My garden includes a slight slope with a few stairs, made with 8” x 8” x 48” wooden highway ties. Earlier, garden stairs might have been made with railroad ties, which were often soaked in creosote as a preservative. Today, highway ties are pressure-treated with chemical preservatives, many of which are too toxic to be used near edible plants.
Happily, my short flight of aging stairs was not treated with a preservative, but consequently it is deteriorating and needs replacement. Wood is suitable for stairs that do contact soil, but in this case I will avoid rot by using flagstones or other natural stone. There are manufactured stone-like materials that also are available for such a project.
When planning garden stairs, first determine your preferred dimensions for the risers and treads. A six-inch riser with a fifteen-inch tread is a recommended combination, but other combinations also can work well. A steep flight of stairs might have seven-inch risers and eleven-inch treads, while a gentle flight might have four-inch risers and twenty eight-inch treads. See on gardening.com for the range of other good combinations.
Then, use a straight board and a carpenter’s level to measure the change in level from the bottom to the top of the slope. For a longer slope, use a garden hose, taking advantage of the fact that water seeks it own level. Hold the hose in a U-shape, with one end near the top of the slope and the other end near the bottom. Fill the hose with water, and adjust it so water is at the opening of each end. When this condition has been met, the two ends will be at the same elevation, and the distance of the lower end to the ground, minus the distance of the upper end to the ground equals the change in level. See ongardening.com for an illustration of this method.
Divide the change in level by your preferred height for the riser to determine the number of steps needed for that particular slope.
Then, measure the horizontal distance from the bottom of the slope to the top. Your preferred dimension for the tread times the number of stairs should equal that distance. If it does not, modify the riser and tread dimensions (using one of the good combinations) or include a curve in the flight of stairs or reshape the slope.
The width of the stairs is the next design issue to be addressed. The narrowest width could be two feet, which might be sufficient for a utility stairway. A one-person stairway should be four feet wide, which is generally considered the minimum for a garden path. A two-person stairway should be five feet wide.
Wider stairways, in scale with the landscape, can provide a visually striking appearance. This stairway at Les Quatre Vents, an estate near Quebec, is designed for grand entrances. (Click to enlarge)