A nearly fail-safe approach to laying out a deck stairway.
Designing a stairway can be complicated and confusing. The trickiest part is making sure that all the steps -- including the bottom and top steps -- are the same height.
A standard set of stairs has rises of about 7 inches and runs of 11 inches, but you may want deeper stairs. If the treads will be 2xs, stringers can be spaced as much as 28 inches apart; if you will use decking boards, space stringers 18 inches apart or closer. Interior stringers must be notched; outside stringers may be notched or closed. For each tread use a single 2x12 or 2x10, or use two or more boards spaced as you would decking boards. Risers are not necessary, but they do hide the underside of the deck and offer a more finished look.
A couple of hours to partially cut a stringer, calculate rise and run, and design a stairway
Level, circular saw, tape measure, calculator, framing square with set stops, carpenter's pencil
Basic mathematics, measuring and cutting, checking for level
Finish the decking, and decide on the type of landing (if any)
Straight board, 2x12 for the first stringer
The vertical distance that a stairway travels from the landing pad to the top of the deck is the total rise. The horizontal distance it travels is the total run. The vertical and horizontal distances traveled by each step are the unit rise and unit run.
A landing must be level (even if the yard is not) and may be made of concrete, pavers, or crushed stone. Stringers are the stair's joists, and these may be either notched or closed, with brackets. Stringers must be firmly attached to the deck; an extra brace may be needed. A toe-kick anchors the stringers to the landing pad. Treads, which may be composed of a single board or several pieces, are the parts that you walk on. Optional risers cover the vertical spaces between treads.
If the yard is level, find the total rise by measuring down from the deck to the ground. If the yard slopes, use a level and a long board to extend the deck level out to a point where you estimate that the stairway will end, and measure down from there. Calculate rise and run. You may decide that you need to move the landing pad closer to or farther from the deck. If so, repeat this step and recalculate.
Double-check the stringer layout, thinking through how it will go together. The bottom rise should be 1-1/2 inches shorter than the other rises if you are using 2x treads, and 1 inch shorter if you are using 5/4 decking for treads. Visualize how the stringer will attach to the deck. Cut the stringer at both ends only.
Hold the partially cut stringer against the deck in the position where it will be attached. Check that the treads will be level. Mark the ground for the location and height of the landing pad. The landing pad should support the stringers fully and should extend outward a comfortable distance for walking.
Stair gauges (sometimes called set stops or buttons) clamp to the edges of a framing square. They are especially useful when laying out stringers. Tighten them at the marks for your rise and run and set them against the edge of the stringer. Slide the square up the board as you lay out each step.