The parts and what they do
| Component | Job | Typical spec |
|---|---|---|
| Drain rock (“wall rock”) | Lets water fall to the pipe instead of building pressure | Clean crushed aggregate about 1/4–1-1/2 in, typically a 12 in column behind the face |
| Perforated drain pipe | Carries water out along the base | 4 in perforated pipe, at least 1% slope |
| Outlet | Gets water out of the wall system | To daylight or a drain system; Allan Block suggests outlets about every 50 ft |
| Filter fabric | Keeps fine soil from clogging the rock | Between native soil and rock, and under the topsoil cap |
| Weep holes | Pressure relief through solid concrete or CMU walls | Per the wall design; useless without rock behind them |
| Surface drainage | Keeps runoff from pouring over or behind the wall | Swale or catch basin above the wall |
Signs the drainage is blocked
- Water or mud seeping out at the base or through joints after storms.
- Weep holes that stay dry even when the yard is soaked, or are packed with soil.
- White mineral stains (efflorescence) on block or concrete faces.
- Soggy ground or sinkholes just above the wall.
- A lean or bulge that appeared or grew after a wet winter.
Seeing these on your wall? A local contractor can check the drainage and say whether it can be fixed in place.
Fixing a wall that wasn’t drained
You can’t add drain rock behind a wall without digging out the backfill. On a short run, that can mean excavating behind the wall, installing rock, fabric and pipe, and rebacking it; Angi’s San Francisco data puts drainage excavation at $90–$105+ per linear foot (Angi). If the wall has already moved, rebuilding is often better value. See repair vs. replacement and, for taller walls, hillside walls.
When the real problem is water across the whole yard or slope, not just behind the wall, a yard drainage specialist is the right call.
Common questions
Does a 4 ft retaining wall need drainage?
Yes. Manufacturer guidance for segmental walls calls for drainage aggregate behind the wall and a 4-inch perforated drain pipe that exits to daylight or a drain system (Allan Block best practices). Taller and engineered walls follow the engineer’s drainage details.
Do retaining walls need weep holes?
Solid concrete and CMU walls commonly use weep holes or pipe outlets through the wall to relieve water pressure; segmental block walls drain through the joints and the drain rock column. Weep holes only work if drain rock behind them keeps soil out.
What gravel goes behind a retaining wall?
Clean, well-graded crushed aggregate, roughly 1/4 to 1-1/2 inches with few fines, is what Allan Block specifies for its “wall rock” column, typically about 12 inches deep behind the face (Allan Block).
Sources
- Allan Block: Best practices for segmental retaining walls (PDF)
- Angi: Retaining wall cost in San Francisco (updated Feb 2026)
Checked October 2026. Rules and prices change; confirm with the agency or a contractor before you build.