How to Build a Stone Wall Using Flagstones

Mark the shape of the wall on the ground with spray paint.

Dig a trench along the line of spray paint several inches wider than the width of largest flagstones you will use. Dig down into the subsoil; the change from topsoil to subsoil is evident by a change in color from dark brown soil to a lighter material. Use a tamper to compact the base of the trench to avoid settling.

Spread 3 to 4 inches of crushed rock in the bottom of the trench and compact it with a tamper.

Lay the first course of flagstones in the trench. Use the largest flagstones in this course and level each stone in the base of crushed rock.

Lay the second course of stones over the first so the seam between each of the foundation stones is bridged by a stone above. Choose a straight, flat edge of each stone to face outwards on the wall and use complimentary shapes from one stone to the next so the pieces fit together as tightly as possible. Set each stone so that it rests one-quarter-inch back from the front edge of the one underneath to make the wall pitch slightly into the slope. Each individual stone should be either perfectly level front to back or be slightly tilted into the slope. If needed, use small chips of flagstone as shims to stabilize stones with uneven shapes and prevent them from wobbling.

Continue to lay more courses of flagstone to build up the height of the wall. Once the wall is 6 or 8 inches tall, lay a sheet of drainage fabric across the slope behind the wall and fill in the cavity between the wall and the slope with gravel. Continue building the wall and backfilling with gravel as it goes up.

Use larger pieces of flagstone for the top course of stones to make a stable cap. Fold the drainage fabric over the gravel 3 or 4 inches below the top of the wall and spread topsoil to fill the remaining space. This area can then be planted with grass or other groundcover plants.