Retaining Walls: Types, Materials, Drainage, and Building Codes
FriendsWithTools.io earns a commission from qualifying purchases made through links on this page, at no additional cost to you. We do not test these tools ourselves — all claims are sourced from manufacturer specifications, retailer listings, and aggregated user reviews, each linked inline. Prices and ratings were verified on May 2026 and may have changed.
A retaining wall holds back soil that would otherwise slide or erode downhill. Walls under 4 feet tall are typically DIY-friendly with segmental block or timber. Walls over 4 feet require an engineer in most jurisdictions because the soil forces increase dramatically with height. A 6-foot wall handles roughly four times the lateral earth pressure of a 3-foot wall, and the consequences of failure scale accordingly.
Types of Retaining Walls
Gravity walls rely on their own mass to resist the outward push of the retained soil. Segmental concrete block (interlocking landscape blocks from manufacturers like Belgard, Pavestone, and Anchor Wall) and stacked natural stone are the most common gravity wall materials for residential work. These walls lean back slightly into the hillside, a design feature called setback or batter, which directs the wall's weight into the slope to counteract the soil pressure.
Timber walls use pressure-treated landscape timbers (typically 6x6 or 6x8 inches) stacked horizontally and pinned together with lengths of 1/2-inch rebar driven through drilled holes into the ground. Timber walls are simpler to build than block and cost less per linear foot, but they have a shorter lifespan of 15 to 20 years before the wood deteriorates even with pressure treatment. CCA-treated timbers (common before 2004) lasted longer but are no longer available for residential use. Current ACQ and CA-B treated timbers are safe for soil contact but less durable than the old formulations.
Cantilevered walls use engineering principles rather than mass alone to resist soil. Made from poured concrete or concrete masonry units (CMU) reinforced with rebar and built on an L-shaped concrete footing, these walls use the weight of the retained soil sitting on the footing's heel to stabilize the structure. Cantilevered walls are the standard for heights over 4 to 6 feet and require engineered drawings from a structural engineer.
Gabion walls are wire mesh baskets (typically galvanized or PVC-coated steel) filled with stone or crushed rock. They drain freely because water passes through the stone fill without building hydrostatic pressure behind the wall. Gabions are common for erosion control on slopes and along waterways, and they work well in residential applications where drainage is a primary concern. Gabion basket systems from Hesco and Modular Gabion Systems are available through landscape supply dealers.
Site Preparation
Excavate a trench for the base course. The trench should be as wide as the wall blocks plus 6 to 8 inches behind the wall for drainage gravel backfill. Depth depends on wall height: 6 to 8 inches below finished grade for a 3-foot wall, and proportionally deeper for taller walls. A common rule is to bury the first course at least one-eighth of the total wall height.
Compact the trench bottom with a plate compactor (available for rent at Home Depot and Sunbelt Rentals for about $80 to $100 per day) or a hand tamper for small projects. Spread 4 to 6 inches of compactable gravel, specifically 3/4-inch crushed stone with angular edges, not round pea gravel. Crushed stone interlocks when compacted; round gravel shifts under load. Compact the gravel layer in two passes.
This base must be level. Every error in the base course multiplies as the wall rises. A wall that starts 1/4 inch off level at the bottom will be visibly crooked by the fourth or fifth course. Spend the time to get the first course right, even if it means re-doing the gravel layer.
Before digging, check for underground utilities. Call 811 (the national "Call Before You Dig" number) for a free locate. Retaining wall trenches are typically 12 to 18 inches deep, which is well within the range of buried gas, electric, cable, and water lines. A shovel through a gas line is an emergency; a shovel through a fiber optic cable is an expensive repair.
Building with Segmental Block
Set the first course of blocks in the compacted gravel base. Use a 4-foot level on every individual block and across multiple blocks in the run. Shim low spots with small amounts of gravel under the blocks and tap them into position with a rubber mallet. Blocks from Belgard, Pavestone, and Anchor Wall typically weigh 30 to 80 pounds each depending on the size, so plan for the physical effort involved.
Stack subsequent courses with the built-in setback. Most segmental block systems have a lip, pin, or channel system that automatically offsets each course 3/4 to 1 inch back from the course below. This batter angle leans the entire wall face into the hillside. The setback is engineered into the block design, so no measuring or guessing is needed for the angle.
Stagger the vertical joints. Center each block over the joint of the course below, similar to a running bond brick pattern. This ties the wall together structurally and distributes point loads across multiple blocks. Avoid stacking joints directly on top of each other, which creates a vertical weakness line.
For walls over 3 courses tall (roughly 2 feet), install geogrid reinforcement every 2 to 3 courses. Geogrid is a polymer mesh (products from Tensar, Strata, or Mirafi are common at landscape supply yards) that extends back into the retained soil, anchoring the wall to the earth mass behind it. The geogrid should extend at least 3 to 4 feet behind the wall face. Without geogrid, tall segmental block walls bulge outward under sustained soil pressure, especially when the soil is saturated after rain.
