This tutorial is on building retaining walls out of natural stone, 3’ in height or less, using a dry-walling or "dry-stack" approach. Other materials for building retaining walls include timbers or salvaged railroad ties, concrete and landscaping block (such as you see at Home Depot and Lowe’s). The dry-stack method is not appropriate for terraces greater than 3 feet in height.
Difficulty: Average
Time Required: 2 Days for a 10'-Long Retaining Wall
Here's How:
Check local codes before you begin digging (you may need a permit), although often you won’t be bothered if you keep the retaining wall 3’ high or less. If your slope is too large for a 3’-high retaining wall, you could terrace the slope by building retaining walls in 2 or more places, rather than trying to do the whole job with just one retaining wall. Building terraces greater than 3’ in height is trickier. The mortar-less method discussed in this tutorial is not intended for such structures.
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 retaining wall). The heavier the stones, the more stability you’ll have -– but also the harder the work (but it will be worth it).
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”-10” deep, so that the first course of stone is 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 retaining wall should be half the wall’s height. Angle the trench so that it inclines back slightly into the slope (2” for every 1’ of terrace height) – this will provide greater stability.
When you’ve almost penetrated down to the required depth, use a skimming motion to remove the remaining soil, so that you don’t 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,” i.e., a 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’s 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’re 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 aren’t uniform enough.
Check to ensure that the stones run level left to right. But because you’ve built a slight backwards slope into the trench’s base, your stones will slope down slightly 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.” When you’re finished building your retaining wall, you can root plants into this chinking and bring life to the retaining wall. Cascading plants, such as thyme and the annual, lobelia are very attractive in stone retaining walls.
As you place each stone, check that there’s 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 retaining wall into the slope in back of it.
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’ve 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’ve almost reached the desired height for your terrace, it’s 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 retaining wall (thus the importance of their being flat).
Tips:
After you've built your stone retaining wall, you'll want to plant something in the soil being retained, to further anchor it. An excellent low-maintenance choice for sunny areas is "Blue Rug" juniper shrub, a low-growing evergreen.
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