Estimate blocks, gravel backfill, and cost for retaining walls.
Retaining Wall Planning Guide
Retaining walls manage slopes, prevent erosion, and create usable flat areas in your landscape. Proper planning and drainage are critical to a long-lasting wall.
| Wall Height | Base Depth | Geogrid Needed? |
|---|---|---|
| Under 2 ft | 4-6 inches gravel | No |
| 2-3 ft | 6 inches gravel | Usually no |
| 3-4 ft | 6-8 inches gravel | Recommended |
| Over 4 ft | Engineer required | Yes, required |
Frequently Asked Questions
How do I calculate blocks for a retaining wall?
Multiply the wall length by the height to get the face area in square feet. Divide by the face area of one block. A standard 6×16 inch block covers 0.67 sq ft. Add 10% for cuts and waste.
How tall can a retaining wall be without engineering?
Most building codes allow retaining walls up to 4 feet tall without engineered plans. Walls over 4 feet typically require a structural engineer’s design, permits, and professional installation.
What type of block is best for a retaining wall?
Interlocking concrete retaining wall blocks are most popular — they’re easy to install and don’t require mortar. Gravity walls use their own weight for stability. For taller walls, consider geogrid-reinforced systems.
How much gravel do I need behind a retaining wall?
Place 12 inches of drainage gravel behind the wall, running the full height. For a 20-foot long, 3-foot tall wall, that’s about 1.5-2 cubic yards of gravel. Proper drainage prevents hydrostatic pressure buildup.
Do I need a footer for a retaining wall?
Yes, a compacted gravel base (6-8 inches deep) is essential. The first course should be partially buried. For walls over 3 feet, a concrete footer may be required. The base must be level for a stable wall.
How much does a retaining wall cost per foot?
Basic concrete block walls cost $15-30 per square foot of face area. Natural stone walls run $25-75 per sq ft. Timber walls cost $15-25 per sq ft. Add 30-50% for professional labor.
How long do retaining walls last?
Concrete block walls: 50-100 years. Poured concrete: 50-100 years. Natural stone: 100+ years. Timber (pressure treated): 15-25 years. Proper drainage is the key factor in longevity.
What is geogrid and do I need it?
Geogrid is a high-strength polymer mesh layered between block courses and extended back into the soil. It’s required for walls over 3-4 feet tall and dramatically increases stability by reinforcing the soil behind the wall.
Can I build a retaining wall on a slope?
Yes, but the base must be stepped to remain level at each section. Each step should match the block height. Walls on slopes require extra attention to drainage and may need engineering for taller designs.
What causes retaining walls to fail?
The #1 cause is poor drainage — water buildup creates hydrostatic pressure that pushes the wall over. Other causes: inadequate base, no setback (batter), missing geogrid on tall walls, and frost heave in cold climates.