How Deep Should a Retaining Wall Footing Be?
Quick Answer
For a segmental block wall, dig 6 inches of compacted gravel base and bury the bottom course about one block height (roughly 10% of wall height) below grade. Poured concrete footings must extend below the local frost line, typically 12 to 48 inches deep.
- Block base: 6 inches compacted gravel + 1 buried course
- Rule of thumb: bury 10% of the wall height below grade
- Poured footing: below the frost line, 12–48 inches by region
- Always compact the base in lifts before setting the first course
Formula
For block walls, bury roughly 10% of the finished wall height below grade. For poured concrete, the footing depth is set by the local frost line rather than the wall height.
Step-by-Step Calculation
- 1
Measure finished wall height
Example: 3 ft (36 inch) block wall
- 2
Apply the 10% rule
36 × 0.10 = ~3.6 inches, so bury at least one full block course
- 3
Add gravel base
Dig 6 inches deeper for compacted gravel under the buried course
- 4
Total trench depth
6 in gravel + 8 in buried block = ~14 inches for a 3 ft wall
Base Trench Depth by Wall Height (block wall)
| Wall Height | Buried Course | Gravel Base | Total Trench Depth |
|---|---|---|---|
| 2 ft | 1 course (~6 in) | 6 in | ~12 in |
| 3 ft | 1 course (~8 in) | 6 in | ~14 in |
| 4 ft | 1 course (~8 in) | 6–8 in | ~16 in |
| Over 4 ft | Engineered | Engineered | Per design |
*Poured concrete footings must reach below the local frost line regardless of wall height.
Frequently Asked Questions
Why bury the bottom course of a retaining wall?
The buried course anchors the base against the soil pushing from behind and stops the wall from kicking out at the bottom. Skipping it is the most common cause of block walls that lean or slide forward.
How thick should the gravel base be?
Use at least 6 inches of 3/4 inch crushed gravel, compacted in 2 to 3 inch lifts. A deeper base of 8 inches helps on soft or clay soils where drainage and settlement are a concern.
Does a block wall need a concrete footing?
No. Segmental block walls sit on a compacted gravel base, not a poured footing. Only poured concrete or masonry block walls with rebar require a concrete footing below the frost line.