How to Build a Retaining Wall with Landscape Timbers

Building a retaining wall with landscape timbers is one of those weekend projects that looks straightforward until you're partway in and realize you skipped a step. The soil is moving, the timbers are shifting, and what started as a clean garden edge is starting to lean. Most of those problems trace back to a handful of decisions made before the first timber ever goes in the ground: sizing, drainage, and how the layers are tied together.

Here's a practical walkthrough of how to do it right the first time.

Choosing the Right Landscape Timber Size

Not all landscape timbers are the same, and the size you pick will affect both how the wall performs and how long it lasts. For low garden borders, a 4x4 or 4x6 timber works fine. If you're building a wall that needs to hold back a significant amount of soil, you’ll want something heavier, typically a 6x6. The added mass keeps the wall stable and gives you more surface area for the tie-backs that anchor the structure into the hillside behind it.

Treated timbers are the standard for ground contact, and the treatment retention level matters. Look for lumber rated for ground contact (UC4A or UC4B, depending on your soil conditions and regional moisture). Our landscape timber products are treated for exactly this kind of use, so you're not guessing at whether the wood can handle long-term exposure to soil and water.

Length also plays into your planning. Standard timbers run 8 feet, which makes cutting and staggering your joints fairly predictable. Map out your wall dimensions before ordering so you minimize waste and keep your joints offset between courses.

Getting Drainage Right Before You Build

Drainage is where most timber retaining walls fail. Wood handles moisture reasonably well when it can dry out between wet periods. What it can't handle is sitting in water-saturated soil with nowhere for that pressure to go. Hydrostatic pressure builds behind a wall without drainage, and eventually, something gives.

Before you set your first timber, excavate a few inches below grade and lay a base of compacted gravel. Aim for at least 6 inches of crushed stone as your foundation layer. Along the back of the wall, add a gravel backfill zone rather than packing native soil directly against the timbers. A perforated drain pipe running along the base of the wall and daylighting at one end of the run is worth the extra hour of work on any wall taller than two courses.

Weep holes are another option for shorter walls: drill or leave gaps between timbers at the base of each course so water has somewhere to exit. It's a simple addition that takes pressure off the structure over time.

Anchoring and Stacking for a Wall That Holds

A timber retaining wall is only as strong as how its layers are connected. Stagger your vertical joints the same way a mason staggers bricks, so no two courses share a seam in the same spot. Use galvanized spikes or rebar driven through the timbers vertically to pin each course to the one below it. Rebar is especially useful for the bottom two courses where the soil pressure is highest.

For walls taller than three or four courses, you'll need dead-man anchors. These are timbers laid perpendicular to the wall, running back into the hillside, that tie the face of the wall to the fill behind it. Skipping them on a taller wall is how you end up with a structure that looks fine in year one but starts bowing by year three.

Setting posts for corners and ends is a common place to reach for treated lumber fence posts, which pair well with timber walls where you need a structural vertical element. If you're driving posts alongside your timber project, a Montana post driver makes that work significantly faster and cleaner than hand-setting.

What to Buy and Where to Source It

Sourcing quality treated timber locally matters more than people expect. Timbers from big-box stores are often wet, heavy, and inconsistently treated. Working with a supplier that specializes in treated wood products means you're getting material that's been properly handled and dried to a workable moisture content.

At Legacy Post & Supply, we supply landscape timbers along with the full range of posts and wood products you'd need for a project like this. If you're also planning a fence alongside your wall, we carry everything from board fence materials to privacy fence components. You can also take a look at how landscape timbers compare to other materials in our post on landscape timber vs. railroad ties if you're still deciding on your approach.

Ready to get started? Request a quote from our team or reach out directly, and we'll help you figure out exactly what you need for your project.