Retaining Walls: Types, Materials, Drainage, and Building Codes
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A retaining wall holds back soil that would otherwise slide or erode. 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 pressure of a 3-foot wall.
Types of Retaining Walls
Gravity walls rely on their own weight to resist soil pressure. Segmental concrete block (interlocking landscape blocks) and stacked stone are the most common. They lean back slightly into the hillside (setback or batter) to counteract the outward push.
Timber walls use pressure-treated landscape timbers stacked horizontally and pinned together with rebar driven through drilled holes into the ground. Simpler to build than block but shorter lifespan (15-20 years before the wood deteriorates).
Cantilevered walls (poured concrete or concrete block with rebar and a footing) use engineering leverage rather than mass to resist soil. The L-shaped footing extends back under the retained soil, using the weight of the soil itself to stabilize the wall. Required for walls over 4-6 feet.
Gabion walls are wire baskets filled with stone. Good for drainage because water flows through freely. Common for erosion control on slopes and waterways.
Site Preparation
Excavate a trench for the base course. The trench should be as wide as the wall blocks plus 6-8 inches behind for backfill gravel. Depth: 6-8 inches below grade for a 3-foot wall, deeper for taller walls.
Compact the trench bottom with a plate compactor or hand tamper. Spread 4-6 inches of compactable gravel (3/4-inch crushed stone, not round pea gravel) and compact again. This base must be level — every error here multiplies as the wall rises.
Check for underground utilities before digging. Call 811 for a locate. Retaining wall trenches are typically 12-18 inches deep, which is well within the range of buried gas, electric, cable, and water lines.
Building with Segmental Block
Set the first course in the compacted gravel base. Use a level on every block and across multiple blocks. Shim with gravel under low spots. The first course determines every course above it — spend the time to get it right.
Stack subsequent courses with the built-in setback. Most segmental blocks have a lip or pin system that automatically offsets each course 3/4 to 1 inch back from the course below. This batter angle leans the wall into the hillside.
Stagger the joints — center each block over the joint of the course below, like a brick pattern. This ties the wall together structurally and distributes loads.
For walls over 3 courses, install geogrid reinforcement every 2-3 courses. Geogrid is a polymer mesh that extends back into the retained soil, anchoring the wall to the earth behind it. Without geogrid, tall block walls bulge outward under soil pressure.
Backfill behind the wall with drainage gravel (clean 3/4-inch stone) as you build each course. The gravel should extend at least 12 inches behind the blocks. Do not backfill with the excavated clay or topsoil — these hold water and increase the pressure the wall must resist.
Drainage Is Not Optional
Water is the primary cause of retaining wall failure. Saturated soil behind a wall weighs significantly more than dry soil, and the water itself exerts hydrostatic pressure against the wall face.
Install a perforated drainage pipe (4-inch corrugated with a filter sock) at the base of the wall, behind the first course, sitting in the drainage gravel. The pipe should slope at least 1/4 inch per foot toward a daylight outlet at one or both ends of the wall.
The drainage gravel backfill acts as a filter — water from the retained soil drains through the gravel and into the pipe, which carries it away. Cap the top of the gravel backfill with landscape fabric before adding topsoil to prevent soil from migrating into and clogging the gravel.
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 require permits at 3 feet. Check your local building department before starting.
Walls that require a permit typically also require engineered drawings — a structural engineer calculates the soil loads, specifies the wall design, and stamps the drawings for the building inspector.
Even for walls under the permit threshold, follow sound engineering practice: proper drainage, compacted base, setback, and backfill with gravel. A 3-foot wall that fails and slides into a neighbor's property is still your liability.
Walls near property lines, near structures, on slopes above structures, or supporting driveways may have additional requirements regardless of height. The building department can clarify.
Tools for Retaining Wall Construction
Shovel and mattock for excavation. Plate compactor for base and backfill (rent). 4-foot level and string line. Rubber mallet for adjusting blocks. Masonry saw or diamond blade on a circular saw for cutting blocks. Wheelbarrow for moving gravel and blocks.
For larger walls: a small excavator saves days of hand-digging. A laser level or transit for establishing grade over long runs. Geogrid, landscape fabric, and a perforated drain pipe with fittings.
Frequently Asked Questions
How long do retaining walls last?
Segmental concrete block walls with proper drainage last 50+ years. Poured concrete: 50-100 years. Pressure-treated timber: 15-20 years. The most common cause of premature failure is inadequate drainage, not material deterioration. A well-drained timber wall outlasts a poorly-drained concrete wall.
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. Step the wall in increments if the grade change is significant — multiple shorter walls are more stable than one tall one.
What is the cheapest retaining wall material?
Pressure-treated timber is the lowest material cost per linear foot for walls under 3 feet. Segmental block costs more per block but installs faster and lasts much longer. For very short walls (under 2 feet), stacked natural stone without mortar is cheapest if you have a local source for stone.