Guidance on use of rainwater tanks

7. Construction material, size and installation

Page last updated: March 2011

Galvanised steel tanks

The most common material used in the manufacture of rainwater tanks is galvanised steel. Galvanised steel is not inherently resistant to corrosion but it is available with rust-resistant coatings such as Zincalume or Aquaplate (see below). Initial corrosion of galvanised steel normally leads to the production of a thin adherent film that coats the surface of the tank and provides protection against further corrosion. It is important when cleaning such tanks not to disturb this film.

New tanks may leach excess concentrations of zinc, which could affect the taste of the stored rainwater, but is not a health risk. These tanks may need to be flushed before use.

Aquaplate steel has a food-grade polymer skin that complies with Australian Standard AS 2070, bonded to a corrosion-resistant galvanised steel base. A number of precautions need to be taken with tanks manufactured using Aquaplate:
  • the polymer coating is not resistant to prolonged exposure to sunlight so tanks must have a top cover in place at all times
  • kerosene or similar chemicals used as mosquito larvicides can cause degeneration of the polymer coating and should not be added to water in the tank
  • the polymer coat should not be damaged when cleaning or installing the tank. If the coating is damaged, it should be repaired immediately using an appropriate sealant to prevent corrosion of the metal portions of the tank.
Additionally, copper or copper alloy fittings (brass and bronze) should not be connected directly to steel tanks as this causes corrosion. A minimum of two metres of plastic pipe should be used between the tank and copper fittings.

Concrete tanks

Concrete and ferro-cement tanks are strong and long lasting and can be installed underground.

New tanks may impart tastes and may leach lime, thereby increasing the pH of water. These tanks may need to be flushed before use.

Fibreglass tanks

Fibreglass tanks suitable for collecting rainwater are available. These tanks are manufactured with a food-grade coating on their interior surface. The coating is cured before the tanks are offered for sale. The tanks should also be manufactured to prevent the entry of light, which could encourage algal growth.

Plastic tanks and tanks with plastic liners

Increasing ranges of tanks manufactured from synthetic polymers including polyethylene are becoming available. Plastic tanks and plastic liners should be constructed of materials that are at least of a food-grade standard (compliant with AS 2070) and preferably that comply with the requirements of AS/NZS 4020. Plastic tanks should be manufactured to prevent the entry of light.

Size of tanks

Where a rainwater tank is intended to provide a supplementary source of water, the size of the tank will be determined by balancing cost against the range of uses required (drinking, food preparation, bathroom, laundry, toilet etc.).

If the rainwater tank is to represent the only source of drinking or domestic water, cost will be less important than the size of tank needed to provide security of supply. In this circumstance the size of the tank will depend on:
  • the volume of water needed
  • the amount and pattern of rainfall
  • the area of the roof catchment
  • the security of supply required.
The amount of rain combined with the area of the roof catchment will determine the maximum volume of water that can be collected. If this is not sufficient, then either a greater catchment area will be needed (for example, garage or shed) or water demand will need to be reduced. A number of water conservation measures could be applied, including dual flush toilets or dry toilets (if permitted), and water efficient devices, such as reduced-flow shower heads and washing machines with suds saver cycles. If, after implementing these measures, the volume of rainwater that can be collected is still not sufficient to meet demand, water will need to be obtained from an additional source (see Section 9).

Experience is always a useful guide and advice should be sought from neighbours particularly in areas where reliance on rainwater tanks is common. Some state and territory government departments have tables of calculated tank sizes based on local rainfall patterns. In Queensland this information is available from Water Wise (Department of Environment and Resource Management), in South Australia from the South Australian Water Corporation or the Department for Water and in Western Australia from the Department of Agriculture. Other departments with responsibilities for water resources or water supply may also be able to provide this information.

In some areas local authorities specify minimum requirements for water storage and there may also be storage requirements associated with firefighting. The local council or local fire authority should be contacted to determine whether such requirements apply.

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Volume of water needed

The volume of water needed may vary from one area to another. Water demand will depend on:
  • the number of people using the water
  • average consumption per person
  • the range of uses (drinking, food preparation, bathroom, laundry, toilet etc.)
  • the use of water conservation devices.
In areas supplied with mains water, the average indoor use per household is estimated to be in the range of 300-740 L per day or alternatively about 100-200 L per person per day. These volumes are steadily decreasing with the application of water conservation measures. Advice on water usage could be sought from the local water or water resources authority.

Average rainfall

In general, the most accurate source of rainfall information is the Bureau of Meteorology. In addition to average rainfall (annual and monthly) it is important to consider yearly variations, seasonality of rainfall, and the occurrence and length of recent dry spells.

