How to Build a Retaining Wall with a Terrace
Integrate stone steps and a stone retaining wall into a challenging backyard slope to make the space more usable.
The first step of the project is to clear the area where you'll be building your tiered wall. If there is a preexisting stone wall, break it up to make room for the new tiered walls. You can save on your budget by reusing some old stones in your new project. You'll want stones with color and character for the wall face, and flat, level stones for the cap.
Next, dig the footings for the new tiers. Each tier will be 20 inches high, and will need a footing that is 12-inches deep. (A 12-inch footing can support a wall that is up to 3 feet high.) A wall 20 inches tall should be 14 to16 inches thick. Anything wider would look out of proportion. If you find some rocks in your footing that you can't move, don't panic. They can be incorporated into the footing. If the rock looks good enough, you can use it in the face of the wall. Otherwise, you can pour concrete around the rock and above it, and incorporate it into the wall. For each footing, follow the same procedure: let the earth tell you where to go.
Next, add pre-mixed concrete to the footings. (Pre-mixed concrete is concrete mixed with sand, and also contains small pebbles for strength.) Pour the concrete into a wheelbarrow and add in water, then use a hoe to mix the concrete to a consistency of peanut butter. Remember to always wear a safety mask while mixing and concrete or mortar to avoid breathing concrete dust particles (Image 1). Once your concrete is ready, pour it into the 12-inch deep row for the footing. For reinforcement, place half-inch rebar along each footing (Image 2). Lay the rebar into the wet concrete like railroad tracks, and then push them into the concrete until they're about halfway down. (The rebar will keep the footing from breaking and will prevent frost heaves in the winter time.) Wait until the concrete has set up (usually in about 12 hours,) before beginning to set the stone.
Before setting any stone, your first step is to mix the mortar. There are two different ways to mix mortar — by hand or by machine. For a large job, consider renting a mortar mixer. A "one bagger" will hold one bag of mortar, plus sand and water. First, add the water. Generally speaking, one bag of Type S mortar will need one 5-gallon bucketful of water. However, the amount of water necessary can vary, depending on the wetness of the sand, so first put in about 3/4 of your 5-gallon bucket of water. Next, add in the sand and mortar. One bag of Type S mortar will need about 16 shovels-full of mason sand. With the mixer going, you can add the rest of the water as needed. For a smaller job, you can mix the mortar by hand. In a wheelbarrow, dry mix Type S mortar with mason sand, then slowly pour in the water. Use the same rations of sand and water, (about 16 shovels-full of sand, and about 5 gallons or water).
Before actually building the wall, you'll want to make sure it is level and straight. First, set up steel bars or wood stakes for posts at each end of the wall, and run a string line between them. Remember that it is important for your string to be square against any adjacent walls or structures (Image 1). Now you're ready to start laying down stone. If you're building next to a wall, then the best place to start is against that wall. If you're not against a wall, then it's best to start in the middle (Image 2).
Whenever you're building a retaining wall, you need to be sure to create a path where the water will go. If not, the water can build up and damage your wall. Take broken stone and pile it up to create a "beehive" near the base of the wall every five feet or so. Build each beehive at the rear of the wall, so that a stone can be set in front of it. Use just enough mortar to contain the broken stones. Set a stone in front of the "beehive," and as you're setting it, be sure to leave a joint dry. Rainwater will be attracted to and flow through the stone beehive, and exit the dry joint in front. Build up additional layers of stone, always keeping an eye on the colors and sizes of stone. You'll want a good amount of variety and contrast for a rustic look.
The next step is to cap your wall. Even if you're building a rustic farm wall, you still want a nice cap. For capping, you'll need stones that have a smooth and flat top. With a stone like Connecticut green, you can create capstones by tapping a thick stone with a carbide chisel along the stone's veins, to split the stone into flat pieces (Image 1). To ensure that your cap stones are all at a level height, run a taut string between the two crowbars at capstone level. Run a line level across it to make sure the capstones are level. Some stones may have a nice smooth top, but will sit at an angle, with one end thicker than the other. Don't worry: Set the stone to your string, so that the top is level. Then add a small flat stone, known as a shim, underneath your larger stone, and that will make it level all around (Image 2). If you don't have any shims lying around, you can chip away at some large stones with your chisel to create some.
The final step to building each wall is jointing the top cap. The jointing is very important because you don't want any water sitting in open joints. Start by filling in the joints with mortar. Use a trowel and jointer to pack the joints until they are slightly recessed (figure H). Then smooth the mortared joints by brushing them with an inexpensive paintbrush. Also remove excess mortar from the joints on the face of the wall. This step should be done when the mortar has set a bit, but before it dries. Use the jointer to dig out the mortar until it is uniformly recessed about 1-2 inches. This will give you a wall that looks like a dry-stack farm wall, but that has the strength of concrete. Smooth the joints with a paintbrush, after they have been uniformly recessed. Once your first wall is completed, you can repeat the same steps for your additional walls.
Since your new retaining walls will need something to retain, spread loam behind each tier and add landscaping. Replant the plants that you had saved from the original landscaping.