How to Set Flagstone Steps

Determine the size and number of flagstone steps you will need. Outdoor steps should have a rise of 4 to 6 1/2 inches and a comfortable tread, or the depth of the step. Decide on either the tread or the rise and use the following formula to determine the ideal corresponding tread or rise: the tread plus two times the riser should equal 26 to 27 inches. Most landscape architects recommend a 15-inch tread with a 6-inch riser, according to the Sustainable Urban Landscape Information Series. Purchase enough flagstones to cover the total area and riser stones in the determined size.

Dig the basic shape and size of the steps into the slope. Mark the base of the stairs where you will place your first flagstone. Dig a 12-inch-deep trench to the length and depth of the step. Pour in 4 inches of crushed rock and tamp firmly. Add 4 inches of concrete over the rock, and allow it to dry for at least 24 hours.

Dig a small hole on each side of the step with a depth of two to three inches. Angle a support stone in each hole. Spread a two-inch layer of mortar on top of the concrete and to the sides of the support stones to set them. Press flagstones into the mortar, trying to fit them snugly across the full area of the step. Make sure the flagstones are level, and mortar between them. Wipe away the excess mortar and allow it to set for three days.

Apply a layer of mortar to the back of the first tread. The layer should be 1 inch thick and the same width as the riser stones. Press the riser stones into the mortar and add compacted soil between the riser stones and the previously excavated rough foundation. Add mortar between the riser stones, wipe away any excess, level the stones and allow the mortar to set.

Dig a 6-inch trench the length of the tread behind the riser stones after the mortar has set. Angle support stones on each side of the step as done in Step 3. Add 3 inches of crushed rock, tamp down and apply 2 inches of mortar. Arrange the flagstones on the mortar, fitting them snugly and allowing a 1-inch overhang above the previous step. Level the flagstones, apply mortar over the tread and wipe away any excess.

Repeat Steps 4 and 5 until you have constructed as many steps as you need. Allow the flagstone steps to set for at least two weeks before using them.