How to Build Flagstone Steps on a Slope

Determine the slope and the tread-to-riser ratio. The slope consists of the vertical rise of the terrain divided by the horizontal distance. Set a level board at the top of the planned steps extending over the slope, then measure the vertical distance between the bottom of the board and the planned location of the bottom step. This is the rise. Hold the board vertically and level at this point, then measure the horizontal distance from the top of the hill to the same height at the board. Decide on a desired tread or riser size or number of steps and calculate the accompanying measurement. The tread size plus two times the riser height should equal 26 or 27. If the desired riser height is 5 inches, the tread should be 16 or 17 inches. Use these measurements in combination with the slope measurement to determine how many steps to construct.

Excavate the rough shape of the steps including a base. The base for the first step should have a depth of 12 inches.

Tamp about 4 inches of gravel or crushed rock into the area excavated for the base.

Pour 4 inches of concrete on top of the gravel. If necessary, make a wooden form to hold the concrete. Use a trowel or board to level the top of the concrete and let the concrete dry for at least 24 hours before proceeding.

Place support stones on both sides of the step. Choose thicker stones that may not be suitable for treads. Dig a space a few inches deep and wide enough to accommodate the stone. Place the stone at an angle to be in contact with two steps.

Prepare the stones for the tread. Lay out the stones on the tread without mortar to determine the best arrangement. Move the stones off the tread but maintain their relative positions. The stones should be clean and dry.

Spread 2 inches of mortar on top of the concrete. Spread the mortar far enough to reach the support stones on either side to hold them in place.

Press flagstones into the mortar. Use a level to ensure the stones are even and adjust the stones by adding additional mortar underneath them or pushing down to force mortar out from underneath. Mortar between the stones and wipe any excess mortar off the stones, using a damp sponge or cloth.

Build the riser for the next step. Apply an inch-thick layer of mortar to the back of the tread that's wide enough to accommodate the riser stones, then press in one or more riser stones of the desired riser height minus the thickness of the flagstone. If multiple stones are required, mortar them together and allow the mortar to dry before proceeding.

Excavate the area behind the riser for the next step to a depth of 6 inches.

Tamp about 3 inches of gravel into the excavated area, adding more gravel as needed so the top of the gravel layer is 2 inches below the top of the riser in front. Place support stones on the sides of the step and lay out flagstone for the tread in the same manner as for the first tread.

Put 2 inches of mortar on top of the gravel. The mortar should be flush with the top of the riser in front.

Lay the flagstones on top of the mortar layer. Lay the flagstones so that there is 1 inch of stone hanging over the front of the riser. Level the stones, mortar the spaces between the stones and clean off any sullied stone.

Repeat riser construction, support stone setting and building steps using gravel, mortar and flagstones until the stairs are completed.