How Agricultural Drains Wreck Houses (and How to Install Them Properly)

An agricultural drain, usually called an ag drain, agi pipe, ag line or French drain, is a slotted pipe laid in a gravel-filled trench to collect water from the ground and carry it away. Installed well, it protects a house. Installed badly, it can do the opposite: feed water into the ground beside the footings and cause movement and cracking.

We regularly investigate cracked houses where a drain installed to “fix” a moisture problem has become part of the cause. This article explains how ag drains work, how they go wrong, and how to install one so it actually helps.

What Is an Ag Drain?

An ag drain is a subsoil drain. It is made up of:

• a trench dug to the required depth;
• a slotted, flexible pipe (agi pipe) laid in the bottom of the trench, often with a geofabric sock to keep soil out;
• clean gravel backfill, commonly 10 mm or 20 mm aggregate, around and above the pipe; and
• in better installations, geofabric wrapped around the whole gravel trench to stop fine soil washing in and clogging it.

Water in the surrounding soil seeps into the gravel, enters the pipe through the slots, and flows along the pipe by gravity to an outlet.

How an Ag Drain Works: Diagram

How Agricultural Drains Wreck Houses

An ag drain is meant to take water out of the ground. It causes damage when it starts putting water in.

On reactive clay sites this matters a great deal. Clay swells when it gets wetter and shrinks when it dries. A drain that concentrates water beside part of a house can make that part of the footing rise or move differently from the rest, and the result is cracking. The NCC’s guidance recognises the same risk: in clay soils, subsoil drains can change long-term soil moisture under a building, by removing water or by introducing it.

The most common ways an ag drain becomes a water source are:

• Connecting it to roof downpipes. Every time it rains, roof water can surge back into the slotted pipe and flood the trench it was meant to drain. The drain becomes an irrigation line beside your footings.
• No fall, or a fall the wrong way. Ag pipe does not make water flow uphill. If the pipe or trench base is flat, dips, or falls towards the house, water sits in the trench and soaks into the ground.
• A blocked or buried outlet. A drain with nowhere to discharge simply stores water.
• Silted or root-filled pipe. Without geofabric, fine soil washes in and the drain gradually stops working.
• Placing it right against the footing, or running slotted pipe under the house, where any water it releases goes straight to the foundation.

How to Install an Ag Drain Properly

• Manage surface water at the surface. If water is flowing over the ground, use grading or spoon drains, not ag pipe. Ag drains are for water that is already in the ground.
• Keep it separate from roof water. Never connect ag pipe into roof downpipe lines. Run it as its own system.
• Grade the pipe and the trench base. Both must fall consistently in the direction of flow, so water in and around the pipe drains by gravity along the trench. Tamp the trench base firm.
• Keep it away from the footings. We recommend keeping ag drains at least 1 m from house footings, with the ground between graded away from the house or covered with a sealed path.
• Never run slotted pipe under a house. Use solid pipe under the building, and avoid introducing water under the slab or footings.
• Keep silt out. Use socked pipe and wrap the gravel trench in geotextile, such as a non-woven geotextile drainage fabric.
• Provide flushing points. Bring the upper end of the pipe to the surface and cap it so the line can be flushed out, for example with a drain jetter hose kit on a pressure washer. Include silt pits where needed and keep them clean.
• Discharge downhill of the house. The outlet must be lower than the house it is protecting, at a legal point of discharge.

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Can You Connect Agi Pipe to Stormwater?

Ag drains often do discharge to the property’s stormwater system. The connection point is what matters. It should be downhill of the house and arranged so stormwater cannot back up into the slotted pipe, for example through a silt pit at the low end. It should never tie into roof downpipe lines beside the house.

Stormwater connections are plumbing work, so have a licensed plumber confirm the arrangement and the legal point of discharge for your property.

When Should You Use an Ag Drain?

Treat an ag drain as a tool for subsoil water, not a first response to puddles. Surface water should be dealt with at the surface wherever possible. Subsoil drains are appropriate where water is already moving through the ground, such as:

• uphill of a house, where groundwater is moving through the soil towards it;
• behind retaining walls;
• beside basement walls and deep footings; and
• on the uphill side of cut-and-fill sites.

Ag Pipe Behind Retaining Walls

Behind a retaining wall, the ag drain relieves water pressure that would otherwise build up against the wall. It is normally laid at the base of the wall in free-draining gravel, wrapped in geofabric, and graded to an outlet at the end of the wall.

The same rules apply: it needs a consistent fall, a free outlet and protection from silt. A retaining wall drain that is blocked or has no outlet can lead to water staining, efflorescence, leaning and, in the worst cases, failure of the wall.

Fixing Poor Drainage Around Your House

Keeping water away from footings is one of the most effective ways to improve how a house performs, especially on reactive clay where slab heave is a risk.

The starting point is surface drainage: the ground around a house should fall away from it, with a fall of at least 50 mm over the first metre. On small or sloping sites that is often hard to achieve:

• houses built close to the boundary depend on ground levels next door; and
• on cut sites, water that falls between the house and the retaining wall has nowhere to go without good surface or subsoil drainage.

Before deciding on ag drains, work out where the water is actually coming from. Our video on drainage for houses on sloping sites walks through that process. There is more in our article on houses on sloping blocks with drainage problems.

For new builds, our video on waffle slabs and site drainage covers how to get drainage right at the construction stage.

A Typical Ag Pipe Question

I have an existing agi pipe that slopes towards the house to a small drain that was, until I cleaned it out, full of soil. The pipe is also full of soil and plant roots are growing through it.

Our answer was that the first question is whether the pipe is needed at all. Ag pipe is used either to collect water from the ground or, in other applications, to spread it into the ground. Decide which job this pipe is meant to be doing.

If it is needed, replace it with socked pipe in a geofabric-wrapped trench. Make sure the trench base falls away from the house to a proper outlet. A pipe that falls towards the house is delivering water to it.

Where the water is coming from a roof or a paved area, we generally prefer to keep it on the surface and direct it away. As this case shows, buried pipes tend to block up.

Ag Drain Checklist

• Do manage surface water at the surface first.
• Do use ag drains to remove water that is already in the ground.
• Do grade the pipe and trench base so water flows away from the house.
• Do keep ag drains at least 1 m from house footings.
• Do use socked pipe and geofabric, and provide flushing points.
• Do discharge downhill of the house.
• Don’t connect ag pipe to roof downpipes.
• Don’t run slotted pipe under a house.

Cracking or Movement Linked to Drainage?

If your house is cracking and you suspect drainage, including a drain that was meant to fix the problem, the cause needs to be established before more money is spent. Cornell Engineers investigates moisture-related building movement and gives practical advice on what needs to change. See our forensic structural engineering page for more.

Send us the property address, photographs of the cracking and the drainage, a short description of what has happened and any reports you already have. We will review the information and let you know whether the matter is a good fit for Cornell Engineers before preparing a fee proposal.

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