Flagstone steps are perfect structures for managing natural slopes. Our design consists of broad flagstone treads and blocky ashlar risers, commonly sold as wall stone. The risers are prepared with compactable gravel beds on which the flagstone treads rest. For the project featured here, we purchased both the flagstone and the wall stone in their natural split state (as opposed to sawn). It may seem like overkill, but you should plan on purchasing 40 percent more flagstone, by square foot coverage, than your plans say you need. The process of fitting the stones together involves a lot of cutting and waste.
The average height of your risers is defined by the height of the wall stone available to you. These rough stones are separated and sold i n a range of thicknesses (such as 3 to 4 inches), but hand-picking the stones helps bring them into a tighter range. The more uniform the thicknesses of your blocks, the less shimming and adjusting you’ll have to do. (Remember, all of the steps must be the same size, to prevent a tripping hazard.) You will also need to stock up on slivers of rocks to use as shims to bring your risers and returns to a consistent height; breaking and cutting your stone generally produces plenty of these.
Flagstone steps work best when you create the broadest possible treads: think of them as a series of terraced patios. The goal, once you have the stock in hand, is to create a tread surface with as few stones as possible. This generally means you’ll be doing quite a bit of cutting to get the irregular shapes to fit together. For a more formal look, cut the flagstones along straight lines so they fit together with small, regular gaps.
Building Flagstone Garden Steps
1. Measure the height and length of the slope to calculate the rise and run dimensions for each step. Plot the footprint of your steps on the ground using marking paint. Purchase wall stones for your risers and returns in a height equal to the rise of your steps. Also buy flagstone (with approx. 40% overage) for the step treads.
2. Begin the excavation: for the area under the first riser and return stones, dig a trench to accommodate a 4″- layer of gravel, plus the thickness of an average flagstone tread. For the area under the back edge of the first step’s tread and the riser and return stones of the second step, dig to accommodate a 4″- layer of gravel, plus a 1 “- layer of sand. Compact the soil with a 2×4 or 4×4.
3. Add a layer of compactable gravel to within 1″ of the planned height and tamp. Add a top layer of compactable gravel and level it side to side and back to front. This top layer should be a flagstones thickness below grade. This will keep the rise of the first step the same as the following steps. Leave the second layer of gravel uncompacted for easy adjustment of the riser and return stones.
4. Set the riser stones and one or two return stones onto the gravel base. Level the riser stones side to side by adding or removing gravel as needed. Level the risers front to back with a torpedo level. Allow for a slight up-slope for the returns (the steps should slope slightly downward from back to front so the treads will drain). Seat the stones firmly in the gravel with a hand maul, protecting the stone with a wood block.
5. Line the excavated area for the first tread with landscape fabric, draping it to cover the insides of the risers and returns. Add layers of compactable gravel and tamp down to within 1″ of the tops of the risers and returns. Fill the remainder of the bed with sand and level it side to side with a 2×4. Slope it slightly from back to front. This layer of sand should be a little above the first risers and returns so that the tread stones will compact down to sit on the wall stones.
6. Set the second group of risers and returns: first, measure the step/run distance back from the face of your first risers and set up a level mason’s string across the sand bed. Position the second-step risers and returns as you did for the first step, except these don’t need to be dug in on the bottom because the bottom tread will reduce the risers’ effective height.
7. Fold the fabric over the tops of the risers and t r im off the excess. Set the flagstone treads of the first step like a puzzle, leaving a consistent distance between stones. Use large, heavy stones with relatively straight edges at the front of the step, overhanging the risers by about 2″.
8. Fill in with smaller stones near the back. Cut and dress stones where necessary using stone chisels and a maul or mason’s hammer. Finding a good arrangement takes some trial and error. Strive for fairly regular gaps, and avoid using small stones as they are easily displaced. Ideally, all stones should be at least as large as a dinner plate.
9. Adjust the stones so the treads form a flat surface. Use a level as a guide, and add wet sand under thinner stones or remove sand from beneath thicker stones until all the flags come close to touching the level and are stable.
10. Shim between treads and risers with thin shards of stone. (Do not use sand to shim here). Glue the shards in place with block and stone adhesive. Check each step to make sure there is no path for sand to wash out from beneath the treads. You can settle smaller stones in sand with a mallet, but cushion your blows with a piece of wood.
11. Complete the second step in the same manner as the first. The bottoms of the risers should be at the same height as the bottoms of the tread on the step below. Continue building steps to the top of the slope. Note: The top step often will not require returns.
12. Fill the joints between stones with sand by sweeping the sand across the treads. Use coarse, dark sand such as granite sand, or choose polymeric sand, which resists washout better than regular builder’s sand. Inspect the steps regularly for the first few weeks and make adjustments to the heights of stones as needed.