How to Build a Retaining Wall on a Slope
Quick Answer
On a slope you step the base trench: dig a level footing, lay the first course, then step up one block height and start a new level section wherever the ground rises past the top of a block. Each stepped section stays perfectly level while the wall climbs the grade.
- Dig a level base trench 6 inches deep plus one buried block course
- Step up one full block height each time the slope rises past the current course
- Keep every course level — never follow the slope angle with the blocks
- Backfill with 3/4 inch gravel and run a perforated drain pipe at the base
Formula
Measure the total vertical rise across the wall length, then divide by your block height to find how many level steps the base course needs as it climbs the slope.
Step-by-Step Calculation
- 1
Measure the slope rise
Example: ground rises 24 inches across the wall run
- 2
Divide by block height
24 ÷ 8-inch block = 3 stepped sections
- 3
Set the base trench depth
6 inches of compacted gravel + 1 buried block course for stability
- 4
Step each section
Start each new level run one block higher where the grade rises
Stepped Base — Sections by Slope Rise (8-inch block)
| Slope Rise | Base Steps Needed | Buried Course | Gravel Base Depth |
|---|---|---|---|
| 8 inches | 1 step | Yes | 6 inches |
| 16 inches | 2 steps | Yes | 6 inches |
| 24 inches | 3 steps | Yes | 6 inches |
| 32 inches | 4 steps | Yes | 6 inches |
*Bury the bottom course roughly one block height below grade for every wall.
Frequently Asked Questions
Do the blocks follow the slope angle?
No. Every course must stay level. The wall climbs the slope in stepped level sections, not by tilting the blocks to match the ground. Following the slope angle makes the wall unstable and it will lean over time.
How deep should the base be on a slope?
Dig a level trench with 6 inches of compacted gravel base, then bury the bottom course about one block height below grade. On the downhill end this means digging deeper so the first level section starts below the finished grade.
Do I need drainage on a sloped retaining wall?
Yes, and it matters more on a slope because water runs downhill and collects behind the wall. Backfill with 3/4 inch clean gravel and run a perforated drain pipe along the base that daylights at the low end.