Site drainage and stormwater: where the water goes
Water should move away from a house and stay away. Current housing provisions require the ground around a slab to fall at least 50 mm over the first metre in most cases, and the slab to sit at least 150 mm above finished ground, or 50 mm above paving that falls away. Every downpipe should discharge into a working stormwater system, not onto the soil by the footings. Ponding, damp walls, cracking near downpipes and water under the floor all point to drainage problems, which are usually cheaper to fix than the damage they cause.
- 01FallGround slopes away, 50 mm over the first metre
- 02HeightSlab 150 mm above ground, 50 mm above paving
- 03DischargeEvery downpipe into stormwater, not the soil
Many cracks and damp walls start in the garden.
Why drainage sits behind so many problems
Water that collects against a house wets the soil under the footings, which moves on reactive clay. It feeds damp walls, decays timber and attracts termites. Poor drainage is a root cause in many of the issues covered in the cracks, damp and termites guides, so it is worth reading the site before reading the building.
The levels and slopes that matter
The ABCB Housing Provisions set the basic requirements for new homes. Older homes were not always built to them, and later landscaping often undoes them.
| Requirement | Minimum | Why |
|---|---|---|
| Ground slope away from the building | 50 mm over the first metre in most cases (less in some low-rainfall areas with impermeable paving) | Surface water flows away rather than towards the footings |
| Slab height above unpaved ground | 150 mm (100 mm in low-rainfall or sandy, well-drained areas) | Keeps water, splash and termites away from the building edge |
| Slab height above paving that slopes away | 50 mm | Paving sheds water but must not trap it at the wall |
| Ground under suspended floors | Graded so it sits above the outside ground and does not pond | Keeps the subfloor dry |
| Overflow relief gully | At least 150 mm below the lowest sanitary fixture | A blocked sewer overflows outside, not into the house |
QBCC's maintenance guide adds the everyday rules: never let soil or paving cover weep holes, keep paths at least 75 mm below the termite barrier, and do not run sprinklers against the walls.
Roof water and the stormwater system
A typical roof sheds a large volume of water in a storm, and all of it arrives at a few downpipes. Each downpipe should connect into a stormwater pipe that carries it to the street, a legal point of discharge or a tank, not onto the ground beside the house. Look for downpipes discharging onto the soil, broken or disconnected pipes at ground level, and pits full of silt or leaves.
| Sign | What it can mean | Check next |
|---|---|---|
| Ponding near walls after rain | Ground falls towards the house, or the soil is saturated | Ground levels and where the water comes from |
| Downpipe discharging onto soil | A disconnected or missing stormwater connection | Whether a stormwater pipe exists and is clear |
| Cracking near a downpipe or corner | Wet, moving soil under the footing | Read with the cracks guide |
| Water entering the subfloor after rain | Surface water draining under the house | Grading, ag drains and the subfloor guide |
| Damp, mould or salts at the base of walls | Ground or paving too high, or water pooling | Slab or floor height against finished ground |
| Gurgling drains or sewage smells | A partly blocked sewer, often tree roots in older clay pipes | A plumber and a drain camera |
Sloping sites and neighbours
On a slope, water from higher ground arrives at the uphill wall. A cut-off or agricultural drain behind the house, or behind a retaining wall, intercepts it before it reaches the building. Water flowing between properties is also a neighbour issue, with rules that vary by state, so keep photographs and records if it causes damage.
Common misreads
| What people say | What is closer to right |
|---|---|
| "New paving will keep the area dry" | Paving laid too high or falling towards the house traps water at the wall and can cover weep holes. |
| "The downpipe goes into the ground, so it is connected" | It may run into a broken pipe or soakage pit beside the footings. Check where it ends up. |
| "Garden beds against the house are fine" | They raise ground levels, hold water and hide termite entry. |
| "It only floods in big storms" | Big storms reveal the design limits; small repeated wetting often does the slow damage. |
Worked example: cracks at one corner only
Illustrative reasoning example, not an inspected property. A 1970s brick-veneer house has stepped cracking at one rear corner and nowhere else. A downpipe at that corner discharges onto a garden bed that is level with the slab.
The location points to drainage before footing failure. The first steps are to reconnect the downpipe to the stormwater system, lower the bed below the slab and regrade it to fall away, then monitor the crack through a full wet and dry season. If it stabilises, the repair is drainage and making good. If it keeps moving, an engineer assesses the footing with the drainage already corrected.
Frequently asked questions
How much should the ground slope away from a house?
Current housing provisions require the ground around a slab to slope away by at least 50 mm over the first metre in most cases, with a smaller fall allowed in some low-rainfall areas with impermeable paving.
How high should a slab be above the ground?
At least 150 mm above finished ground in most cases, 100 mm in low-rainfall or sandy, well-drained areas, and 50 mm above paving that slopes away from the building.
Where should downpipes discharge?
Into a stormwater system that carries water to the street, a legal point of discharge or a tank, not onto the soil beside the house.
Can poor drainage cause cracks in a house?
Yes. Water collecting beside footings wets the soil, and reactive clay soils swell and shrink with moisture, which can move footings and crack walls.
What is an overflow relief gully?
A grated outlet on the sewer line outside the house, set at least 150 mm below the lowest sanitary fixture, so a blocked sewer overflows outside rather than into the house.
Why does water get under my house after rain?
Usually because surface water flows towards the building instead of away. Regrading the ground, connecting downpipes and adding an agricultural drain on the uphill side often fixes it.
How this guide was researched
We read the site before the building: where water arrives, where it should go, the levels that keep it away, and the symptoms that show it is not, with a fictional example.
Limits: Older homes may predate current requirements. Stormwater discharge and neighbour rules vary by council and state.
Sources
- Housing Provisions Part 3.3: DrainageAustralian Building Codes Board · accessed
- Your essential home maintenance guideQueensland Building and Construction Commission · accessed
- Building Technology Resources (foundation maintenance, condensation, managing termites, asbestos)CSIRO · accessed
- Housing Provisions Part 6.2: Subfloor ventilationAustralian Building Codes Board · accessed
- Protect against termitesQueensland Building and Construction Commission · accessed