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A stable wall needs a solid foundation – for this wall, that means concrete. Dig a trench for the concrete foundation about one foot wide and 4 to 5 inches deep, running the full length of the seating wall. Use a square shovel to level out the bottom of the trench.
Mix concrete according to the manufacturer's instructions and pour the foundation. A portable cement mixer makes this job easier, but mixing by hand in wheelbarrows also works well. Always wear safety glasses and a dust mask when mixing concrete or mortar. Use extreme caution if working with power tools such as a portable concrete mixer or a drill with a paddle mixing bit.
Before the concrete dries, place the first layer of stones and press them into the wet concrete. Use the largest stones for the bottom of the wall, and choose the most attractive ones for the front face. Set the stones as close together as possible.
Different sizes of stones give this wall design much of its charm, but that great look requires some work. Stack different stones on top of the base layer to find the right fit. Again, use the most attractive stones for the front of the wall, and use the larger stones on the lower layers of the wall. Fill in any gaps with smaller stones as needed. A good fit for the stones makes the wall look more natural and makes the overall structure stronger.
Wearing safety glasses and a dust mask, mix mortar to the consistency of brownie batter. As with the concrete, mortar can be mixed by hand or with power tools. Use mortar to attach the stones at all points. Use a generous amount of mortar, especially at the center of the wall. Once a section is complete, use a wet sponge to clear away any excess mortar that has squeezed out between the stones. The mortar will be hidden on the completed wall, which will look as though it's held together by its own weight.
Keep installing stones until the wall reaches the desired height. Continue to dry-fit stones before mortaring in place, and wipe away excess mortar with a sponge.
The Yard Crashers crew topped off this low wall with a mortared slate top for a comfortable and decorative seating surface that matches the border on the gravel patio. It's important to keep the slate as level as possible for a comfortable seat; dry-fit the slate, with the smoothest edges toward the front of the wall, to check for level. Leave a small overhang over the edge of the seating wall to help hide the mortar. Use stone-shaping tools to score and chip away excess slate; remember to wear safety glasses when chipping or cutting stone.
Once the slate pieces are fitted, mortar them in place. For each piece, add mortar to the top of the seating wall and place the slate on top, pushing it into place. Check each new piece for level, removing or adding mortar as needed. Leave a small gap – less than an inch – between the slate pieces.
Once the top is mortared into place, go back along the wall and mortar in between the slate pieces. Fill the gaps with mortar and smooth down with a wet sponge so the mortar sits flush with the top of the stones. Let all of the mortar dry completely before placing any weight on the wall.