We all know the ups and downs of our homes by heart. But approach an unfamiliar set of steps and most people look down to check the first footfall. After that, reflexes take over, and our expectation is that once we negotiate the first step, all the others will be the same.
It makes sense — the same height, same angle and same tread size each time. It’s also code, and what keeps people from tumbling onto and off of porches and decks, even if the transition is no more than a single platform. If you have to tell friends to watch out for that big step onto the grass, you need a transition level to split that distance in half.
Doing the math
It takes some basic math to sort out the dimensions and proportions. They apply to stairs that climb a full story or a platform that eases a 12-inch drop from a deck — and are worth checking on plans before drawings become reality. There are design guidelines, and also parameters that come from the local municipal building department. You should check locally, but you’re likely to find these three key limits.
Risers
Maximum unit rise is 8 inches, referring to the vertical distance from one tread to another, or how high you have to step as you climb. You might want to match the rise of interior stairs to keep everything the same. Outside, though, a rise of 5 or 6 inches is more in tune with the relaxing nature of a deck — and just easier to climb.
But reduce the rise under 4 inches and climbing can become too much like walking, and is often disorienting — the last thing you want to build into a set of steps.
Depth
Minimum unit run is 8.25 inches, referring to the horizontal distance from one tread to another not including the nosing, or overhang. Increase the run, and stairs stretch further into the yard, reducing the overall pitch at the same time. Small unit runs are closer to climbing a ladder. Large unit runs are closer to walking a ramp.
Width
Minimum tread width, including the overhang, is 9.25 inches, which works out just right for 2-by-10 lumber. That’s more than enough room for little feet, but a minefield for a guy with size 12s.
Design help
You can sketch different combinations, see how the code limits would work, then change one or more of the dimensions to see the effect on the stairs overall. On the Web, try the interactive stair planner at blocklayer.com/stairs. The page comes up metric, but after switching to “stairs English” (at top of the page) you can type in different rise and run dimensions, hit calculate and watch the side view of the stairs change shape. The free site also does some math, for example, deducing from your inputs the exact amount of rise needed to make sure each step is equally spaced.
You can also make practical tests using stairs inside the house.
Try a few trips to the approximate height of the stairs you’re planning outside, up and down. Does it feel like a chore and seem hard to negotiate with a tray full of glasses? Outside, you probably want a greater run and less slope. Do you sometimes bump your foot into the riser (the vertical board between treads) on the way up? Outside, you probably want a wider tread. Instead of one 2-by-10 (actually 9.5 inches), maybe two 2-by-6s (actually 11 inches) or even four 2-by-4s (14 inches).
But at about twice the size of a standard tread, the normal pace of taking one step at a time becomes a stretch and starts to call for two steps — more disorientation to avoid. If you want a larger tread, make it significantly larger so people won’t get caught between strides.
