Building deck stairs is ordinary stair work with one hard part: nobody can agree where the bottom is. Measure from the deck surface to the top of the finished landing pad, not to the dirt, and the rest is the same arithmetic as an inside flight. A 36 inch deck comes out at 5 risers of 7.2 inches, 4 treads, a 40 inch run and a 53.8 inch stringer, which is 6 foot stock.
Measure to the pad, not to the ground
The total rise is the vertical distance from the walking surface of the deck to the walking surface at the bottom. If a patio slab, a paver pad or a set of concrete blocks goes in first, the rise is measured to the top of that, and it is usually two to four inches less than the hole you are looking at today. Get this wrong and every riser in the flight is wrong by a fifth of the error.
Build the pad before you cut the stringers. It is the only way to measure the thing you are actually going to land on, and a stringer is cheap to cut twice only in the sense that you have to buy another board.
Work out your deck stair
The arithmetic on a typical deck
- 36 in deck: 5 risers at 7.2 in, 4 treads, 40 in run, 53.8 in stringer, 6 ft stock
- 48 in deck: 7 risers at 6.86 in, 6 treads, 60 in run, 76.8 in stringer, 8 ft stock
- 60 in deck: 8 risers at 7.5 in, 7 treads, 70 in run, 92.2 in stringer, 8 ft stock
Notice what happens to the run. Every extra riser adds a tread, so a stair that climbs two feet higher reaches a foot and a half further out into the garden. The run is the thing that catches people out when the stair has to miss a path, a fence or a neighbour.
What the code adds when you build deck stairs
- A handrail from four risers up. The IRC wants one on at least one side of any flight with four or more risers, at 34 to 38 inches above the nosing line. A 36 inch deck is five risers, so it needs one.
- A landing at top and bottom. Not less than 36 inches measured in the direction of travel, which is what the pad has to be.
- Guards. 36 inches on the open side of the deck, and not less than 34 inches on the open side of the stair measured from the nosing line.
Riser and tread limits are the same outdoors as in, and so is the 3/8 inch uniformity rule. Weather is the only thing the code treats differently, and that shows up in the material rather than the geometry. The American Wood Council collects the outdoor rules in one free document, DCA 6, its prescriptive deck guide (PDF), which is what many building departments hand out.
Attaching the stringers
The stringers hang off the rim joist or off a ledger bolted to it, and the top riser is measured to the finished deck surface, so if the decking is not down yet, allow for its thickness. At the bottom the stringers sit on the pad, and the bottom notch is cut short by the thickness of one tread for the same reason it is on an inside stair.
Frequently asked
Five risers at 7.2 inches each, with four treads. The run works out at 40 inches on a 10 inch tread, and the stringer is 53.8 inches, so 6 foot stock.
From the walking surface of the deck to the top of the finished landing pad, not to the ground as it stands. Build the pad first, because its thickness comes off the rise and changes every riser in the flight.
The IRC requires one on at least one side of a flight with four or more risers, at 34 to 38 inches above the nosing line. Most deck stairs clear that threshold, since a 36 inch deck is already five risers.