Stairs are the one part of a deck or a house where small arithmetic errors cause falls. The rule is simple: divide the total rise by a target riser height, round to a whole number, then divide back to get the exact riser.
Measure the total rise correctly
Total rise is from finished lower surface to finished upper surface. For a deck that is from the ground or patio to the top of the decking, not to the top of the frame.
Getting this wrong by the thickness of the decking is the classic error, and it shows up as a first or last step that is different from the others.
Riser and tread limits
Most residential codes cap risers at 7.75 inches and require treads of at least 10 inches. Seven inches of rise and eleven of run is a comfortable stair.
The variation rule matters as much as the maximum: all risers in a flight must be within about 3/8 inch of each other.
The bottom step and the tread thickness
If you cut a stringer with equal risers and set it on the ground, the bottom riser ends up taller by the thickness of the tread once the treads go on.
The fix is to cut the bottom of the stringer down by exactly one tread thickness. Every stringer layout guide covers this and it is still the most common mistake.
Stringer stock
The stringer length above is the diagonal — the minimum board length before any cutting. Buy the next standard size up, because a stringer laid out on a board with no margin cannot be adjusted.
Use 2×12 for cut stringers. Once the notches are cut there is very little wood left, and anything narrower is not enough.
Cutting the first stringer as a pattern
All the stringers in a flight are cut from one pattern. Cut the first, test fit it in place before cutting any more, and only then trace it onto the rest.
Test fitting means checking that the top riser and the bottom riser come out the same as the others once the stringer is resting where it will live. This is where the tread thickness adjustment at the bottom shows up as right or wrong.
Cutting three stringers at once from a traced pattern and discovering an error afterwards wastes three long boards, which on pressure treated 2x12 is not trivial.
How many stringers, and how they are supported
Spacing follows the tread material. Standard decking wants stringers no more than sixteen inches apart; some composite treads require twelve. A three foot wide stair usually takes three stringers, sometimes four.
The cut-out for each step removes a lot of the board's depth, so what is left below the notches — the throat — is what actually carries the load. Keeping at least five inches of throat is the common rule.
At the top, stringers hang from the framing on hangers or a ledger rather than being toe-nailed. At the bottom they sit on a solid pad, not on soil, and are fastened so they cannot kick out.