Do I Need Drainage Behind a Retaining Wall?
Quick Answer
Yes. Water pressure behind the wall is the number one reason retaining walls fail. Backfill with a 12-inch zone of clean 3/4 inch gravel and run a perforated drain pipe along the base that daylights or ties into a drain at the low end.
- Use a 12-inch wide zone of clean 3/4 inch gravel behind the wall
- Run a perforated pipe along the base, sloped to daylight
- Wrap the gravel in filter fabric to keep soil from clogging it
- A 20 ft × 3 ft wall needs about 2.2 cubic yards of gravel backfill
Formula
A 12-inch (1 foot) wide gravel drainage zone equals wall length times height times 1 foot, divided by 27 to convert cubic feet to cubic yards.
Step-by-Step Calculation
- 1
Set the drainage zone width
Standard is 12 inches (1 foot) of gravel behind the wall
- 2
Find the wall face area
Example: 20 ft long × 3 ft high = 60 sq ft
- 3
Multiply by zone width
60 × 1 ft = 60 cubic feet of gravel
- 4
Convert to cubic yards
60 ÷ 27 = 2.22 cubic yards
Gravel Backfill Volume (12-inch drainage zone)
| Wall Size | Face Area | Backfill (cu ft) | Backfill (cu yd) |
|---|---|---|---|
| 10 ft × 3 ft | 30 sq ft | 30 | 1.11 |
| 20 ft × 3 ft | 60 sq ft | 60 | 2.22 |
| 30 ft × 4 ft | 120 sq ft | 120 | 4.44 |
| 40 ft × 4 ft | 160 sq ft | 160 | 5.93 |
*Add a separate 6-inch compacted gravel base under the wall on top of this backfill.
Frequently Asked Questions
What happens if I skip drainage behind a retaining wall?
Water saturates the soil behind the wall and hydrostatic pressure builds until the wall bulges, cracks, or topples. Poor drainage is the single most common cause of retaining wall failure, so it is never optional.
What size gravel goes behind a retaining wall?
Use clean, angular 3/4 inch crushed stone with no fines. It drains fast and locks together. Avoid pea gravel or sand, which hold water and shift under load.
Do I need a drain pipe or is gravel enough?
On a low wall under 2 feet, a gravel zone alone may cope. For anything taller, add a perforated drain pipe at the base, sloped at least 1% to daylight, so collected water actually leaves the wall.