Block Types: CMU vs Landscape
The two main categories of retaining wall blocks are standard concrete masonry units (CMU) and interlocking landscape blocks. Your choice depends on wall height, aesthetic preference, and whether the wall will be structural or decorative.
Standard CMU (8x8x16)
Standard CMU blocks are the workhorse of masonry construction. At 8 inches high, 8 inches deep, and 16 inches long, they provide a stable, strong wall that can be reinforced with rebar and grout for structural applications. CMU walls typically require mortar (unless used as dry-stack with surface-bonding cement) and are best for walls that need engineering for height or surcharge loads.
Landscape Blocks (Interlocking)
Interlocking landscape blocks are designed specifically for retaining walls. They come in various sizes (common dimensions include 12x4x12 and 18x6x12 inches) and feature a built-in lip or pin system that creates a slight setback with each course. This setback improves stability without mortar or rebar for walls under 4 feet.
Landscape blocks are the preferred choice for DIY retaining walls under 4 feet. They require no mortar, no special skills, and can be installed in a weekend. For walls over 4 feet, consult an engineer regardless of block type.
Drainage: The Most Critical Component
More retaining walls fail from water pressure than from structural overload. Water that accumulates behind the wall creates hydrostatic pressure that can push the wall outward or cause it to overturn. Every retaining wall needs a drainage system.
Gravel Backfill
Install at least 12 inches of clean, free-draining gravel (typically 3/4-inch crushed stone) behind the wall. This gravel zone allows water to flow downward to the drain pipe rather than building pressure against the wall face. Use the gravel calculator to estimate the backfill volume.
Perforated Drain Pipe
A 4-inch perforated drain pipe sits at the base of the wall, embedded in the gravel backfill. The pipe collects water and directs it to a daylight outlet at the end of the wall. The pipe should extend 3-4 feet beyond the wall end for the outlet. Wrap the pipe in filter fabric to prevent fine soil from clogging the perforations.
Filter Fabric
Landscape filter fabric (geotextile) separates the native soil from the gravel backfill. Without it, fine soil particles migrate into the gravel over time and clog the drain pipe. Install the fabric against the native soil face of the excavation, wrap it up and over the top of the gravel backfill.
Base Preparation
A compacted gravel base is essential for wall stability. Excavate a trench wider than the block and 4-6 inches deep. Fill with crushed stone and compact in 2-inch lifts using a plate compactor. The base must be level — check with a string line and level at multiple points. The first course of blocks sets the alignment for the entire wall.
Bury the first course partially below grade (at least 1 inch per 8 inches of wall height). This provides lateral resistance at the base and prevents the wall from sliding forward.
Permits and Engineering
Permit requirements vary by jurisdiction, but the general thresholds are:
- Under 4 feet: Usually no permit required for simple gravity walls. Check local codes.
- 4-6 feet: Most jurisdictions require a building permit. Some require engineering drawings.
- Over 6 feet: Almost universally requires a permit AND engineering design by a licensed professional engineer.
- Any height with surcharge: Walls supporting driveways, structures, or slopes above may require engineering regardless of height.
Cap Blocks
The top course of a retaining wall uses cap blocks (also called capstones). These are flat-topped blocks that provide a finished appearance and protect the wall core from water infiltration. Cap blocks are typically glued in place with construction adhesive. Calculate cap blocks separately from wall blocks — you need one row of caps for the full wall length.
Calculate Your Wall Materials
Use our retaining wall calculator to estimate wall blocks, cap blocks, backfill gravel, drain pipe, and filter fabric from your wall dimensions and block type selection. For masonry walls using mortar, see the block wall calculator.