How to Build a DIY Timber Retaining Wall

Building a timber retaining wall is a good way to create a flat area in your landscape for a patio or flower bed, and it's sometimes necessary just to control erosion or to protect a small, existing structure or hardscape feature. When speaking of "timber" as opposed to the more generic term "wood," landscapers are usually speaking of 6x6 pressure-treated lumber rather than 2x6 or 2x8 pressure-treated boards, which require posts for support. Timber is beefy enough to stand on its own, although it needs to be pinned together and supported with braces known as deadmen to effectively hold back dirt.

If you intend to DIY your own timber retaining wall, plan on keeping the height below 3 feet. Most building authorities require a permit for anything higher than that, says Retaining Wall Companies. The permitting process involves a plan drawn up in conjunction with an engineer, and the wall likely needs deep postholes that may require heavy machinery to dig. A 3-foot wall is still a major home improvement project that's going to involve a lot of digging, shoveling and heavy lifting; you can do it yourself, but you'll definitely want the help of at least one other person.

Unlike a decorative garden wall, a retaining wall has a job to do, which is to hold back soil, so it needs to be strong and built of long-lasting materials. While they don't last as long as concrete, brick or stone, 6x6 landscaping timbers are pressure-treated with a wood preservative that fights rot and confers a typical service life of 20 years or more, and they weigh enough to provide good inertial resistance.

Contractors increase the ability of a retaining wall to hold back dirt by constructing it with a slight batter, which means it slopes toward the dirt it's holding back. They also take advantage of the weight of the dirt being held back by attaching the wall to anchors that extend under the dirt. These anchors can be geogrids made of high-tensile woven fabric, as described by CRH, or some other material, but for a timber retaining wall, the most common ones are wooden braces known as deadmen that extend laterally behind the wall and are usually anchored by crosspieces at their ends.

A good wood retaining wall sits on a solid footing of compacted gravel, and it has sufficient drainage behind it to protect the wood from moisture deterioration. The timbers are pinned to one another and to the ground, usually with steel reinforcement bar, or rebar, or with landscape screws. Adding a corner or a zigzag pattern gives the wall more strength. One of the most important characteristics is that the wall is high enough to prevent dirt from spilling over the top. If you don't have a permit, remember to keep it under 3 feet.

Like a fence, a retaining wall should never encroach on a neighbor's property, so you need to stake out the property line and ensure that you have enough room for the wall, the backfill material and the deadmen, which typically means the wall should be several feet from the property line.

And speaking of backfill, every cubic foot you add will require the removal of the same amount of native dirt, so if you're planning a long wall, plan ahead so you know what to do with all of that dirt. You can replace most of it, but you may prefer to backfill with planting soil instead. In that case, you may have a place for the dirt in another part of the yard, but if you don't, you'll need a truck to haul it away (or a willing neighbor with their own way of picking it up).

The project involves digging a trench for a gravel footing, which needs to be at least 12 inches deep. Before you start digging, call 811, a national hotline set up by utility companies, and request to have the underground utility lines marked on your property (a free service). This is a requirement that prevents you from slicing an electric or telephone cable and necessitating an expensive repair for which you'll get the bill.

Pressure-treated timbers make good steps, and while you can put steps directly in front of the wall, they look better and are more useful if you do as Allan Block recommends and leave a space for them when building the wall. This would consist of two corners formed by perpendicular walls extending toward the hillside. This creates an alcove into which you can stack timbers or build a conventional staircase with stringers and treads.

To determine the number of steps, measure the vertical rise from the bottom to the top of the wall using a tape measure and divide that number by the height of the treads, which is 5 1/2 inches for 6x6 timbers. If you flatten out the ground inside the alcove, you can place timbers on top of one another, anchoring them to the ground and to one another, or you can construct a pair of stringers with a 7-inch rise, fasten them to the sides of the alcove and screw treads to the stringers.

Stacking timbers is probably the easier method. Create the base by laying timbers side by side and anchoring them to the ground with rebar and then place a second layer and screw them down with landscape screws, leaving the first two timbers in the base exposed to act as the first step (this makes each step about 11 inches deep). Continue adding layers in this way, leaving the two timbers in the layer underneath exposed until you reach the last step, which is ideally at the same level as the ground at the top of the stairway.