Subsoil drainage around footings and slabs: when it is needed and how to build it properly

Water is behind a surprising share of the call-backs on residential jobs. A slab can be poured to the right level, a footing can be sized correctly, and the wall can still start to stain, lift or crack because the ground beneath it stays wet for longer than the design assumed. Subsoil drainage is the part of the job that deals with that water before it reaches the footing zone.

This article covers when a subsoil line is needed around footings and slabs, how the pipe should be bedded and graded, how the filter fabric and backfill should be made, and how the outlet should be connected. It does not cover surface stormwater, roof drainage or retaining walls, which each have their own design and approval path. Where the engineer or the certifier has set out the drain, that design governs, and what follows is a guide to building what the design asks for.

The aim is to give an experienced builder enough of the mechanism to reason about a site the drawings did not anticipate. A subsoil drain that is installed correctly is almost invisible once the house is finished. A drain that is installed badly looks exactly the same from the surface until the wet season arrives, and by then it is under the paving.

When subsoil drainage is needed around footings

Subsoil drainage is not a default item on every house. Whether it is needed depends on what the ground and the site are doing. The usual triggers are a site where groundwater sits close to the level of the footings for part of the year, a block that falls toward the house, a cut into a slope where water running down the hill meets the footings, or a soil that holds water and does not let it drain away.

The site classification and the footing design will often tell you whether the ground is likely to move with changes in moisture. Reactive clays and some fill sites make drainage more important, because moisture change is what moves the footings in the first place. The link between site classification and slab choice is worth reading alongside this article, because the same soil conditions drive both decisions.

Check the uphill side of the block, not just the spot where the house will sit. A neighbouring block that sits higher can send runoff and seepage toward your footings even when your own ground looks dry. A site walk after rain, with a torch and a careful look at the lowest points, will tell you more than most desktop searches. Note where the water goes, where it stands and how long it stays there.

How water reaches the footing zone

Water in the ground moves in two broad ways. It falls through the soil under gravity until it reaches a layer it cannot pass, and then it sits or runs sideways along the top of that layer. It also moves laterally through fill, trench backfill and loose granular layers that act as a channel. A footing that sits in a wet pocket, or beside a service trench full of loose material, is in the path of that water whatever the rest of the block looks like.

The effect on a footing is the change in moisture, not the water itself. When the soil under and beside a footing gets wetter, a reactive soil swells and a non-reactive soil can soften and lose bearing. When it dries, reactive soils shrink. Repeated cycles of that movement are what produce cracking, sticking doors and lifting corners. Subsoil drainage slows the cycle by keeping the zone around the footings at a more constant moisture level, which is why the drain is judged on how steady the ground stays rather than on how wet the trench looks on the day it is dug.

Two things commonly go wrong at this stage. The first is a drain set at a level that does not intercept the water it is meant to catch, usually because the line was set from the top of the trench instead of from the design reference. The second is a drain placed so close to the footings that a leaking service or a backfill of poor material feeds the very water the drain was meant to remove. Both mistakes come down to not understanding how the water moves, and both are easier to avoid at the planning stage than to fix afterwards.

Where the drain line sits relative to the footings

The line normally runs along the outside of the perimeter footings, on the side the water comes from, rather than under the slab. Its job is to catch seepage before it reaches the trench and carry it away from the building. On a sloping block the line is usually placed on the uphill side and runs around the house toward the low side, where the outlet sits. Where water comes from more than one direction, the design will show the line running on each side that needs it.

The level matters more than most builders expect. A line set too high will catch only the upper part of the seepage and allow the lower water to pass beneath it. A line set well below the footings can be difficult to fall and to connect, and it can end up in material that is harder to drain. The design usually specifies the depth relative to the underside of the footing, and that is the reference you measure from. Do not set it by eye from the top of the trench, and do not assume the excavator operator has found the right level.

Keep the drain clear of the footing concrete and the wall. A clearance of soil or clean aggregate should separate the two, so that the drain bed and the footing do not share one bulk of material. The strip and pad footing article covers the trench conditions the footing itself needs. If the drain trench and the footing trench are dug in the same operation, dig and place them in a sequence that keeps the drain bed clean and stops spoil falling in from the footing excavation.

Pipe bedding, grading and fall

The pipe is normally a perforated, corrugated plastic pipe with the perforations arranged so that water enters from the sides and the bottom as it passes through the bed. Lay it with the perforations facing down. A pipe laid with the perforations facing up will still drain, but it collects silt in the lower part and loses capacity sooner than it should.

Bedding is the material the pipe sits on. A clean, free-draining granular material is the normal choice, placed on a level base that is firm enough to stop the pipe settling once it is loaded. Avoid laying the pipe directly on spoil, clay lumps or rubble. Any point where the pipe rests on a hard or uneven base will lose its fall, and a pipe that has lost its fall holds water rather than shedding it. The bed is cheap compared with the cost of lifting a finished drain to correct it.

