Stairs, decks and fences
Deck stairs calculator: steps, risers and stringers
Deck stairs follow the same rules as any stair, with two catches: the rise is measured to the ground where the stairs will land, and the deck itself is the top step. Here is the whole calculation for a short deck stair.
How do you measure the rise for deck stairs?
Measure from the top of the deck boards straight down to the spot where the bottom step will land, not to the ground at the deck's edge. Ground usually slopes away, so run a level or a straight board out from the deck to that spot and measure down from it. If the stairs land on a concrete pad or pavers, measure to the top of the finished pad.
How many steps do deck stairs need?
Divide the rise by a riser height near 7 in and round to a whole number: that is the number of risers. Divide the rise by that number for the exact riser height. The deck surface is the top step, so the number of treads is one fewer than the number of risers.
Example. The rise is 30 in. 30 ÷ 7 = 4.29, so 4 risers of 30 ÷ 4 = 7 1/2 in each. That leaves 3 treads to build.
What tread depth for deck stairs?
Treads are often made of two deck boards side by side. Two 2x6 boards (5 1/2 in each, as actually sized) make an 11 in tread. Under the US International Residential Code (IRC) a tread is at least 10 in deep and a riser at most 7 3/4 in; your local code is the one that counts.
How long are the stringers?
The total run is the number of treads times the tread depth. The stringer's line length is the long side of the triangle formed by the total rise and the total run.
Example. 3 treads of 11 in make a run of 33 in. The line length is √(30² + 33²) = 44.6 in, about 3 ft 8 5/8 in. The stair angle is arctan(7.5 ÷ 11) = 34.3°. Buy stringer stock a good deal longer than the line length, because the top and bottom cuts need room.
Cut the first riser short
The bottom of the stringer is cut shorter by the thickness of one tread, so the first step is the same height as the rest once the treads are on.
How many stringers do deck stairs need?
That depends on the tread material. Decking makers set the stringer spacing for their boards on stairs, often 12 in or closer for composite, and many builders keep wood treads to 16 in or less; read the instructions for the boards you buy.
Example. A 48 in wide stair with stringers at most 16 in apart: 48 ÷ 16 = 3 spaces, so 4 stringers. At 12 in it is 4 spaces and 5 stringers.
What do the stringers stand on?
Set them on something firm and level: a concrete pad, pavers on a compacted base or a footing your code allows, never on bare soil, which settles and rots the wood.
Deck stairs in Clad
Clad's stairs tool takes the total rise and your preferred riser and tread, and gives the risers, the treads, the total run, the stringer length and the angle, with the options for one riser more or fewer. Saved to a job, the result sits next to the decking and concrete for the same deck.