Deck stair calculator: risers, treads, stringers and code checks

Pour or set the pad first whenever you can, then measure. The total rise runs from the deck surface to the finished top of the landing, so a pad that ends up 2 in above the ground you measured to shortens the bottom riser by 2 in, far outside the ⅜ in the IRC allows within a flight. If the stringers must be cut first, drive a stake where the stair lands, mark the planned top of the pad on it with a level, and measure to that mark. In the calculator, enter how far the pad stands above grade and the rise is reduced for you.

Under AWC DCA 6 they do. The guide has the stringers bear on a 2x4 block fixed to the stair guard posts at the bottom of the flight, and those posts bear on footings at least 12 in below undisturbed ground or below the frost line, whichever is deeper. It also says stringers shall not bear on a new or existing concrete pad or patio that is not founded to that depth. Where no guard is required the posts may stop below the bottom tread. The calculator counts two footings at the bottom plus one for each post under the stringers; your building department decides what it accepts for a short stair.

Almost always because the total rise was measured to the wrong surface. Three things move it: the ground slopes, so the height at the house is not the height where the stair lands; a landing pad or paver was added after the stringers were cut; or the tread thickness was not taken off the bottom of the stringer. Each one changes only the first riser, and the IRC allows just ⅜ in between the tallest and shortest riser in a flight. Measure at the landing point to the finished surface, and use the stair stringer calculator for the tread-thickness drop on the bottom cut.

A 4 foot high deck needs 7 risers and 6 treads. 48 in divided by a 7½ in target is 6.4, which rounds up to 7 risers of 6 7/8 in each; the deck surface is the top step, so there are 6 treads to build. With two 2x6 per tread and a 1 in overhang the run is 10¼ in, the stair reaches 5 ft 1 1/2 in out from the deck, and each 2x12 stringer comes from a 10 ft board. Six risers would be 8 in each, over the 7¾ in limit, so seven is the fewest the code allows.

Three for a 36 in wide stair with cut stringers, and one more each time the spacing would pass 18 in on center. AWC DCA 6 asks for a minimum of three where only cut stringers are used and a maximum spacing of 18 in on center, which works out to four stringers at 42 and 48 in wide and five at 60 and 72 in. Two solid stringers can carry a 36 in stair, but then each tread spans the full width and has to meet the larger sizes in the guide's tread table. The IRC stair section sets no stringer count or spacing.

A cut 2x12 stringer spans 6 ft 0 in and a solid 2x12 stringer spans 13 ft 3 in in AWC DCA 6 Figure 28, measured horizontally and not along the slope. With two 2x6 per tread and a 1 in overhang the run is 10¼ in, so seven treads (5 ft 11 3/4 in) fit and eight do not: a cut-stringer stair needs a post once it has more than eight risers, which is a deck higher than 5 ft at a 7½ in riser. The post is a 4x4, notched and through-bolted to the stringer, centered on a 12 in round or 10 in square footing 6 in thick.

Yes on cut stringers, no between two solid stringers. AWC DCA 6 Table 6 lists a 2x4 or a 5/4 board as the minimum tread on cut stringers, which sit no more than 18 in apart, so two 2x6 per tread is the common build and gives 11¼ in of tread with a ¼ in gap. Where the tread spans 36 in between two solid stringers the same table asks for a 2x8 in southern pine, a 2x8 or 3x4 in Douglas fir-larch, hem-fir or spruce-pine-fir, and a 2x10 or 3x4 in redwood, western cedars, ponderosa pine or red pine. The guide's commentary notes that no species works at that span as a No. 2 grade 2x6.

Between ¾ in and 1¼ in. IRC R311.7.5.3 requires a nosing in that range wherever the tread depth is under 11 in, and none where it is 11 in or more. On a deck stair the overhang also sets the run, because the boards are a fixed size: two 2x6 with a ¼ in gap cover 11¼ in, so a 1¼ in overhang leaves exactly the 10 in minimum run, 1 in leaves 10¼ in and ¾ in leaves 10½ in. An overhang larger than 1¼ in pushes the run under 10 in and fails on both counts.

Open risers are allowed, but only where a 4 in sphere cannot pass through any opening that is more than 30 in above grade (IRC R311.7.5.1). The opening is the riser height less the tread thickness, so a 7 in riser with a 1½ in tread leaves 5½ in and fails. With 2x treads the riser would have to be under 5½ in to pass, which a normal deck stair is not. In practice the steps within 30 in of the ground can stay open and the ones above need a riser board or a filler strip that closes the gap to under 4 in.

Each riser may be at most 7¾ in under IRC R311.7.5.1, and the tallest and shortest riser in a flight may differ by no more than ⅜ in. The whole flight may rise 12 ft 7 in between landings under IRC R311.7.3; AWC DCA 6 is stricter for decks and asks for an intermediate landing once the stairway is over 12 ft 0 in high. The calculator divides the deck height into equal risers at or under the deck step height you ask for, never above 7¾ in, and warns when the total rise passes 12 ft.

A handrail is required on at least one side of a flight with four or more risers, set 34 to 38 in above the line of the tread nosings (IRC R311.7.8 and R311.7.8.1). Four risers is a deck only about 2 ft high, so most deck stairs need one. It has to be graspable: AWC DCA 6 describes a Type I rail with a perimeter of 4 to 6¼ in, and it runs from above the lowest riser to above the highest, returning to the guard at each end. A plain 2x6 cap is 14 in around, so it does not count; add a graspable rail on the stair side.

Not a handrail: the IRC asks for a handrail on at least one side of a flight with four or more risers. Guards are a separate rule. Any open side of the stair that is more than 30 in above the grade below needs a guard (IRC R312.1.1), so a stair that is open on both sides and that high gets a guard on both, at least 34 in above the nosing line. One of them can double as the handrail if its top is graspable and sits between 34 and 38 in. A low stair, 30 in or less above grade, needs no guard by height.

Always at the bottom, and partway up once the stair is tall. The IRC requires a floor or landing at the top and bottom of every stairway (R311.7.6); the deck is the top one, and the bottom one must be at least 36 in in the direction of travel and as wide as the stair. AWC DCA 6 requires an intermediate landing where the stairway is more than 12 ft 0 in high, and allows one at any height to shorten the stringer span. The guide has an intermediate landing built as a non-ledger deck, with its own posts, beam and footings.

Not as the support for the stringers, under AWC DCA 6. The guide says stringers shall not bear on new or existing concrete pads or patios unless they are founded at least 12 in below undisturbed ground or below the frost line. Its detail carries the stringers on posts with their own footings, and the patio or pavers then serve as the landing surface only. Whether a short stair may rest on a slab is a local decision, so ask the building department before relying on one. Either way, measure the rise to the finished surface the bottom step lands on.

At least 36 in, under IRC R311.7.1 and AWC DCA 6. A wider stair only adds stringers and tread length. Deck stair width choices and other sizing questions are answered in deck stair questions.

Set a framing square to the riser and the run from this page, step off each tread along a 2x12, and shorten the bottom cut by the tread thickness. The stair stringer calculator gives the cut diagram, the throat left in the board and the length to buy.