Tips & Tricks

How Agricultural Drains Wreck Houses

Agricultural drains can wreck the houses they are meant to protect. If agricultural drains are specified or installed incorrectly, they can easily be the cause of uncontrolled movement and cracking in your house.

Agricultural drains (also known as ag pipes, agi pipes, ag line, french drains (in France?!?), soakage drains and slotted drainage pipes – phew!!) are sometimes used by homeowners and engineers to improve house subsoil drainage.
(Breaking news – check out our brand new article about improving drainage on sloping sites)

Sometimes agricultural drains are used to fix slab heave and reduce cracking in houses.

When agricultural drains are installed improperly, they can become a problem.

What is an Agricultural Drain?

An agricultural drain is a slotted PVC pipe placed into a trench that is backfilled with rocks or sand. The purpose of the agi pipe is to drain away any water that is in the ground.

Agricultural pipe without a geofabric sock

Groundwater or surface water that seeps into the trench falls through the rocks and is carried away by the slotted PVC pipe.

An agricultural drain is constructed by

  • digging a trench
  • placing a slotted PVC pipe in the bottom of the trench
  • sometimes the agi pipe is protected with a geofabric filter (a sock that keeps the dirt out)
  • backfilling the trench either to the surface or just below the surface with 10mm or 20mm gravel
  • sometimes the entire trench is wrapped in geofabric filter material for maximum protection from silt

How Agricultural Drains Wreck Houses

Agricultural drains and agricultural pipes are meant to remove water from the ground. Agricultural drains wreck houses when they become the source of moisture in the ground.

The National Construction Code says it in this round-about complex way:

In clay soil, subsoil drains can alter the long-term moisture content in the soil, adversely affecting the building foundation by removing or, in some cases, introducing water. In such conditions, subsoil drains should only be used where there are no other options for dealing with subsoil water.

Let me make it easy for you!

Do not connect your ag pipes to your stormwater pipes. Run separate pipes.

If your agricultural pipes are connected to your roof downpipes, every time it rains water can flood back into the agricultural pipe and flood the trench that is meant to be collecting water!

That water then soaks into the ground and can affect your house footings.

Never connect agricultural pipes to your roof stormwater system!

The Best Way to Install Agricultural Drains

This is the best way to install agricultural pipes:

  • If water is already on the surface, use spoon drains rather than aggi-drains. Surface water should be managed at surface level. Ag pipes are for water that is already in the ground.
  • Connect agricultural drains to their own pipework system. Do not connect agi pipes and soakage trenches to the roof stormwater system.
  • Ensure the agricultural pipe AND the bottom of the trench are graded in the direction of water flow. Water in and around the pipe should always be able to drain by gravity along the trench.
  • I’ll say that again! Ag pipes DO NOT magically make water flow uphill. The bottom of the trench has to be graded in the direction of the water flow.
  • Position agricultural drains no closer than 1m from your house footings. Use solid pathways or grade the ground away from your house for the first metre.
  • Never run slotted pipe drains UNDER your house. Use only solid pipes under your house. Try not to introduce water under your house.
  • Use silt pits where required by the National Construction Code. Don’t forget to keep silt pits clean of silt and debris.
  • Use geofabric and geotextiles to keep silt out of the soakage drain and pipe.
  • Bring the end of an ag pipe to the surface and cap it so that the pipe can be flushed out from time to time.
  • Discharge the low end of the agi pipe downhill of the house you are protecting or into the stormwater system downhill from the house.

More About House Drainage

Keeping water away from your house footings is one of the best ways to improve the performance of your house. This is especially true if your house is built on reactive clay soils and you are trying to avoid slab heave.

The Australian standard for residential footings and slabs (AS2870) stipulates 50mm ground fall away from your home over the first metre.

This is called surface drainage and it is getting harder to achieve on small sites.

  • Some buildings are built right to the boundary and you don’t have control of the ground levels or surface drainage next door.
  • On sloping blocks that are dug out to create a flat block, stormwater that falls between the house and the retaining wall has nowhere to go unless there is good surface or sub-surface drainage.

In these cases, your engineer and plumber should have a chat.

When Should Agricultural Pipes Be Used?

Agricultural drains should be used as a last choice when the management of surface water isn’t enough to improve the performance of your house.

A good way to manage surface water is with spoon drains.

Check out our explainer video on why we prefer spoon drains to agricultural drains and grated drains.

Sometimes water is already in the ground and it needs to be removed. This is the true benefit of soakage trenches and agricultural drains.

Subsoil drainage systems should be used when:

  • Water is soaking into the ground uphill from your house and running through the ground towards your house.
  • Behind retaining walls.
  • Adjacent to basement walls.
  • Adjacent to deep footings.
  • On the uphill side of cut and fill sites.

If water is on the ground try your hardest to achieve drainage by overland flow rather than letting the water soak into the ground and be collected by an agi pipe.

A Quick Summary of Agricultural Pipes

  • Don’t connect agi pipes to the roof stormwater system. Run separate pipes.
  • Do make sure soakage trenches slope downhill.
  • Do keep agi pipe trenches at least 500mm from a house.
  • Do use soakage trenches to remove water from the ground
  • Do remember that ag drains do not magically make water flow uphill
  • Do discharge water from soakage trenches downhill of the house you are protecting
  • Do not run agi pipes under houses.

Can We Help?

Contact Cornell Engineers to help improve the drainage around your home.

