An agi drain, short for agricultural drain, is one of the simplest and most cost-effective tools available for protecting a Wagga Wagga foundation from reactive clay movement. Paired with correct surface grading, it is often the difference between a footing that moves evenly with the seasons and one that cracks because water is concentrating unevenly around it. This guide explains what an agi drain actually is, how it works alongside surface grading, when it is enough on its own, and what it typically costs.
What is an agi drain?
An agi drain is a length of slotted or perforated pipe, usually 90-100 mm PVC or corrugated poly pipe, wrapped in a geotextile sock and laid in a gravel-filled trench. Water in the surrounding soil enters through the slots and gravel, and gravity carries it along the pipe to a safe discharge point, typically the stormwater system, a soakage pit well clear of the house, or a lower point on the property.
The name comes from agricultural drainage, where the same technology has been used for over a century to lower water tables in farmland. Around a house footing, the job is narrower but the principle is identical: intercept water before it reaches the zone where it would otherwise saturate the clay under or beside the footing.
Why agi drains matter on Wagga’s reactive clay
Wagga Wagga sits on shrink-swell clay that changes volume with moisture content. It is not the presence of water itself that damages a footing, it is uneven moisture. A footing that stays consistently damp, or consistently dry, tends to move as a whole, which is manageable. A footing where one side is wet and the other is dry experiences differential movement, and differential movement is what cracks brickwork, jams doors and cracks slabs.
Water reaches the soil next to a footing from several directions: rainfall running off the roof and yard, groundwater tracking downhill from a neighbouring or higher block, and sometimes leaking pipes (our guide on checking for a plumbing leak affecting your foundation covers that specific cause). An agi drain is aimed squarely at the first two: it intercepts water travelling through the soil, whether from rain infiltration or subsurface flow from higher ground, before it reaches the footing zone.
Our broader guide on how drainage affects foundation stability in Wagga Wagga sets out the full mechanism connecting moisture variation to cracking; this page focuses specifically on the ag-drain and grading side of the fix.
Where an agi drain is typically installed
The most common position is along the uphill or higher side of a house, running parallel to the footing at a shallow offset (commonly 300-600 mm out from the wall), deep enough to sit below the topsoil and intercept water moving through the subsoil before it reaches the footing. On a sloped block, a single agi drain along the uphill side can intercept most of the water that would otherwise track downhill and pool against that wall.
On a flat block, or where drainage problems are affecting more than one side of the house, a perimeter ag drain running most or all of the way around the footing may be recommended instead. This is more work and cost than a single-side drain, but it is the appropriate response where moisture variation is affecting multiple elevations rather than one.
Ag drains are also commonly installed:
- Alongside retaining walls, where soil behind the wall would otherwise stay saturated and push water toward nearby footings
- Around subfloor perimeters in older stumped homes, to keep the crawl space drier
- Below garden beds built hard against a wall, intercepting water that would otherwise sit against the footing
Surface grading: the drain’s essential partner
An agi drain manages water that has already entered the soil. Surface grading is what stops water getting into the soil next to the footing in the first place, and the two need to work together. A subsoil drain installed under a yard that still slopes toward the house will keep working overtime, and eventually be overwhelmed in heavy rain, because it is fighting a surface problem with a subsurface fix.
The general benchmark used across our published guidance is a minimum fall of 50 mm over the first metre away from the external wall, ideally maintained for at least 2 m where the yard allows it. Where paths, patios or garden edging have settled or been built without this fall, water can pool against the wall or drain inward, exactly the differential moisture pattern that drives cracking.
Practical grading corrections include:
- Reshaping garden beds so the soil slopes away from the wall rather than being mounded against it
- Re-laying or packing paving that has settled and now tilts toward the house
- Removing or lowering garden edging that dams water against a wall after rain
- Extending downpipes so they discharge well clear of both the footing and any newly graded fall
Does an agi drain replace structural repair?
No, and this is worth being direct about. An agi drain and correct grading address the moisture cause of foundation movement. They do not lift a dropped corner, repair a footing that has already settled, or straighten a wall that has already cracked. For a home where movement is mild, recent, and hasn’t produced significant structural symptoms, correcting drainage can arrest further movement and may be the only intervention needed. Where a footing has genuinely dropped or a stump has failed, drainage correction is usually recommended alongside underpinning or restumping, not instead of it, because removing the moisture cause protects the repair once it’s made.
A foundation inspection is the right way to find out which situation applies before spending money either way.
Indicative costs
| Scope | Typical work | Indicative range |
|---|---|---|
| Single-side ag drain | 6-10 m run, one wall | $800-$2,500 |
| Perimeter ag drain | Full perimeter, average home | $3,500-$9,000 |
| Surface regrading (moderate) | Garden bed and path adjustment, one or two sides | $600-$2,000 |
| Combined drainage correction | Ag drain, regrading, downpipe work | $2,000-$12,000 |
These figures are indicative only, drawn from the ranges published across this site, and confirmed after a site inspection since soil access, existing paving and discharge point availability all move the price.
Frequently asked questions
How do I know if I need an agi drain or just better surface grading?
A site inspection is the reliable way to tell, but a rough guide: if water visibly pools on the surface against the wall after rain, grading is likely the priority. If the ground around the footing stays damp for days after rain has stopped, or the house sits below a sloping neighbouring block, subsurface water is likely involved and an agi drain is worth adding.
Will an agi drain stop my existing cracks from getting worse?
If the crack is being driven by ongoing moisture variation, and this is the case for most Wagga foundation cracking, correcting the drainage typically arrests further movement. It will not close or repair a crack that already exists; that comes later, once movement has stabilised.
Can I install an agi drain myself?
A shallow ag drain along a garden bed is within reach of a competent DIYer. Around a footing, getting the depth, fall, gravel specification and discharge point right matters more than it looks, get it wrong and water can pool at the low end of the trench right next to the footing, making the problem worse rather than better. Where the drain is intended to protect the structure rather than just a garden bed, professional installation is worth the modest extra cost.
Does an agi drain need council approval in Wagga Wagga?
Generally not for a standard subsoil drain discharging to an existing stormwater connection or approved soakage point. Where the work involves new stormwater connections or significant re-grading near a boundary, it’s worth checking with the specialist arranging the work.
Not sure whether your site needs an agi drain, better grading, or both? Get a free quote and an independent licensed contractor can assess your site and recommend the right combination before any work begins.