A segmental (interlocking block) retaining wall is one of the most DIY-friendly hardscape projects, but it lives or dies on the part you can’t see: the base. Here’s the workflow pros follow.
1. Plan and estimate
Decide the wall’s length and exposed height. Most manufacturers cap unreinforced DIY walls at about 3 feet — taller walls usually need engineering, geogrid reinforcement, or a permit. Check local codes first. Estimate your blocks, cap blocks, and base gravel with the retaining wall calculator, and your base stone with the gravel calculator.
2. Dig the base trench
Excavate a trench roughly twice the block depth wide and deep enough for 6 inches of compacted gravel plus half the first course buried below grade. Burying the first course is what keeps the wall from kicking out.
3. Build a level, compacted base
Add gravel in 2–3 inch lifts and compact each lift with a plate compactor. Level it carefully — the base sets the accuracy of every course above it. A flat, firm base is the single biggest predictor of a wall that stays straight for decades.
4. Set the critical first course
Place the first row of blocks on the compacted base, checking level front-to-back and side-to-side on every block. Tap down high spots; add base material under low ones. Spend the time here — errors multiply as you stack.
5. Stack, backfill, and drain
Stagger the joints like brickwork as you add courses. Behind the wall, backfill with free-draining gravel and run a perforated drain pipe at the base to daylight. Trapped water is the number-one cause of retaining-wall failure.
6. Cap it
Glue the cap blocks on with masonry adhesive for a finished top that also locks the wall together.
Common mistakes
- Skipping compaction or building on soft soil.
- No drainage gravel or drain pipe behind the wall.
- Building too tall without reinforcement.
- Not burying the first course below grade.
Ready to price it out? Start with the retaining wall block calculator.