- It's possible to build retaining walls with everyday landscaping materials such as lumber and stones, but you can also build more complicated versions that entail the use of concrete, reinforcing steel and support systems. Height is the main consideration for retaining walls; the more soil you need to hold in place, the higher the wall should be. Lumber and stone are enough for smaller retaining walls, known as gravity walls, but larger walls such as cantilever, counterfort and buttressed walls need concrete and reinforcing steel.
- Pressure-treated posts and boards and some concrete are sufficient for smaller gravity walls. Wood walls start with digging a 2-foot-wide trench, deep enough to hold a layer of compacted and leveled sand or gravel and one row of cross-member boards. Posts of at least 4-by-4 inches, footed in concrete, should be placed every 4 feet along the length of the wall. The cross members on short walls are usually made from 2-by-6-inch boards, whereas 2-by-8 or 2-by-10 boards are more suitable for taller gravity walls up to 3 or 4 feet high. Cross-member boards are bolted horizontally in tight-fitting adjacent rows across the posts, top to bottom. Carriage bolts, rather than nails or screws, are needed to secure wood retaining walls.
- Stone is a popular material in retaining wall construction, because the type of stone used can add an attractive look to beds and planters. Stone walls, otherwise known as segmental walls, also begin with a trench with a sand or gravel base that needs to be compacted and leveled to ensure a straight wall line. Rows of stone are laid from the trench upward, using mortar to hold them in place if needed. Backfill aggregate is added with each row until the wall is complete, and drain tile is placed at grade level behind the wall, with outlets every 50 to 75 feet and at each end, for drainage.
- Larger retaining walls require poured or formed concrete. This type of retaining wall construction is generally best left to professional licensed contractors, due to the size of these retaining walls and the specifics of pouring and strengthening concrete. Proper footings, drainage and support are needed here, just like they are for foundation wall construction. This is why, unless you're a trained and knowledgeable concrete worker, you really should leave this job to the pros.
- Drainage is crucial in constructing retaining walls. Drain tiles and fabrics are used to accomplish this, along with sufficient aggregates in the base trench and backfill areas. Compacting the base trench as well as the backfill material is also important to provide a solid fill that won’t move during ground shifts and freezes. Lateral ground pressure is the No. 1 cause of failures in retaining walls; hiring a licensed engineer to study the soil and environmental status of the area is a safety measure you should keep in mind. Building codes are likely to be in play as well, so be sure to contact your local code agency before starting the project.
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