How to Make Mortared Flagstone Walls

Excavate a trench for the wall's footing, or foundation. The footing should extend below the frost-line to avoid frost heave and should be at least as deep as the wall is wide. The footing should protrude at least 1 foot behind and in front of the wall and the bottom of the excavated trench should be roughly level.

Pour the concrete footing. If the soil collapses into the trench easily, build a wooden form to shape the sides of the footing. Use a wooden board or trowel to smooth the top of the concrete footing. Let the concrete cure, or dry, for about a week before proceeding.

Plan out and prepare the first layer of stones. Snap chalk lines on the footing where the front, back and sides of the wall will be. Perform a "dry run" by arranging the first layer of stones within the chalk outline, placing larger stones at the end and every 4 to 6 feet and filling in the spaces between these large stones with smaller ones. Keep the fattest side of the stone facing down for the first course. Once the stones have been arranged, place them off to the side while maintaining their relative positions and make sure that the stones are clean and dry.

Spread a layer of mortar about 2 inches thick onto the foundation within the chalk outline. Start with a small, 3- to 4-foot section.

Set the stones into the mortar bed in their planned position. Wiggle each stone slightly when placing it, then tamp it gently with the handle of the trowel to remove air bubbles from the mortar bed.

Pack mortar between the stones using a downward flicking motion with the trowel. Use the trowel to remove any excess mortar from the joints and wipe off any mortar that gets on the stone face with a damp sponge. Leave about an inch of space without mortar between joints at the front of the wall.

Mortar and set stones, working in sections, until the entire first course is completed.

Set up guides that will help to keep the courses level. Pound wooden stakes into the ground at the ends of the wall and tie level strings to the stakes a few inches above the course.

Measure down from the string to the stones in several places to make sure that the stones reach roughly the same height and adjust the stones as needed by placing additional mortar under the stone or push down on the stone to squeeze excess mortar out.

Prepare to place the next course. Dry-fit the next layer of flagstones. Make sure that vertical joints are staggered so that one stone is set over where two come together below it. Adjust the guide strings and set the stones aside, keeping them in their planned relative positions.

Lay the next course. Spread 2 inches of mortar over the first course, then place the stones in the mortar and fill in joints.

Continue building the wall course by course by adjusting the guide strings, laying mortar, placing stones, filling in joints and checking for level until the desired height is reached.

Place capstones on the wall, if desired. Capstones are generally large, flat rocks that can extend the entire width of the wall and may be cut to have square edges to give the wall a more finished appearance. Make a level, inch-thick mortar bed on top of the last course and set the capstones in the mortar. Fill the gaps with mortar and tool the joints when the mortar has dried slightly.