Fall is the reason the drain works. Water only moves through a subsoil line if the line has a continuous gradient toward the outlet. The minimum fall is set out in the standard the designer or certifier works to, so check the figure in the current document rather than working from memory or from a supplier’s brochure. The principle is simple. No low spot should be left anywhere in the run, because standing water in the pipe is what carries silt into the perforations and reduces flow. Check the fall with a level or a laser at the bedding stage, before the pipe is covered, and again at the outlet.

Filter fabric and the backfill around the pipe

The pipe needs two things around it. It needs an aggregate that lets water into the line easily, and it needs a filter fabric that stops fine soil migrating into the aggregate and blocking the perforations. Without the fabric, a drain fills with the finer material from the surrounding ground over a relatively short period. The pipe is still there, but the water has nowhere to go, and the first sign is usually damp at the bottom of the wall rather than anything visible in the trench.

The usual practice is to wrap the pipe and its bed in a geotextile filter fabric, with the fabric lapped over the top of the aggregate before the trench is backfilled. Lap the fabric generously so that no edge of the aggregate is exposed to the soil. Check the product data for the fabric suited to your ground conditions, since not every fabric suits every soil, and a fabric that is too fine for a silty soil can blind just as quickly as no fabric at all.

The backfill over the drain should be placed in layers and compacted to suit the location. Do not tip heavy spoil straight onto the pipe, because the impact can shift the line and open a joint. Where the drain runs under a paved area, the compaction and the surface finish have to match what the design calls for. Keep the top of the drain trench sealed against surface water running straight down into it, since an open trench top is a common way for a drain to become a channel for silt and sediment.

Connecting the outlet to a legal point of discharge

Subsoil water has to go somewhere, and the outlet is where most disputes start. The water from a subsoil line is directed to the stormwater system through a legal point of discharge, which is usually a stormwater pit, a kerb outlet or another discharge point that the approved stormwater design permits. A subsoil line is not connected to the sewer, and it should never be run to a point where it will discharge onto a neighbour’s land or back against the building.

Check the stormwater plan approved for the site before you connect. The approved plan shows where the discharge point is and how it is to be formed. If the plan is silent, or the point is not where the drain naturally falls, stop and get the design amended rather than choosing a convenient point on site. A change to the outlet is far cheaper to resolve on paper than a drain that backs up into a finished floor, and the council or certifier will want to see the change regardless.

The outlet itself should be protected from silt and from vermin. A simple grate or a screened end at the discharge point keeps leaves and animals out without stopping the flow. Make sure the outlet sits above the level of any water it might have to discharge into, so that a flooded stormwater pit does not force water back up the drain line. Where the outlet has to sit below the stormwater level, the design should show how that is handled, and it may need a non-return arrangement or a different discharge point.

Worked example: a sloping block in western Sydney

A builder in western Sydney is building a new home on a block that falls gently from the rear boundary. The footing design includes perimeter footings and a slab, and the site classification calls for attention to moisture. During excavation the rear footing trench fills with water after a short rain, and there is seepage on the uphill side of the trench that was not visible during the dry weeks before the dig.

The builder checks the design and finds that a subsoil line is specified along the uphill and rear sides, with an outlet to the approved stormwater point at the low corner. The line is installed the following week. The fall is checked at the bedding stage with a laser, a low spot is found near the middle of the run, and the bedding is lifted and re-levelled before the pipe goes in. The filter fabric is wrapped and lapped, and the backfill is placed in layers with no spoil dropped directly onto the pipe.

The mistake the builder avoids is the common one of connecting to the approved outlet without checking its level. The stormwater pit at the low corner sits at a level where a heavy storm would back water into the drain, so the builder raises the outlet slightly and adds a screened end, then notes the arrangement in the job file for the certifier to see. The next rain produces no standing water in the trench, and the seepage stops at the uphill side.

What to check before the trench is closed

A subsoil line is one of the few items on a residential job that gets built, covered and forgotten in the same afternoon. That is why it needs a deliberate hold point. Before the backfill goes over the drain, stop and check the work against the design and against the things that cannot be checked once the trench is closed. The checklist below is the minimum a supervisor should be able to sign off.

Record the fall at the start, the middle and the end of the run, with the date and the reading. Photograph the bedding, the joints and the fabric before the aggregate and the backfill go in. Photograph the outlet and its connection to the approved point. These photographs cost very little, and they are the only evidence a future owner or a future builder will have if the drain fails and somebody asks what was done.

The mistakes that come back to the builder

Most subsoil failures are not dramatic. The drain blocks, the water finds another route, the footing moves a little, and the owner notices damp at the base of a wall or a door that sticks in winter. By the time the problem is visible, the drain is under the finished garden, the paving and the landscaping, and the cost of opening it up is far larger than the cost of doing the job properly the first time. A defect claim is not limited to the footing that failed, and the owner will ask what else was done wrong.

Subsoil drainage is one part of managing moisture around a building, and it sits alongside the footing design, the slab choice and the site drainage that the rest of the project relies on. If you want a structured refresher on the site work around this, PointCPD’s NSW builder CPD courses are written for licensed builders who want the mechanism, not just the rule. Working through the design and the records together is what turns a drain from a guess into a defensible piece of the build.

Questions builders ask

• Drainage
• Footings
• Moisture

Published 7 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.