Storing rainwater on a residential lot for use in the house and garden provides the occupants with a water source additional to potable or recycled water from the mains. This bulletin examines the appropriate design standards for the collection and use of rainwater on residential lots.
Types of tanks
Rainwater tanks come in different sizes and shapes to suit the space available. Most off-the-shelf tanks are made from sheet metal or polyethylene plastic, with a range of colours to complement the colour of the adjacent house wall. A metal or concrete tank can be customised to suit the size of the space available. If the house is within a bushfire prone area, a plastic tank is not suitable, and a metal or concrete tank must be used instead.
Slimline tanks, made from plastic, are the most popular type of tank for suburban lots. They can fit in the typical one metre wide passageway along the side of a house, having a width of 500 mm to 800 mm. This uses space that is otherwise often unused. Capacity ranges from 300 litres to 5000 litres. While the typical height is 2.1 m, some are short enough to fit under windows, and others are narrow enough to fit between windows.
Cylindrical tanks, either metal or plastic, have capacities ranging from 1000 L to a massive 50,000 L. While the cheapest option per litre of water stored, they tend to take up valuable yard space that can be used in other ways. They are too wide to be placed in the typical suburban side passageway. For example, a 2000 L tank has a diameter of 1300 mm.
Storage walls, sometimes called thin tanks, are very slim small capacity tanks that fit into tight spaces. For example, a 1000 L thin tank has a width of only 260 mm. Thin tanks can be connected to each other to provide a larger capacity. While thin, these tanks are typically the most expensive type of tank per litre stored.
Underfloor bladders, also called pillow tanks, are flexible sacks that are used in underfloor spaces. They require a space at least 600 mm high. These tanks are slightly more expensive than above ground tanks.
Underground tanks save yard and passageway space. While usually sited below ground, they can also be installed below a deck or raised platform. Polyethylene tanks typically have a 1000 to 5000 L capacity. Concrete tanks can be much larger.
An Aquacomb system is a system of fully modular underground tanks. The minimum capacity is 250 L. The tanks can be installed under the house slab, under a driveway, inside retaining walls or in wall cavities, freeing up outdoor space. It can be used to store rainwater or greywater.
Tank monitoring systems
Tank monitoring systems range from a basic volume indicator to those that monitor inflows and outflows. Some water tank providers offer remote online monitoring. The most advanced technology provides real time tracking of inflows, outflows, usage, storage volume and maintenance data.
The Aquarevo estate in Lyndhurst, Victoria uses a tank monitoring system called Tank Talk. Using weather forecasts, it drains water from rainwater tanks into the stormwater system in advance of a storm. This reduces the peak loads in the stormwater system, including in any natural waterways downstream.
Tank Talk technology monitoring.
Tank capacity
Water tanks may be required by the local or state government. For example, Queensland requires a 5000 litre tank in QDC councils and South Australia requires a minimum of 1000 litres. Within a designated bushfire prone area in Victoria, houses must provide a minimum tank capacity for personal and fire authority use.
Government regulations sometimes set a minimum tank capacity. However, additional storage capacity may be in order. This is determined by potential supply and likely demand. One square meter of roof collects one litre per millilitre of rainfall. Depending on where the tanks are located, and their height, the runoff from some parts of the roof area may not be collectable.
Demand depends on the number of occupants, how they use water, and the area of garden to be irrigated. Also relevant is the likely maximum number of days between rainfall recharging the tanks. Some localities can have months with hot and dry periods when the demand for water in the house and garden increases, but no rain falls.
This metal water tank is colour matched to the roof. Once the boundary fence is erected, the tank will leave enough space in the side passageway between the house and the lot boundary to squeeze by.
Siting
On a suburban lot, the best space for a water tank is typically in the side passage between the house and the boundary fence. A typical residential lot has a 1.0 m wide passage beside the house. Other ancillary items are also often placed in this passage, such as hot water services. At least 400 mm is needed between the tank and the fence for pedestrian access and maintenance. This is adequate if another wider pathway between the front and rear yards provides wheelbarrow access. In a 1 m wide passage, a 2000 litre slimline tank will leave 400 mm clear. Another possible location is at the rear of the house, but placing a tank here is likely to cramp the most useful area for private open space.
Tanks can also be placed underground. This saves yard space, insulates the water, and reduces the opportunity for vandalism. However, it is more expensive, requires additional plumbing, and is more difficult to maintain.
When siting tanks, their height and prominence when seen from the public realm and from the neighbouring lots should be considered. On a corner lot, a water tank and associated plumbing sitting above the fence by the secondary street is unsightly. The tank may instead be able to be placed in the other side passage, where only the neighbour will see it.
Plastic water tanks in side passages.
Design standards
The Design Review Group recommends a minimum setback distance between the water tank and side fence to allow for pedestrian access and maintenance.
“The water tank must be set back from the side boundary at least 400 mm.”
Visibility of the tank and infrastructure from the public realm and from neighbouring lots can be a concern.
“All water tanks must be no more than 2.2 m high and must be located out of public view.”
For sustainability or to avoid needing to upgrade a stormwater system, a minimum tank size that is larger than government requirements may be desirable. The Design Review Group can help select the best minimum size tank for your estate.
“If the lot's area is larger than 300 m2, a rainwater storage tank must be installed with a capacity of at least 2,500 litres.”
If mandating a certain capacity of water tank, the roof area should be adequate to fill the tank.
“The water tank must be fed by a minimum 120 m2 roof catchment area.”
Design standards can be used to reduce the load on the stormwater system. This may save an estate developer the cost of increasing infrastructure capacity.
“Water tanks must be fitted with technology to predict rainfall events and empty the tank appropriately into the legal point of discharge prior to the rainfall event.”
Conclusion
Water storage requirements reduce potable water consumption and reduce the capacity needed in the stormwater system. By including design standards for capacity, siting, size and colour of water tanks, the estate can be more attractive for lot owners and potential buyers.
