 8″ or 8¼″ with a 9″ tread. Build to the code your inspector is holding.

Where these numbers came from

Riser, tread, nosing, width, headroom, rise-per-flight, landing, walking-surface and handrail rules are from IRC 2021 Chapter 3 R311.7.1–R311.7.9, fetched from the up.codes IRC 2021 viewers on 2026-09-10. Guards are R312.1.1–R312.1.4, and the footing rules R403.1.4 / R403.1.4.1 / R507.3.2 / R507.3.3, both fetched 2026-09-15. The state riser and tread amendments in the table were verified 2026-09-10. Stringer figures — the 5.1 inch effective depth used to analyse cut stringers at a 7¾:10 rise-to-run ratio, the 13 foot 3 inch solid-stringer span limit, the 6 foot cut-stringer limit, the three-cut-stringer minimum, the 18 inch maximum stringer spacing, the 12 foot stairway height at which DCA 6 requires an intermediate landing, Table 6 tread stock, and the stair-footing depth rule — are from the American Wood Council's DCA 6 Prescriptive Residential Wood Deck Construction Guide (2015 IRC basis, © 2018), fetched and read from the AWC PDF on 2026-09-28. Local amendments and the authority having jurisdiction always win.

Related pages

• Stair stringer calculator — riser height, tread run and stringer length from your total rise
• Stair math guide — the same rules with the arithmetic shown explicitly
• Deck footing calculator — footing size from tributary area, and the depth minimums that also apply to the stair pad
• Deck beam & joist span calculator — IRC Table R507.6 and R507.5(1) spans for the deck the stairs are attached to
• Reference tables — the deck span tables are in the build queue, with the ampacity and breaker charts already live