Table of Contents
Retaining Wall Basics
A retaining wall holds back soil on a slope, creating a level area on one or both sides. Properly built retaining walls require adequate drainage, a compacted gravel base, and the correct number of interlocking blocks to resist the lateral earth pressure.
Most municipalities allow homeowners to build retaining walls up to 4 feet tall without a permit or engineering. Walls taller than 4 feet typically require an engineered design, a building permit, and possibly geogrid reinforcement layers.
Calculation Formulas
Material Guide
| Material | Purpose | Quantity Guide |
|---|---|---|
| Retaining Wall Blocks | Wall structure | See calculator above |
| Crushed Gravel (3/4") | Backfill behind wall | ~1 ton per 20 sq ft of wall face |
| Compactable Base (QP) | Leveling pad | 6" deep x (block depth + 6") wide |
| Landscape Fabric | Prevent soil migration | Wall area + 2 ft on each side |
| Drain Pipe (4") | Water drainage | 1 length = wall length + exits |
| Construction Adhesive | Cap block attachment | 1 tube per 20-25 linear feet |
Drainage Requirements
- Always install a 4-inch perforated drain pipe at the base of the wall behind the first course of blocks.
- The drain pipe should have at least 1% slope (1/8 inch per foot) toward an exit point.
- Use 3/4-inch crushed angular gravel for backfill, not round river rock, as it compacts better and drains well.
- Wrap the drain pipe in filter fabric to prevent clogging from fine soil particles.
- Backfill gravel should extend at least 12 inches behind the wall from bottom to within 6 inches of the top.
- Top the gravel with landscape fabric, then 6 inches of native soil and sod to prevent surface water infiltration.
Frequently Asked Questions
How many blocks do I need for a 30-foot retaining wall?
For a 30-foot long, 3-foot tall wall using standard 16x8x12" blocks: You need approximately 4.5 courses (round to 5) and about 23 blocks per course, totaling 115 blocks. Add 5-10% for cuts and waste, bringing the total to about 125 blocks.
Do I need a permit for a retaining wall?
Most areas require a permit for retaining walls taller than 4 feet (measured from the bottom of the footing to the top of the wall). Some municipalities set the threshold at 3 feet. Always check with your local building department before starting construction.
How deep should the base trench be?
Dig the trench 6 inches deep for the compacted gravel base, plus enough depth to bury the first course of blocks at least halfway. For a standard 8-inch tall block, the trench should be about 10-12 inches deep total. The base should extend 6 inches beyond the wall face on each side.
Do I need geogrid?
Geogrid reinforcement is recommended for walls taller than 3 feet and required for walls taller than 4 feet. Install geogrid layers every 2 courses, extending at least 3 feet behind the wall into the compacted backfill. The geogrid creates a reinforced soil mass that resists the lateral earth pressure.