Area of roof catchment

Calculate the area covered by the parts of the roof from which the water is to be collected. It is the flat or plan area (including eaves) that should be determined. The slope or pitch of the roof and the actual area of tiles or metal is not important.

The average roof area for a small house is about 100-150 m2, for a medium house about 150-200 m2, and for a large house it can be 200 m2 or greater.

Security of supply

The size of the tank needed will be influenced by the degree of security required. As used in this guidance document, securities of 90% or 99% mean the rainwater tank should supply the demand of water calculated for 90% or 99% of the time, respectively.

Maintaining water supply under almost all conditions, including extended dry spells (high security), will require a larger tank than that needed to maintain supply under normal or average conditions (lower security). Lower security will mean water rationing or alternative sources of supply (see Section 9) may have to be used more frequently.

Calculation of tank size

Methods for determining the maximum amount of water that can be collected as a function of rainfall, and the roof size and tank size needed to provide security of supply throughout the year are provided in Appendix B.

Installation

Rainwater tanks should be installed in a manner that will minimise the risk of contamination from industrial pollutants, dust, leaves, pollens, pesticide sprays, fertilisers, debris, vermin, birds, small animals and insects. Tanks should not be allowed to provide breeding sites for mosquitoes.

Underground tanks require additional protection against entry of surface run-off or groundwater, animal or human faecal material (including septic tank waste) and soil. These tanks need to be properly sealed and access points need to be protected against the ingress of surface run-off. Maintenance and cleaning of underground tanks might be more difficult.

Interconnection with mains water supplies

Rainwater tanks should never be interconnected with mains water supplies without determining local requirements. Protection of mains water distribution systems from other sources of water is an extremely important public health requirement and inappropriate cross-connections have been identified as sources of waterborne disease outbreaks (Craun & Calderon 2001).

Water authorities do not allow direct connection of rainwater systems with mains water supplies. The use of backflow prevention devices, or similar are required to stop rainwater siphoning back into the mains supply.

Information should be sought from the local water authority.

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Covers and lids

Tanks should have impervious covers and all access points, except for the inlet and overflow, should be provided with close-fitting lids which should be kept shut unless in use. The inlet to the tank should incorporate a screen to prevent material, such as leaves etc., which may have collected on the roof or in gutters, being washed into the tank and a mesh covering to prevent access of mosquitoes and other insects. Overflows should also be covered with an insect-proof mesh.

Tanks should be light-proof to minimise algal growth. Most algae will not make water unsafe for human consumption but can adversely affect the taste, odour and appearance of the water.

Bypass or overflow water

Run-off that is not collected in the tank and/or overflows should be diverted away from tank foundations, buildings or other structures. This water should be directed onto gardens or into the stormwater drain; it should not be allowed to pool or to cause nuisance to neighbouring properties or to areas of public access. Local authorities may have regulations or requirements that apply to diverted or excess rainwater flows.

Inlet pipes

Wherever possible, all sections of inlet pipes should be directed down and rainwater should flow into the top of the tank. The inclusion of rising sections will provide potential traps for sediments, biofilms and stagnant water and these should be avoided. Modifications to existing downpipes should not restrict existing water flows from roof gutters. To maximise the collection of rainwater, the downpipes should be of sufficient diameter to accept all water flow from roof gutters, even in heavy rains.

Catchment

Before installing a rainwater tank, the roof catchment should be checked for:
  • overhanging vegetation – if vegetation is overhanging the roof it should be pruned
  • a flue from a slow combustion heater – if possible this section of roof should be avoided; if not ensure the flue is installed in accord with Australian/New Zealand Standards
  • overflows/discharges/bleed-off pipes from roof-mounted appliances, such as evaporative air conditioners, hot water services, and solar heaters – the overflows/discharges/bleed-off pipes should not discharge onto the rainwater catchment area
  • uncoated lead flashing – if it is present it should be painted
  • exposed preservative-treated timber – exposed timber should be sealed or the section of roof containing the timber should not be used for the collection of rainwater
  • solar hot water systems – if possible this section of roof should be avoided or sealed off when maintenance is carried out. Solar systems contain fluids that are hazardous in rainwater tanks.
Gutters should have sufficient and continuous fall to downpipes to prevent pooling of water, which could increase accumulation of material, lead to algal growth and possibly provide a site for mosquito breeding. A fall of one in 100 should be sufficient.

Gutter shielding devices will substantially reduce the amount of larger debris (bark, larger leaves, etc.) but small particles will not be removed. Periodic cleaning will still be needed but at a lower frequency than for gutters without shielding.