Terracing a hillside with wood involves cutting level benches into the slope and building timber retaining walls to hold each bench in place. The basic process is straightforward for walls up to 4 feet tall: excavate a flat shelf, install a pressure-treated timber wall at the downhill edge, backfill with gravel and soil, and repeat up the slope. Taller or steeper projects get more complex and typically require engineering.
Planning Your Terrace Layout
Before you dig, you need to decide how many terraces the slope requires and how tall each wall will be. The simplest approach is to divide the total vertical rise of your hillside by your desired wall height. A slope with 12 feet of total rise, for example, could become three terraces with 4-foot walls or four terraces with 3-foot walls. Shorter walls are easier to build, more forgiving of mistakes, and less likely to need a permit.
Most jurisdictions don’t require a building permit for retaining walls 4 feet or shorter, measured from the lowest ground level to the top of the wall. Once you exceed that height, you’ll typically need both a permit and engineered plans. Even below 4 feet, walls that support a driveway, structure, or another retaining wall directly above them may still trigger permit requirements. If your terraces will be stacked close together, check with your local building department first, since closely spaced walls are often treated differently than standalone ones.
Mark your terrace locations with stakes and string. Each flat bench should be wide enough to be useful, generally at least 3 feet for planting beds and 6 feet or more for usable yard space. On slopes steeper than about 40%, you’ll want benches at least 10 feet wide to give yourself room for excavation equipment and proper compaction.
Choosing the Right Wood
The timber you use will be buried in soil permanently, so standard lumber won’t last. You need pressure-treated wood rated for ground contact, specifically Use Category 4A (UC4A) or higher as defined by the American Wood Protection Association. UC4A covers general ground contact. UC4B is rated for heavy-duty ground contact and is the better choice for retaining walls where the wood bears constant soil pressure and moisture.
The most common choice is 6×6-inch pressure-treated landscape timbers or 6×6 posts, available at most lumber yards in 8-foot lengths. Be careful with thinner stock: treated lumber that’s 2 inches thick or less is often only treated for above-ground use, even if it looks identical. Check the end tag stapled to each piece, which lists the use category and preservative type. Look for treatments like Copper Azole (CA-B or CA-C) or Micronized Copper Azole (MCA), which are the most widely available ground-contact preservatives for residential lumber.
Avoid old railroad ties. They’re treated with creosote, which leaches chemicals into soil and is not recommended near gardens or areas where you’ll spend time.
Excavating the Hillside
Terracing uses a cut-and-fill method: you cut soil from the uphill side of each bench and use that same soil to fill the downhill side, creating a level platform. The goal is to balance the volume of soil you remove with the volume you place so you’re not hauling material off-site or trucking in extra fill.
Start at the bottom of the slope and work upward. For each terrace:
- Strip the organic layer. Remove grass, roots, and topsoil from the bench area. Set this aside to use as a top layer later.
- Cut into the hillside. Dig into the uphill face to create a vertical or near-vertical cut at the back of the bench. The depth of this cut depends on your wall height and bench width.
- Fill the downhill side. Spread the excavated soil on the downhill portion to extend the flat area forward to where your wall will sit.
- Compact in layers. Don’t dump all the fill at once. Spread it in layers about 6 inches thick and compact each layer with a plate compactor or hand tamper before adding the next. Uncompacted fill will settle unevenly over months and pull away from your wall.
On steeper slopes, cut a small shelf (called a toe bench) into the undisturbed hillside before placing fill over it. This keeps the new fill from sliding along the original slope surface.
Building the Timber Wall
Each retaining wall sits at the front edge of its terrace, holding back the soil of the bench above or behind it.
Preparing the Base
Dig a trench where the first course of timbers will sit. The trench should be about 8 inches deep and wide enough to fit your timbers with a few extra inches on each side. Partially burying the base course prevents the wall from sliding forward under soil pressure. Level the bottom of the trench carefully. If the trench follows a slope along its length, step the timbers down in increments rather than angling them.
