Part 3.1.2 Drainage
Acceptable construction manuals
Section 5 of AS/NZS 3500.5.
Acceptable construction practice
Compliance with this acceptable construction practice satisfies Performance Requirement for drainage of—
roofs in areas subject to 5 minute duration rainfall intensities of not more than 255 mm per hour over an average recurrence interval of 20 years (as per Table 184.108.40.206) where a drainage system is required; and
sub-soil areas where excessive soil moisture problems may occur; and
land adjoining and under buildings,
provided the stormwater drainage system otherwise complies with the acceptable construction manual.
- The BCA does not require the installation of drainage systems. Accordingly these requirements need only be applied when these systems are used.
- Information on the need for drainage systems may be obtained from the appropriate authority.
- The legal discharge point from a building site is generally determined by local government authorities.
220.127.116.11 Drainage requirements
Drainage systems must be installed as follows—
excavation for drains adjacent to existing footings must be within the area described in Figure 18.104.22.168 as being safe for excavation.
|EXCAVATION FOR DRAINS ADJACENT TO FOOTINGS|
|Note: Any excavation below the area defined as being safe for excavation will need additional protection measures to be determined by appropriately qualified persons.|
22.214.171.124 Surface water drainage
Surface water must be diverted away from Class 1 buildings as follows:
Slab-on-ground — finished ground level adjacent to buildings:
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
50 mm over the first 1 m from the building in any other case.
Slab-on-ground — finished slab heights:
the height of the slab-on-ground above external finished surfaces must be not less than (see Figure 126.96.36.199)—
100 mm above the finished ground level in low rainfall intensity areas or sandy, well-drained areas; or
50 mm above impermeable (paved or concreted areas) that slope away from the building in accordance with (a); or
150 mm in any other case.
The appropriate slab height above finished ground level and the slope of the external finished surface surrounding the slab may vary depending on:
- 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.
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 188.8.131.52).
|SITE SURFACE DRAINAGE|
|GRADING OF GROUND UNDER SUSPENDED FLOORS|
184.108.40.206 Subsoil drainage
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 220.127.116.11); and
provision for cleaning and maintenance.
|CONSTRUCTION OF SILT PITS|
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 the following diagrams.
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, AS/NZS 3500.5 and AS 2870.
18.104.22.168 Stormwater drainage
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.
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.
- On-site catchment systems, such as stormwater tanks.
- On-site soil drainage systems, such as soaker wells.
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.
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.