Agricultural pipe — commonly called ag pipe — is the primary drainage component in a retaining wall drainage system. It collects water that drains down through the drainage aggregate behind the wall and carries it to a suitable outlet. Without ag pipe, water has no controlled path out from behind the wall and hydrostatic pressure builds until the wall fails.
This guide covers everything you need to know about ag pipe for retaining wall drainage: what size to use, slotted vs solid, sock wrapping, fall requirements, outlet options, and how to install it correctly.
What Is Ag Pipe?
Agricultural pipe (ag pipe) is a flexible, corrugated plastic pipe with slots or perforations that allow water to enter the pipe from the surrounding soil or drainage aggregate. It is available in a range of diameters and in slotted (perforated) and solid (non-perforated) versions.
In a retaining wall drainage system, slotted ag pipe is used in the drainage zone behind the wall to collect water. Solid ag pipe may be used to connect the slotted section to the outlet point, particularly where the pipe runs through soil rather than drainage aggregate.
What Size Ag Pipe for a Retaining Wall?
The standard ag pipe size for most residential retaining walls is 100mm diameter. This provides adequate flow capacity for the drainage loads typical of residential retaining walls in Australian conditions.
When to consider larger diameter pipe:
- Long walls (over 30–40m) where the pipe must carry drainage from a large catchment area
- High-rainfall sites (coastal NSW, Queensland wet season, Victoria's Dandenong Ranges)
- Sites with a large upslope catchment area draining toward the wall
- Where a hydraulic engineer or drainage consultant has specified a larger pipe
For most residential retaining walls under 20–25m in length, 100mm ag pipe is the correct specification.
Slotted vs Solid Ag Pipe
Slotted (Perforated) Ag Pipe
Slotted ag pipe has slots or perforations along its length that allow water to enter the pipe from the surrounding drainage aggregate. This is the type used in the drainage zone behind the retaining wall. The slots should face downward or to the sides during installation — not upward, where soil or aggregate can fall into the pipe.
Solid Ag Pipe
Solid ag pipe has no perforations and is used to carry water from the slotted section to the outlet point. If the outlet is at the end of the wall and the pipe runs through drainage aggregate for its full length, slotted pipe can be used throughout. If the pipe needs to run through soil (e.g., under a path or garden bed to reach a stormwater pit), use solid pipe for the section running through soil to prevent soil ingress.
Ag Pipe Sock (Filter Sock)
Ag pipe is available with or without a geotextile filter sock wrapped around it. The sock prevents fine soil particles from entering the pipe through the slots and blocking it over time.
For retaining wall drainage applications, ag pipe with a sock is strongly recommended. Even with geotextile fabric lining the drainage zone, some fine particles can reach the ag pipe over time. The sock provides an additional layer of protection against pipe blockage.
Most ag pipe sold for retaining wall applications is available pre-wrapped with a sock. If you're purchasing ag pipe without a sock, wrap it with geotextile fabric before installation.
Fall Requirements
The ag pipe must have a continuous fall (slope) from the high end to the outlet. Water in the pipe flows by gravity — if the pipe is flat or has low points where water pools, the drainage system will not work effectively.
- Minimum fall: 1:100 (1cm per metre) is the generally recommended minimum fall for ag pipe in retaining wall applications.
- Preferred fall: 1:50 (2cm per metre) or greater provides better drainage performance and is less susceptible to blockage from minor pipe deflection.
- No flat sections: Avoid flat sections in the pipe run. Even a short flat section can create a low point where silt accumulates and blocks the pipe.
On sloping sites, achieving the required fall is straightforward — the pipe naturally follows the slope. On flat sites, the pipe must be deliberately graded to achieve the required fall to the outlet.
Outlet Options
The ag pipe must connect to a suitable outlet. Options include:
- Daylight outlet at the end of the wall: The simplest option. The ag pipe exits at the low end of the wall and discharges onto the surface or into a garden bed. Suitable where there is a natural low point at the end of the wall and the discharge can be managed without causing erosion or nuisance.
- Stormwater pit: The ag pipe connects to an existing stormwater pit or is connected to the stormwater system via a new pit. This is the preferred option for walls where the discharge volume is significant or where a daylight outlet is not practical.
- Sump: A small sump (pit filled with drainage aggregate) at the end of the wall that allows the water to disperse into the surrounding soil. Suitable for sites with good natural drainage and low discharge volumes. Not suitable for clay soils or high-rainfall sites.
Check with your local council about stormwater connection requirements before connecting ag pipe to the stormwater system. In most Australian states, connecting to the stormwater system requires council approval.
How to Install Ag Pipe in a Retaining Wall
- After setting the posts and allowing the concrete to cure, lay the geotextile filter fabric against the excavated soil face behind where the sleepers will sit.
- Place the ag pipe at the base of the wall, against the geotextile fabric, with the slots facing downward or to the sides. Ensure the pipe has a continuous fall to the outlet from the start.
- Connect the pipe to the outlet before backfilling. Confirm the outlet is clear and the pipe has the required fall.
- Install the sleepers course by course, installing weep holes in the lowest course as you go.
- Backfill with drainage aggregate around and over the ag pipe, filling the drainage zone to 300–500mm behind the sleepers.
- Fold the geotextile fabric over the top of the drainage aggregate before backfilling with soil above the drainage zone.
- Test the drainage system before completing the backfill. Run a hose into the drainage zone and confirm water exits at the outlet within a reasonable time.
Frequently Asked Questions
What size ag pipe do I need for a retaining wall?
100mm diameter slotted ag pipe is the standard specification for most residential retaining walls. For long walls (over 30–40m) or high-rainfall sites, confirm the pipe size with a drainage engineer.
Does ag pipe need to be wrapped in a sock?
Yes, strongly recommended. The geotextile sock prevents fine soil particles from entering the pipe through the slots and blocking it over time. Most ag pipe sold for retaining wall applications is available pre-wrapped with a sock.
Which way do the slots face in ag pipe?
The slots should face downward or to the sides during installation — not upward. Water enters the pipe from below and the sides; facing the slots upward allows soil and aggregate to fall into the pipe.
What fall does ag pipe need?
A minimum fall of 1:100 (1cm per metre) is recommended. A fall of 1:50 (2cm per metre) or greater is preferred. The pipe must have a continuous fall with no flat sections or low points.
Where does the ag pipe drain to?
The ag pipe must connect to a suitable outlet — a daylight outlet at the end of the wall, a stormwater pit, or a sump. It cannot simply terminate in the ground. Check with your local council about stormwater connection requirements.
Order Drainage Materials
Our drainage kits include ag pipe (with sock), geotextile fabric, and weep holes in one order — everything you need for the drainage system in a single purchase. Geotextile fabric is also available separately for larger drainage zones. Or contact our team to confirm the right drainage specification for your project.
