Part 3.3 Drainage
New for 2022
Drainage systems must be installed as follows:
- Areas adjoining and under buildings — surface water drainage in accordance with 3.3.3; and
- Where site conditions exist that create a need for subsoil water to be diverted away from footings, basements, retaining walls etc — sub-soil drainage in accordance with 3.3.4; and
- Where underground drainage from roof areas is required or permitted — underground stormwater drainage in accordance with 3.3.5; and
- Excavation for drains adjacent to existing footings must be within the area described in Figure 3.3.2 as being safe for excavation.
Surface water drainage
Surface water must be diverted away from a Class 1 building as follows:
- Slab-on-ground — finished ground level adjacent to a building: the external finished surface surrounding the slab must be drained to move surface water away from the building and graded to give a slope of not less than (see Figure 3.3.3a) —
- 25 mm over the first 1 m from the building—
- in low rainfall intensity areas for surfaces that are reasonably impermeable (such as concrete or clay paving); or
- for any reasonably impermeable surface that forms part of an access path or ramp provided for the purposes of Clauses 1.1(2) or (4)(c) of the ABCB Standard for Livable Housing Design; or
- 50 mm over the first 1 m from the building in any other case.
- 25 mm over the first 1 m from the building—
- Slab-on-ground — finished slab heights: the height of the slab-on-ground above external finished surfaces must be not less than (see Figure 3.3.3a) —
- 100 mm above the finished ground level in low rainfall intensity areas or sandy, well-drained areas; or
- 50 mm above impermeable (paved or concrete) areas that slope away from the building in accordance with (a); or
- 150 mm in any other case.
- The ground beneath suspended floors must be graded so that the area beneath the building is above the adjacent external finished ground level and surface water is prevented from ponding under the building (see Figure 3.3.3b).
3.3.3 does not apply to a landing area provided for the purposes of Clause 2.3 of the ABCB Standard for Livable Housing Design, except for a channel drain or drainage surface provided under Clause 2.4 of that standard.
The appropriate slab height above finished ground level and the slope of the external finished surface surrounding the slab may vary depending on the following:
- The local plumbing requirements; in particular the height of the overflow relief gully relative to drainage fittings and ground level (to work effectively they must be a minimum of 150 mm below the lowest sanitary fixture).
- The run-off from storms, particularly in areas of high rainfall intensity, and the local topography.
- The effect of excavation on a cut and fill site.
- The possibility of flooding.
- Termite risk management provisions.
Clearances between wall cladding and the finished ground level are provided in 7.5.7.
Where a subsoil drainage system is installed to divert subsurface water away from the area beneath a building, the subsoil drain must—
- be graded with a uniform fall of not less than 1:300; and
- discharge into an external silt pit or sump with—
- the level of discharge from the silt pit or sump into an impervious drainage line not less than 50 mm below the invert level of the inlet (see Figure 3.3.4); and
- provision for cleaning and maintenance.
Subsoil drainage systems may need to be installed where subsurface water movement could damage buildings or cause loss of amenity through the build up of excessive moisture or lateral water pressure. Typical locations of subsoil drainage systems are on the uphill side of cut and fill sites, adjacent to deep footings, behind retaining walls and adjacent to basement walls.
The design and installation of subsoil drainage systems should take into account the nature of the soil and the anticipated water level, quantity and movement. In some cases, detailed investigations involving excavations, field observations and soil tests may be necessary to determine the appropriate solution. Typical subsoil drain configurations are shown in Figure 3.3.4 (explanatory).
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.
Additional guidance on subsoil drainage systems can be found in AS/NZS 3500.3 and AS 2870.
Where a stormwater drainage system is installed, it must comply with the following:
- The position and manner of discharge of the stormwater drainage system must be to the satisfaction of the appropriate authority.
- The stormwater drainage system must be designed so that any overflow during heavy rain periods is prevented from flowing back into the building.
- Cover to stormwater drains: the cover to 90 mm Class 6 UPVC stormwater drains installed underground must be not less than—
- under soil — 100 mm; or
- under paved or concrete areas — 50 mm; or
- under areas subject to light vehicle traffic—
- reinforced concrete — 75 mm; or
- paved — 100 mm.
Explanatory information: Discharge points
The manner of discharge of stormwater drainage systems includes consideration of discharge points. Some examples of discharge points which may be acceptable to the appropriate authority are—
- a legal discharge point at the allotment boundary; or
- on-site catchment systems, such as stormwater tanks; or
- on-site soil drainage systems, such as soaker wells.
Explanatory information: Depth of cover
Different depths of soil cover (or no cover at all) can be achieved using other types of pipes. The cover specified is measured from the top of the pipe to either the finished ground level or, in the case of paved or concreted areas, to the underside of the paving or concrete.