Backfill behind the wall with drainage gravel (clean 3/4-inch stone) as you build each course. The gravel zone should extend at least 12 inches behind the blocks. Do not backfill with the excavated clay or topsoil. Native soil holds water and dramatically increases the hydrostatic pressure the wall must resist. The drainage gravel lets water pass through to the drain pipe at the base rather than building up behind the wall face.
Drainage Is Not Optional
Water is the primary cause of retaining wall failure. Saturated soil weighs roughly 20 to 30 percent more than dry soil, and the water itself exerts hydrostatic pressure directly against the wall face. A wall designed for dry soil conditions that gets saturated after a heavy rain event is carrying loads far beyond its design capacity. Most retaining wall failures happen during or shortly after prolonged rain.
Install a perforated drainage pipe (4-inch corrugated pipe with a filter fabric sock to prevent soil intrusion) at the base of the wall, behind the first course, sitting in the drainage gravel bed. The pipe should slope at least 1/4 inch per foot toward a daylight outlet at one or both ends of the wall. The daylight outlet is where the pipe exits the hillside at grade level, allowing the collected water to drain away from the wall by gravity.
The drainage gravel backfill works as a filter system. Water from the retained soil drains through the gravel and into the perforated pipe, which carries it to the daylight outlet. Cap the top of the gravel backfill with landscape fabric before adding topsoil on top. The fabric prevents fine soil particles from migrating down into the gravel and eventually clogging it. Without the fabric barrier, the gravel fill becomes soil-clogged within a few years and stops draining effectively.
Permits and Engineering
Most jurisdictions require a building permit for retaining walls over 4 feet tall, measured from the bottom of the footing to the top of the wall. Some municipalities set the threshold at 3 feet. A few (notably parts of California and areas with known landslide risk) require permits for any retaining wall regardless of height. Check with your local building department before starting any wall project.
Walls that require a permit typically also require engineered drawings stamped by a licensed structural engineer. The engineer calculates the lateral earth pressure based on soil type, slope angle, drainage conditions, and any surcharge loads (driveways, structures, or slopes above the wall), then specifies the wall design, reinforcement schedule, and drainage requirements. Engineering fees for a residential retaining wall typically run $500 to $2,000 depending on complexity.
Even for walls under the permit threshold, following sound engineering practice protects you. Proper drainage, a compacted base, adequate setback, geogrid where needed, and gravel backfill are not optional extras. A 3-foot wall that fails and slides onto a neighbor's property or into a walkway is still your liability, permit or not. Building it correctly costs marginally more than building it poorly.
Walls near property lines, near structures, on slopes above occupied structures, or supporting driveways and parking areas may have additional requirements regardless of height. Setback requirements from property lines vary by jurisdiction. The building department can clarify what applies to your specific situation before you start digging.
Tools for Retaining Wall Construction
The excavation phase requires a shovel, a mattock or pick for breaking compacted soil, and a wheelbarrow for moving dirt and gravel. A plate compactor is essential for the base and backfill layers. Renting a plate compactor runs about $80 to $100 per day from Home Depot or Sunbelt Rentals and is money well spent compared to trying to compact a base with a hand tamper alone.
For block setting, you need a 4-foot level, a string line for maintaining alignment over long runs, a rubber mallet for adjusting block positions, and a masonry saw or a diamond blade on a circular saw for cutting blocks at corners and ends. A standard circular saw fitted with a 7-inch diamond masonry blade handles most segmental block cuts. Wet cutting reduces dust significantly.
For larger walls, a small excavator (mini excavator rentals start around $250 to $350 per day) saves days of hand-digging. A laser level or builder's transit establishes grade over long wall runs more accurately than a bubble level. Geogrid, landscape fabric, 4-inch perforated drain pipe with fittings, and plenty of 3/4-inch drainage gravel round out the materials list. Budget for more gravel than you think you need. Running short mid-project means stopping work and making an extra delivery run.
Frequently Asked Questions
How Long Do Retaining Walls Last?
Segmental concrete block walls with proper drainage last 50 years or more. Poured concrete and CMU walls last 50 to 100 years. Pressure-treated timber walls last 15 to 20 years. Gabion walls last 30 to 50 years depending on the wire basket coating. The most common cause of premature failure is inadequate drainage, not material deterioration. A well-drained timber wall outlasts a poorly drained concrete block wall because the drainage prevents the saturated-soil overload condition that causes structural failure.
Can I Build a Retaining Wall on a Slope?
Yes, but the excavation is more complex. You need to bench-cut into the slope to create a level trench for the base course. The uphill end of the wall will be taller (retaining more soil) than the downhill end. If the grade change is significant, stepping the wall in increments is more practical and more stable. Two 3-foot walls with a terrace between them handle the grade change better than one 6-foot wall, and the shorter walls are within DIY range without engineering.
What Is the Cheapest Retaining Wall Material?
Pressure-treated timber is the lowest material cost per linear foot for walls under 3 feet. Landscape timbers (6x6, 8-foot lengths) run $12 to $20 each at Home Depot and Lowe's. Segmental block costs more per unit but installs faster and lasts much longer, making it the better value over a 20-year timeframe. For very short walls under 2 feet, stacked natural stone without mortar is the cheapest option if you have a local source for fieldstone or recycled stone.