Drop a level out from the deck surface to where the stairs will land, then measure straight down to the finished grade or pad. This vertical distance is your total rise. Measure it at a couple of points, because grade is rarely flat, and use the larger figure so the bottom step is never short.
Divide the total rise by a comfortable target riser — around 7 to 7.5 inches for exterior stairs. Round to a whole number of risers, then divide the rise back by that count for the actual uniform riser. The IRC caps riser height at 7.75 inches and requires a tread depth of at least 10.0 inches.
A cut stringer must retain at least 3.5 inches of solid wood behind every notch, called the throat. For typical residential step geometry that means a 2x12; a 2x10 only holds enough throat for shallow steps. If your throat falls below 3.5 inches, step up board size rather than risk a weak stringer.
Space stringers about 16 inches on-center for standard decking, closer for thinner treads. A 36-inch-wide stair usually needs three to four stringers. Wider stairs add stringers proportionally.
A single flight may not exceed 147.0 inches of vertical rise without a landing under the IRC. Tall decks usually need an intermediate landing, which also breaks a long run into safer segments.
Run your exact numbers in the deck stair calculator, which sizes the board, counts stringers, and flags a required landing automatically.
Mark the stringers with a framing square set to your riser and tread, using stair gauges to lock the two numbers so every step is identical. Cut the notches with a circular saw and finish the inside corners with a handsaw — overcutting with the circular saw weakens the throat. Remember to deduct one tread thickness from the bottom riser ("dropping the stringer") so the first step isn't tall once the treads go on.
Deck stair stringers carry more load than people expect. At 16-inch on-center spacing a 36-inch stair typically needs three stringers; wider stairs or thinner decking treads need more. Attach the top securely to the deck framing with a hanger or ledger, and set the bottom on a concrete pad or footing — never directly on soil, which heaves and rots the wood.
Fasten treads working from the bottom up. Once the steps are in, code requires a graspable handrail 34 to 38 inches above the nosing line on stairs of four or more risers, and guards where the drop alongside exceeds 30 inches. These aren't optional add-ons — they're inspected, and a deck stair without a compliant handrail fails.
Size the whole flight, including stringer board and throat, in the deck stair calculator.
There are two ways to attach the top of a deck stair stringer. A flush (top) mount finishes the top step level with the deck surface, so the deck acts as the final tread. A hung mount hangs the stringer off the face of the rim joist, with the top tread below the deck level. The choice changes the top riser calculation, so decide it before cutting — and remember to drop the stringer by one tread thickness at the bottom either way.
Fasten the top of each stringer to solid framing — the rim joist or a dedicated ledger — with structural connectors or hangers rated for the load, not just deck screws into end grain. The bottom must land on a concrete pad or footing, never bare soil, which shifts and rots the wood. A galvanized or stainless connector at the base keeps the end grain off the concrete and out of standing water.
Cut stringers are almost always 2x12 stock. The reason is the throat — the solid wood left behind each notch must be at least 3.5 inches, and a 2x10 usually can't keep that much behind a normal step notch, so it splits under load. Buy 2x12s, and make each board the diagonal length plus 12 to 16 inches for the top and bottom cuts.
Laying Out a Stair Stringer · Rise and Run Explained · Stair Landing Requirements