Deck Stair Code: 7¾″ Max Rise, 10″ Min Tread, 36″ Wide (IRC 2021)

7¾ inches, under IRC 2021 R311.7.5.1, and the number that catches more decks than the maximum itself is the uniformity rule beside it: every riser in the flight must be within 3/8 inch of every other riser. Most builders aim for 7 to 7.5 inches, which leaves margin and produces a flight that feels comfortable rather than steep. The arithmetic is straightforward once the total rise is measured from the finished deck surface to the finished landing surface, including the thickness of the landing pad. Divide that total by 7.5 and round to a whole number for the riser count, then divide the total rise by the riser count to get the actual riser height. A 36-inch deck gives 36 ÷ 7.5 = 4.8, so five risers at 7.2 inches each — comfortably under the maximum, and identical to each other, which is what the inspector measures.

10 inches, measured nosing to nosing, under IRC 2021 R311.7.5.2 — and like the riser, treads must be within 3/8 inch of each other across the flight. Most builders run 10.5 to 11 inches because the extra depth makes descending noticeably more comfortable, which matters more than it sounds on an exterior stair that gets used in the wet. Tread depth also sets how far the stairs project into the yard: the total run is the tread count times the tread depth, and the tread count is always one less than the riser count because the deck surface itself serves as the top tread. Five risers therefore means four treads, and at 10.5 inches each the flight projects 42 inches from the deck — a dimension worth checking against the landing space available before the stringers are cut.

Yes, on at least one side, as soon as the flight has four or more risers — IRC 2021 R311.7.8. Three specifics decide whether it passes. Height: 34 to 38 inches measured vertically from the tread nosings, not from the treads themselves. Graspability: a 1¼ to 2 inch round profile, or an equivalent Type II graspable shape. A 2x4 laid flat on edge is the classic rejection, because a hand cannot close around it, and it is the most common handrail on owner-built stairs. Continuity: the rail has to run the full length of the flight and return into a post or wall at both ends rather than stopping in mid-air. None of these is expensive to get right at build time, and all of them mean rework if they are wrong, because the rail is usually the last thing installed.

2x12, and this is one of the few places where the recommended size is effectively mandatory. Cutting a notched stringer removes material for every rise and run, and what is left behind the deepest notch — the throat — is what actually carries the load. Code practice wants roughly 5 inches of uncut depth remaining, and a notched 2x10 does not leave it. A 2x12 does, with margin. There is an alternative if the lumber cost matters: solid or cleated stringers are not notched at all, with treads bolted to metal cleats fixed to the face of an uncut board. That is stronger and more material-efficient, at the cost of more hardware and more assembly time. Notched stringers remain the standard site-built method because they need nothing but a saw and a square.

Yes, but with a restriction that rules them out on most flights: a 4-inch sphere must not pass through the opening between treads, under IRC 2021 R311.7.5.1. Since typical risers run 7 to 7.5 inches, an open gap of that height fails the test straight away, so open risers in practice need either a partial back-stop behind each tread or a closed riser board. The rule is a child-and-pet safety provision rather than a structural one, and it is not negotiable at inspection. Closed risers are the residential default for that reason and have a practical benefit too — they stop the space beneath the stairs collecting leaves and let the flight shed water in a controlled way. If the modern open look matters, price the back-stop detail into the design rather than discovering it at inspection.

34 inches minimum, measured from the tread nosings — and note that this is a different number, measured from a different reference, than the 36-inch guard on the deck itself, which is measured from the deck surface. Confusing the two is a common rejection. Stairs need a guard wherever they are more than 30 inches above the grade below, under IRC 2021 R312.1.1. The infill rules on stairs are also their own: a 4⅜-inch sphere must not pass through the guard infill, slightly more generous than the 4-inch sphere used on the deck, and the triangular opening formed by the tread, the riser and the bottom rail must not pass a 6-inch sphere. Set the guard out on the drawing with all three spheres in mind; retrofitting infill after the rail is built is fiddly and visible.

Yes, and it is the step most often left out of a plan. IRC 2021 R311.7.6 requires a landing at the foot of the flight at least as deep as the stair is wide, with a 36-inch minimum, and it must be a firm level surface — a concrete pad, a paver pad or compacted gravel all qualify, bare soil does not. The landing also cannot slope more than 2%, about ¼ inch per foot, which is enough to drain without being enough to feel underfoot. There is a second landing rule worth knowing on tall decks: a single flight cannot exceed 12 feet of vertical rise without an intermediate landing, under R311.7.3. Budget $90 to $300 for the pad depending on whether it is gravel or poured concrete.

Three for a typical 36 to 42 inch wide flight, four if the stair is wider — but the spacing between them is set by the tread material, not by the width. Pressure-treated treads allow up to 16 inches on centre. Composite treads usually require 12 inches on centre, because composite deflects more under a point load right at the tread edge, and the brand's stair specification overrides the lumber default in the same way it overrides joist spacing on the deck. That difference is worth pricing early, because it can add a stringer to the flight. Tread material also moves the budget noticeably: 2026 retail runs about $2.95 per linear foot for pressure-treated, $6.50 for composite and $8.95 for hardwood such as Ipe, which needs a pilot hole for every screw.