This tutorial is designed for beginners interested in building stone retaining walls, 3 feet in height or shorter, using a dry-walling or "dry-stack" approach. Other materials that could be used for such a project include landscape timbers or salvaged railroad ties, concrete, and landscaping block (such as you see at Home Depot and Lowe's). But the step-by-step instructions provided below are meant specifically for walls built of stone.
The dry-stack method is not appropriate for walls that are to be greater than 3 feet in height. For taller structures, you would want mortared walls. Beginners should expect to spend two days to construct 10 feet of wall.
How to Build a Stone Retaining Wall
- Check local codes before you begin (you may need a permit), although often you will not be bothered if you keep the structure 3 feet high or less. Also phone the Call Before You Dig number well before starting the project.
- In the planning stages, you will be working out all the logistics, taking all the measurements, gathering materials, etc. If your slope is too large for a 3-foot-high structure, you could terrace the slope by building retaining walls in two or more places, rather than trying to do the whole job with just one structure. Building stone retaining walls greater than 3 feet in height is trickier. The mortar-less method discussed in this tutorial is not intended for such projects.
- The great virtue of stone retaining walls that are mortar-less is that your "drainage system" comes built-in: water will usually seep through the cracks between the stones. When damage from water pressure does occur, it can be repaired easily enough. Another advantage in building terraces of this sort is that you do not have to sink a "footing" beneath the frost line, as you do when using concrete or stone and mortar.
- Select stones that have at least two sides that are flat (what will become the "top" and the "bottom" once in place in the structure). The heavier the stones, the more stability you will have -- but also the harder the work (it will be worth it, though).
- Plot out where the retaining wall will sit at the bottom of the slope, using stakes and string for a straight terrace, a garden hose for a curved one. The advantage of a straight terrace is that you can attach a line level to the string to make sure the courses of your retaining wall are level.
- Dig a trench about 8 inches -10 inches deep, so that the first course of stone will be fully or mostly submerged. This will help your retaining wall withstand the pressure exerted on it by the slope it is holding back.
- To calculate the necessary width of the trench, just remember the base of the structure should be half the wall's height. Angle the trench so that it inclines back slightly into the slope (2 inches for every 1 foot of terrace height) – this will provide greater stability.
- When you have almost penetrated down to the required depth, use a skimming motion to remove the remaining soil, so that you do not end up with a base of loose soil. Keeping the base as solid as possible will reduce the chances of shifting as the retaining wall settles.
- Terraces of natural stone are laid in "courses," that is, one horizontal row at a time. The first course of stones will consist of your largest, widest, longest, flattest stones (but save some for the final course, the "capstones"). They have to be the most stable stones. Take the time to fit them as closely together as possible. Building terraces with natural stone is like fitting the pieces of a puzzle together -– only it is a puzzle that can turn out many different ways.
- In terms of the height of the first course of stones and the following courses, you have two choices. If you are aiming for a look of uniform rows, choose stones of the same height when laying a particular course. The other option is a more random look, in which you play each course by ear, using filler stones wherever necessary to make up for a difference in heights. Sometimes you are forced into the latter option, because the stones you have to work with simply are not uniform enough.
- Check to ensure that the stones run level left to right. But because you have built a slight backwards slope into the trench's base, your stones will slope down slightly from front to back. After completing this first course (the foundation, if you will) backfill with some of your excavated soil and any stones too small to use for building the retaining wall, and tamp it down.
- In laying the next course of stones and those that follow, avoid lining up the joints over the joints of the course underneath. Again, backfill and tamp down after completing the course. Also tuck soil in between any gaps in the terrace, to serve as “chinking" (a term that comes from log-cabin building but which is also apt here). When you are finished building your retaining wall, you can root plants into this chinking and bring life to the structure. Cascading plants, such as creeping thyme, perennial yellow alyssum, and annual white alyssum are very attractive spilling down the sides of stone retaining walls.
- As you place each stone, check that there is as little wobble as possible. To counteract any wobbling, you may have to use small, flat rocks as "shims." Use a mason's hammer to knick off stone fragments so as to achieve a better fit where possible.
- Continue in the same manner with the third course and succeeding courses. By the time your terrace is half its planned height, you should start incorporating what are known as "deadmen." In the case of stone retaining walls, the term refers to long stones laid perpendicularly across the wall, rather than parallel to all the other stones. The idea behind deadmen stones is to tie the structure into the slope in back of it for greater stability.
- A hole is first dug into the slope to incorporate a deadman. Then one end of the deadman is set on the terrace (as part of whatever course you happen to be laying), and the other end placed into the hole you have just created in the slope. The longer the stones you can find to serve as your deadmen, the better. The Colorado State Extension states that "A good rule of thumb is to provide at least one deadman per 16 square feet of exposed wall face."
- When you have almost reached the desired height for your terrace, it is time to place the capstones on top. "Capstones" are similar to the stones used in your first course, in the sense that they should be flat and have significant mass. They serve both to help hold the stones under them in place and to provide a finished look to the structure (thus the importance of their being flat).
A Planting Tip and the Supplies That You Will Need
After you have built your stone retaining wall, you will want to plant something in the soil being retained (on the slope behind the wall), to further anchor it and prevent erosion. An excellent low-maintenance choice for sunny areas is "Blue Rug" juniper shrub, a low-growing evergreen. Japanese pachysandra is a possibility for shade.
In addition to stones, assemble the following supplies ahead of time:
- Line level and string / garden hose
- Mason's hammer
- Carpenter's level