How deep should a retaining wall base be?
6 in of compacted gravel, plus one buried course of block.
For a 3 ft wall in 8 in blocks that is about 14 in of digging. The rule for burial is a tenth of the wall height, and never less than one full course.
Trench depth by wall height
| Wall height | Buried courses | Buried depth | Total dig | Exposed |
|---|---|---|---|---|
| 2 ft | 1 | 8 in | 14 in | 1.33 ft |
| 3 ft | 1 | 8 in | 14 in | 2.33 ft |
| 4 ft | 1 | 8 in | 14 in | 3.33 ft |
| 5 ft | 1 | 8 in | 14 in | 4.33 ft |
| 6 ft | 1 | 8 in | 14 in | 5.33 ft |
Heights of 5 and 6 ft are in the table for scale, not as encouragement. Anything at or above 4 ft, measured from the bottom of the footing, generally needs an engineered design and a permit.
Why the first course decides everything
Every error in the bottom course is repeated by every course above it, and they accumulate. A block 1/8 in out of level at the bottom of a five-course wall is most of an inch out at the top, and there is no way to correct it partway up — the blocks only sit one way.
This is why the base is gravel rather than concrete, and why it is compacted in layers. A compacted crushed base can be screeded dead flat, and it drains. A concrete footing under a segmental wall traps water and gives frost something rigid to lift.
Level the first course side to side AND front to back, and check every block against a string line rather than against its neighbour. Neighbours accumulate error; a string line does not.
Common questions
- How deep should the paver base be for a retaining wall?
- 6 in compacted for walls up to about 4 ft. Deeper walls, soft or clay soil, or anything carrying a driveway above it want more — 8 to 12 in, compacted in two or three lifts rather than all at once.
- Does the base need to be wider than the block?
- Yes — about 24 in, roughly twice a standard block's depth, so the base extends in front of and behind the wall. A base only as wide as the block lets the front edge settle and the wall tips forward.
- Do I need to go below the frost line?
- Not usually, and this is the difference from a fence post or a deck footing. A segmental wall is dry stacked and flexible: it can take a little frost movement and settle back. What it cannot take is water trapped behind it, which is why the drainage matters more than the depth here.
- Can I use sand instead of gravel?
- No. Sand does not compact into a load-bearing layer and it migrates when wet. A thin sand screed on top of a compacted gravel base is a common finishing trick for levelling the first course, but sand alone is not a base.
- How much do I bury on a short wall?
- One full course, even when a tenth of the height would be less. On a 2 ft wall that is 1 course, leaving 1.33 ft visible.
- What if the ground slopes?
- Step the base. Dig level sections and drop each one by exactly one block height where the grade falls, so every course stays level and the steps happen underground. Sloping the base to follow the ground is the classic way to end up with a wall that cannot be levelled anywhere.
Related answers
Or run any size through the retaining wall calculator.