How to Build a Block Retaining Wall (Under 4 Feet)
Planning, trenching, the leveling pad, the buried course, drainage and caps for a small segmental block retaining wall.
Segmental block walls are the most DIY-friendly kind of retaining wall: the blocks stack without mortar and lock together with a lip or pins. A wall up to about 3 feet is a realistic weekend-or-two project. The work that decides whether it lasts is all below ground and behind the wall.
Know your limits. Walls over about 4 feet (measured from the bottom of the base), walls with a driveway, slope or building above them, and walls near property lines often need a permit and an engineered design. Taller walls usually need geogrid reinforcement. Check with your building department and the block maker’s guide before you buy.
Example: a 30 ft wall, 30” tall
| Item | Quantity |
|---|---|
| Wall blocks (6” × 16” × 12”) | 145: 5 visible courses + 1 buried, 23 per course, plus 5% |
| Caps (12”) | 32, with 2 tubes of adhesive |
| Leveling pad gravel | 1.79 tons of crushed stone with fines |
| Drainage gravel | 4.67 tons of clean ¾” stone |
| Drain pipe | 40 ft of 4” perforated pipe |
| Fabric | 150 sq ft |
The blocks alone weigh about 5.1 tons, so arrange delivery as close to the wall as possible.
1. Plan the line and call 811
Mark the face of the wall with string or paint. Gentle curves are easy with these blocks; tight curves need cutting. In the US, call 811 before digging.
2. Dig the trench
The trench needs to be wide enough for the block plus about 6” on each side (24” for a 12”-deep block), and deep enough for 6” of base plus the buried course. For 6” blocks, that’s about 12” deep. Dig into the slope behind the wall as well, to make room for the drainage gravel.
Compact the bottom of the trench.
3. Build the leveling pad
Fill the trench with 6” of crushed stone that has fines, in two layers, compacting each. Screed it perfectly level front to back and side to side. This is the most important step: every error here shows up, magnified, at the top of the wall.
4. Lay the first course
Set the first course on the pad, buried below the finished ground. Level every block in both directions and against its neighbors, tapping with a rubber mallet. Take your time; this course decides how straight the wall is.
5. Drainage, then the next courses
Lay the perforated pipe behind the first course, sloping to an outlet where water can drain away. Fill behind and between the blocks with clean ¾” gravel.
Then build up course by course, staggering joints like brickwork. After each course:
- sweep the top of the blocks clean so the next course sits flat,
- fill the 12” zone behind the wall with clean gravel,
- backfill behind the gravel with soil, compacting in thin layers.
Place fabric between the drainage gravel and the soil so the soil doesn’t wash into the stone. See Retaining Wall Drainage for why this matters.
6. Caps and finishing
Glue the caps on with concrete or landscape block adhesive. Top the drainage gravel with a few inches of soil so surface water runs off rather than straight down behind the wall. Slope the ground in front of the wall away from it.
Common mistakes
- No buried course. The base of the wall can slide out.
- Using soil instead of gravel behind the wall. Wet soil is heavy and pushes walls over.
- A base that isn’t level. It can’t be fixed once the wall is up.
- Going too tall without help. Above roughly 3–4 ft, follow the manufacturer’s geogrid requirements and local rules.
Work out blocks and materials for your wall in the Retaining Wall Planner.