Deck stair code requirements

The numbers that get checked

Local amendments exist and a few jurisdictions are stricter, so confirm before you cut. The stair stringer calculator works your rise and run against these limits and tells you which one you have crossed.

The uniformity rule is the one that fails inspections

Every riser in a flight has to be within three eighths of an inch of every other riser. That sounds generous until you realise where the error comes from: the bottom step.

A stringer cut with equal risers produces a bottom riser that is one tread thickness too tall, because the treads above sit on top of the stringer cuts and the bottom one sits on the landing. The fix is to drop the stringer by the thickness of one tread before cutting — subtract the tread material thickness from the bottom riser cut. Miss it and every step is right except the first, by exactly the thickness of the decking.

The second common source is a landing that is not level, or a deck surface that is not where the drawing said. Measure total rise from the finished deck surface to the finished landing surface, after both exist, not from the framing.

Stringer throat depth

A cut stringer is a board with triangles removed. What is left behind the cuts is the throat, and it is what actually carries the stairs. Prescriptive guidance sets a minimum throat of 5 inches.

This is where 2x10 stringers quietly fail. At a 7 inch rise and a 10.5 inch tread, a 2x10 leaves a throat of about 3.4 inches — well under the minimum — while a 2x12 leaves about 5.4 inches and passes. It is the single most common reason a set of homemade stairs is not code compliant, and it is invisible unless you calculate it.

The stair stringer calculator reports throat depth alongside the rise and run so you can see it before you buy the lumber.

Handrails: height, graspability and continuity

A handrail is not the same thing as a guard. The guard stops you falling off the side; the handrail is what your hand slides along. A flight of four or more risers needs at least one handrail, mounted 34 to 38 inches above a line drawn along the stair nosings.

• It has to be graspable. A round rail between 1 1/4 and 2 inches in diameter qualifies; a flat 2x4 laid on edge does not, and that is the most common failure.
• It has to be continuous for the full flight, with no interruption from a newel post mid-run.
• It has to return to a wall or a post at each end, so a sleeve or a bag strap cannot catch on an open end.
• It needs at least 1 1/2 inches of clearance from any wall behind it.

Stringer spacing and support

Stringer spacing is set by the tread material, not by the code alone. Wood treads generally allow 16 inches on centre between stringers; most composite tread products require 12 inches, and some specify even less on stair applications because a stair tread is loaded differently from a deck board.

The bottom of the flight has to bear on something solid — a concrete pad, a footing, or a landing built for the purpose — not on soil or a couple of pavers. The top is hung on the deck rim with rated hardware, not toe-nailed. Both ends are things inspectors look at closely.