AG pipe is a corrugated polyethylene drainage pipe used to collect and redirect groundwater away from structures, gardens, and waterlogged soil.

If you have pooling water around a retaining wall, a soggy lawn that never dries, or a building pad that needs subsoil drainage before construction, ag pipe is almost certainly part of the fix. It is one of the most common drainage materials used across Australian residential, rural, and civil projects.

This guide covers the three main types of ag pipe, which sizes suit which jobs, how to install it properly, and the mistakes that turn a drainage solution into a drainage problem.

What Is AG Pipe and How Does It Work

AG pipe (short for agricultural pipe, also called agi pipe, ag line, or drain coil) is a flexible corrugated plastic pipe made from high-density polyethylene. It sits in a trench below ground level, surrounded by gravel or coarse sand, and collects water that seeps through the soil. The corrugated profile gives the pipe flexibility so it can follow contours without rigid fittings at every bend.

Water enters through slots cut along the pipe wall, flows along the inside, and drains to a discharge point, typically a stormwater pit, rubble pit, or daylight outlet at the low end of the site. The gravel bed around the pipe acts as a filter, stopping soil particles from clogging the slots while letting water pass freely.

• Residential uses: retaining wall drainage, garden bed drainage, subfloor moisture control, French drains around footings
• Rural uses: paddock drainage for waterlogged pasture, dam overflow channels, stock yard drainage
• Civil uses: road subgrade drainage, embankment toe drains, sports field drainage, construction site dewatering

Ag pipe manufactured in Australia meets AS/NZS 2439.1 for perforated and non-perforated polyethylene drainage pipe. If you are buying pipe for a council or engineering project, check the specification calls for Class 400 (SN8) or Class 1000 (SN20) depending on the load requirements. The Australian Department of the Environment also provides guidelines on drainage and erosion management for projects near waterways or environmentally sensitive areas.

Three Types of AG Pipe You Need to Know

Every ag pipe falls into one of three categories, and picking the wrong one is the single most common mistake on drainage jobs. Each type has a specific purpose, and using slotted pipe where you need solid, or skipping the sock where you need it, will cost you a re-dig within a few years.

Slotted AG Pipe

Slotted ag pipe has rows of narrow cuts along the corrugation valleys that allow groundwater to seep into the pipe from the surrounding soil. This is the standard choice for subsoil drainage where you want to actively collect water, such as behind retaining walls, along the low side of a sloping block, or around building footings.

The slots are small enough to keep out gravel but will let fine sand and silt through over time if the pipe is installed without proper filtration. In clay soils, slotted pipe on its own usually works fine because clay particles are too large to enter the slots. In sandy or silty soils, you need a socked version.

Socked AG Pipe

Socked ag pipe is a slotted pipe wrapped in a geotextile filter fabric (the "sock"). The fabric acts as a secondary filter that stops fine particles from entering through the slots and clogging the pipe. All Stake Supply carries 100mm socked ag pipe in 20m and 100m coils, which covers most residential and civil drainage jobs.

Use socked pipe in sandy soils, coastal areas, fill material, or anywhere the natural soil has a high proportion of fine particles. If you are not sure about the soil type, go socked. The sock adds minimal cost and eliminates the biggest maintenance headache with ag pipe systems.

Solid (Unslotted) AG Pipe

Solid ag pipe has no slots or perforations. It carries water from point A to point B without collecting anything along the way. You use solid pipe to bypass areas where you do not want water entering the pipe, such as under driveways, past tree roots, or through garden beds where irrigation would otherwise soak into the drain.

• Use slotted where you want to collect groundwater
• Use socked where soil is sandy, silty, or mixed fill
• Use solid where you are transporting water past obstacles or connecting to a discharge point

On most residential jobs, you will use a combination of all three. A typical retaining wall drain might run socked pipe behind the wall, transition to solid under a garden path, then daylight into a rubble pit at the low corner of the block.

Which AG Pipe Size Do You Actually Need

AG pipe comes in four standard diameters in Australia: 50mm, 65mm, 100mm, and 160mm. The right size depends on the volume of water you need to handle and the length of the pipe run. Most people default to 100mm because it is the most widely stocked and handles the majority of residential and light commercial drainage.

A 100mm ag pipe running at a 1% grade can move roughly 1.5 litres per second. For a standard house block in a high-rainfall area like coastal NSW or Queensland, that is more than adequate. The Agriculture Victoria drainage guide recommends sizing pipe based on catchment area and expected rainfall intensity. If the pipe run exceeds 50 metres or the catchment area is large (multiple downslope properties draining to one line), step up to 160mm or add a second parallel run.

How to Install AG Pipe Step by Step

Poor installation is why most ag pipe systems fail. The pipe itself is simple. Getting the trench depth, gradient, and backfill right is where the job succeeds or falls apart. Here is the process used by experienced drainage contractors across Australia.