Lay 2 to 3 inches of compacted gravel in the trench before placing the first timber. This provides drainage under the wall and a stable, level base.
Laying and Pinning the Courses
Set your first course of 6×6 timbers in the trench. Pin them to the ground using 1/2-inch diameter rebar spikes, 24 inches long. Pre-drill holes through the timber, then drive the rebar through and into the subgrade below. The spike should penetrate at least 12 inches into undisturbed soil beneath the timber. Space rebar pins about 4 feet apart along each timber and within 12 inches of every joint where two timbers meet end to end.
Stack additional courses on top, offsetting the joints like brickwork so no two seams line up vertically. Connect each course to the one below it using structural timber screws or galvanized spikes at least 10 to 12 inches long, driven every 2 to 3 feet. Stagger your fastener locations so they don’t split along the same grain line.
Installing Deadman Anchors
For walls taller than about 2 feet, you need deadman anchors to keep the wall from tipping forward. A deadman is a timber that runs perpendicular to the wall face, extending back into the hillside, with a cross plate (a short timber) attached at the buried end. Think of it as a T-shaped anchor embedded in the soil behind the wall.
Install deadmen every 8 feet along the wall’s length. Each deadman should be about 6 feet long, extending from the wall face back into compacted soil. Attach the deadman to the wall course with structural fasteners or lag bolts, and bolt a cross plate (a 3-foot section of matching timber) perpendicular to its buried end. The cross plate acts like an anchor fluke, resisting the pull of the wall. Deadmen are typically placed in the middle courses of the wall, not in the very top or very bottom row.
Drainage Behind the Wall
Water pressure is the most common reason timber retaining walls fail. Without drainage, soil behind the wall becomes saturated after rain, dramatically increasing the force pushing against the timbers.
Line the back face of the wall with landscape fabric, then fill the space directly behind the timbers with 8 to 12 inches of clean gravel (crushed stone or pea gravel). Lay a perforated drain pipe at the base of the gravel, sloped slightly to one or both ends of the wall so water can exit. The landscape fabric keeps soil from migrating into the gravel and clogging it over time.
You can also drill weep holes through the face of the lowest course, about every 4 to 6 feet, to let trapped water escape directly through the wall. Angle the holes slightly downward toward the outside face.
Backfilling and Finishing
Once the wall and drainage layer are in place, backfill behind the gravel with the soil you excavated earlier. Add soil in 6-inch lifts and compact each layer. Overfill slightly above grade because even well-compacted fill settles a small amount over the first year. Spread the topsoil you stripped earlier across the top of each finished bench.
If you’re building multiple terraces, repeat the entire process for each level moving up the slope. Leave at least a few feet of horizontal distance between the back of one wall and the base of the next. Stacking walls directly on top of each other creates surcharge loads that a simple gravity wall isn’t designed to handle.
What It Costs
For a DIY timber retaining wall, expect to spend roughly $13 to $35 per square foot of wall face, depending on your region and the grade of lumber you choose. A 20-foot-long wall that’s 3 feet tall has 60 square feet of face area, putting material costs somewhere between $780 and $2,100. Rebar adds about $1.50 to $2.50 per square foot. Gravel for drainage runs $4 to $7 per bag, and you’ll need quite a few bags, so buying gravel by the cubic yard from a landscape supplier is usually more economical for anything beyond a very small wall.
The biggest variable in total cost is whether you can do the excavation by hand or need to rent equipment. A single small terrace on a gentle slope is manageable with a shovel and wheelbarrow. Multiple terraces on a steep hillside almost always justify renting a compact excavator, which typically costs $250 to $500 per day.
How Long Timber Terraces Last
Properly treated 6×6 timbers rated UC4A or UC4B in ground contact generally last 15 to 25 years before they need replacement. Longevity depends heavily on your climate and drainage. Walls in wet climates with poor drainage behind them deteriorate faster. Walls with good gravel backfill and functioning drain pipes at the base can push toward the upper end of that range. You’ll know replacement is coming when timbers become soft enough to push a screwdriver into easily, or when the wall begins to visibly bow outward.