Head over to our Get a Quote page. Let’s see if you need an agricultural drain, a spoon drain or some other way of improving the drainage around your home.

Get a Quote

Agricultural Pipe References

A New House post about drainage

20 replies on “How Agricultural Drains Wreck Houses”

Note: French drains were named after Henry French, a 19th-century judge and farmer, not because they came from France.

This spoils an otherwise good article

I am needing to put agi pipe trenches through my backyard to prevent boggy soil. My backyard is about 10cm of topsoil on top of clay, doesn’t drain and the backyard is almost flat.
I am finding it difficult to find some ‘rules of thumb’ or recommendations or guidance on line as to the spacing apart required for the trenches. Can you help ?
Also is 100mm agi pipe pretty much the standard diameter to use or can narrower pipe be used ?

Hi Matt,

I’ve received the following advice: “Additional surface water runoff drainage is recommended to be installed at the front of the subject property. An agi pipe can be installed to the perimeter of the dwelling, diverting water to the legal point of discharge through the properties current stormwater drainage
system. This will prevent further water erosion causing subsidence.”
For context, my house is on a block with a site slope from sw to nw. Reading your article I’m a little apprehensive about this – how can I talk to the engineer about it?

Hi Matthew,

Great article and Video! I have officially subscribed look forward to share your vids.

I see that you are situated in Brisbane. I was wondering if you guys do any work in Sydney or if you possibly know a reliable drainage engineer?

Hi Pamela
Thanks for following us. We’ve provided advice for drainage job/problems in Sydney and we’ve had engineers in Sydney a few times. Let’s find out if we can help – perhaps send us a Messenger message and let’s see. If we can’t help, I’ll see if I can recommend someone.

What drainage can I do if the subfloor of a lean-to on piers, is around 400mm below the finished ground level of the paving outside? This is proposed new work, and the lean-to is facing a hill. It’s making me nervous to leave the subfloor space with no drainage out of it…? A builder is saying to install an ag pipe which I now realise is wrong from your article

Hi Sue,
I’m glad you are investigating the issues with poor drainage now before troubles begin. So many people get caught out with poor drainage after the builder has already finished.
Surface drainage is my preferred way of avoiding problems if it is at all possible. The ground should grade away from the building on all sides even though the natural grade of the ground is towards the building. Can the builder assist with adjusting the ground around the building as part of the work?
Sometimes there really is no alternative to installing an agricultural drain but it really is not the best way to take care of surface water.
Matt Cornell

Good write-up.

In the how Not to put in Ag-Drain, I thought the drawing as presented may also undermine house footings. As well as introducing moisture to potentially reactive soil.

Good point MD. It may well cause a subsidence problem – all the more reason to be really careful with excavations and the placement of trenches beside a dwelling. Thanks for your comment.

I wouldn’t trust a plumber for good advice; from my experience, they will overcharge and under deliver. I was upgrading my stormwater system and had to get council engineer approval – unfortunately, the engineer made a rookie mistake with his calculations but it would have been obvious to the plumber that built the system, as the pipe should have been 150 mm, not 100 mm as the plan specified – the plumber didn’t say a word, probably laughing behind my back, knowing the system would back up with heavy rain, which it does.
Other plumbers I’ve used have overcharged and also did substandard work, including using second-hand pits with holes from previous use in the wrong place.

Bought a 30-year double house, and discovered mold and damp problems. Terracotta pipes under the house connected to the bathroom were wet and presumed to be leaking. 70K quote to reline.
Further investigation showed previous owner installation of heat pump overflow lines just emptied onto the ground, next to the bathroom.
Connecting the lines to buckets showed over 20 l a week ponding under the house. Connecting heat pump overflow to sewer line fixed the water, and over time the terra cotta pipes dried out.
Added subfloor ventilation fans and the underfloor is completely dry, as proven by regular inspection.
Plumber wanted to install ag lines beside footings as shown in the how not to do ag drain diagram. No need now with HWS fixed.
Excellent article and keep in mind those HWS overflow pipes

Thanks for the information Matt, do you know anybody in Sydney who can do the job of installing the Agi pipe properly as my house is on a slope.

I’ve used the pipe in Western Australia, the area I lived was high in clay and local shire laws stated that down pipes to be connected to soak wells, problem being is that the soak wells filled and blew out. I connected the agi pipe to all 9 soak wells, lots of gravel around the pipe, I then ran the pipe both sides of my property to my front verge where I placed vents for any excess water to escape. Only once did I ever see water come from the vent. Fortunately for me my land was large enough to run a pipe 2m away from my house and my block sloped slightly down to the road giving water a direction to flow.
The combination use of soakwells and agi pipe are fantastic, only mistake I see people make is not enough agrigrate gravel aroung the pipe and soak well.

Thanks for the help on how to set up proper drainage pipes for your property and which ones will help you in getting rid of water that you do not want. Just like you mentioned, keeping the water drainage away from your home is the best idea so that there is no chance that it gets inside. Living on the coast, I think that I will definitely need something like this for when the storms come. Thanks again!

Hi Sophie. I’m not sure what the code requirements are for stormwater plumbing in your state. Speak to a certifier or to your local authority. However when we look for ways to improve drainage around a house I recommend not having roof stormwater pipes connected to agricultural pipes unless they are well away and downslope from the house.
Can anyone else comment?

No not legal and will distribute the stormwater through the soil via the agi pipe causing a major drainage problem with the related potential foundation problems

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