1. Plan the fall. AG pipe needs a consistent downhill grade of at least 1:100 (1cm drop per metre of run). Use a string line, laser level, or dumpy level to mark the trench line from the collection area to the discharge point. Water does not flow uphill, and flat sections create silt traps.
2. Dig the trench. Minimum depth of 400mm below finished ground level. For drains beside footings or slabs, the pipe must sit at least 100mm below the bottom of the footing. In heavy clay, 650mm depth gives better performance. Trench width should be at least 300mm to allow room for gravel around the pipe.
3. Line with geotextile fabric. Lay a strip of non-woven geotextile fabric along the trench before adding gravel. This wraps around the entire gravel bed and stops surrounding soil from migrating into the aggregate over time. Overlap the fabric at least 200mm where sheets join.
4. Add the gravel bed. Pour 50-75mm of 20mm blue metal or washed gravel into the base of the trench. Level it to maintain your grade. The pipe sits on top of this bed, not directly on soil.
5. Lay the pipe. Place the ag pipe on the gravel bed with the slots facing downward (this is counterintuitive but correct, as water rises from below and enters the pipe at the lowest point). Keep the pipe centred in the trench.
6. Backfill with gravel. Cover the pipe with at least 75mm of gravel on all sides and on top. The gravel envelope should surround the pipe completely.
7. Wrap the geotextile. Fold the geotextile fabric over the top of the gravel bed. The wrapped gravel package prevents fine soil particles from silting up the aggregate layer.
8. Backfill with soil. Fill the rest of the trench with excavated soil or clean fill. Compact lightly in 150mm layers.

The entire process takes a day for a standard 20-30m residential drain. Two people, a small excavator or a mattock, and a trailer load of gravel. Not complicated, but every step matters.

Five AG Pipe Installation Mistakes That Cause Flooding

These are the mistakes that drainage contractors see repeatedly on failed systems. Each one can turn a properly planned drainage line into an expensive hole in the ground that does nothing.

• Connecting ag pipe to stormwater or downpipes. This is the big one. Stormwater from a roof delivers a sudden slug of water under pressure. That water floods through the ag pipe slots and saturates the gravel bed and surrounding soil, which is the exact opposite of what the drain is supposed to do. Keep stormwater and subsoil drainage as separate systems.
• No geotextile wrap around the gravel bed. Without geotextile fabric wrapping the aggregate, soil migrates into the gravel within 2-3 years and the system silts up. Replacing a clogged drain means digging the whole trench again.
• Laying pipe with no fall or insufficient grade. A dead-flat pipe collects silt instead of moving water. You need at least 1:100 fall consistently along the entire run. Check grade at both ends AND in the middle, as sagging in the centre is common on longer runs.
• Using slotted pipe near tree roots. Fine tree roots will find and enter ag pipe slots within a single growing season. Use solid pipe for at least 3 metres either side of established trees. Eucalyptus, fig, and willow roots are particularly aggressive.
• Skipping the gravel envelope. Ag pipe sitting directly in clay or loam with no gravel around it works for about six months before the slots clog. The gravel bed is not optional. Use 20mm blue metal or washed river gravel, not crusher dust or road base.

AG Pipe Compared to Other Drainage Methods

Ag pipe is not the only drainage option, and it is not always the right one. Here is how it stacks up against the alternatives for common Australian drainage problems.

For most residential drainage problems, ag pipe in a gravel trench lined with geotextile fabric is the standard solution. It is proven, inexpensive, and does not require power or moving parts. The only situations where ag pipe is not ideal are purely surface water problems (use a channel drain) or sites with no gravity discharge point (you will need a pump).

How to Pick the Right AG Pipe for Your Project

Matching the pipe type to the soil and the pipe size to the water volume is the whole game. Get those two decisions right and the installation is straightforward. Get either one wrong and you are digging it up again in a few years.

• Clay soil: slotted ag pipe works well without a sock. Clay particles are too large to enter the slots. Go 100mm for runs over 5 metres.
• Sandy or silty soil: socked ag pipe is non-negotiable. Fine particles will clog unprotected slots within months.
• Mixed fill or disturbed ground: socked pipe. You do not know what is in fill material, and it often contains fine particles that migrate.
• Next to trees: solid pipe within 3m of any established tree, transitioning to slotted or socked beyond the root zone. Check out tree protection products if you are planting near drainage lines.
• Under driveways or paths: solid pipe. You do not want slots collecting water under hardstand areas.

All Stake Supply stocks 100mm socked ag pipe in 20m coils for residential jobs and 100m coils for larger civil and rural drainage projects. Both ship from the St Marys warehouse with guaranteed complete delivery, so you are not waiting on a partial order to finish the job.

Getting Your Drainage Right the First Time

AG pipe is not complicated, but it does not forgive sloppy installation. Pick the right type for your soil, maintain a consistent grade, wrap the gravel bed in geotextile, and keep stormwater out of your subsoil drain. Do those four things and the system will work quietly for decades.

If you are not sure which ag pipe or drainage products suit your project, contact the team at All Stake Supply. With 48 years of product knowledge and over 1,000 erosion control and drainage products in stock, they can point you to the right solution before you start digging.