Harvesting rainwater

It’s crucial to ensure that the roof, guttering, pipes and other features do not contaminate the water before it goes into the storage tank.

On this page:

• Roofing materials
• Roof paint
• Gutters and downpipes
• Leaf screens and guards
• First flush diverters

Water from roofs can be contaminated by decaying vegetable matter such as leaves, petals and pollen; faecal matter from birds, possums and rats; and dead insects, birds and animals.

Other sources of contamination include particulates from solid fuel flues; pollution; chemical spray drift; and harmful materials – such as lead – in the roof, roof paint, gutters, downpipes etc.

The Building Code requires, among other things, that water supplies must be protected from contamination. See water supply for general Code requirements for water supply, and mains or rainwater for requirements relating to rainwater collection.

To reduce the risk of contamination:

• specify roof,  and spouting and pipework materials that will not contaminate the water supply
• specify leaf guards over the gutters and leaf screens on downpipes
• specify a first flush diverter to prevent the first 20–25 litres of water, which may be heavily contaminated, from entering the storage tank
• do not collect rainwater from a roof that has overhanging trees or a TV aerial – this reduces the risk of leaves and bird droppings getting into the harvested rainwater
• ensure that the flue from a solid fuel burner is located so that soot and other discharges are carried clear of the rainwater collection area
• advise owners that gutters must be kept clean.

If rainwater is being harvested for human consumption, roofing, spoutings, downpipes, and pipework materials must comply with AS/NZS 4020 Testing of products for use in contact with drinking water.

Roofing materials

Roofs suitable for water collection for human consumption include:

• unpainted zinc/aluminium-coated or galvanised steel
• factory-coated or painted zinc/aluminium alloy-coated or galvanised steel
• zinc
• stainless steel
• aluminium
• concrete or terracotta (clay) tiles
• copper
• PVC (without lead stabilisers) or fibreglass sheet
• untreated timber shingles (usually imported western red cedar)
• butyl rubber.

When specifying materials, while the first consideration is health and safety, you may also want to take account of sustainability considerations such as embodied energy. See material use for more.

Leave a new roof for at least one good period of rainfall before connecting the downpipes to the storage system.

Do not use collected water for drinking if it has come into contact with:

• uncoated lead flashings (lead flashings on existing roofs should be coated with a suitable paint; coated lead is available for new roofs)
• treated timber where chemicals leaching out might contaminate the water
• bitumen-based roofing
• asbestos (although no longer used in building, existing asbestos roofs should not be used for collection of rainwater).

Roof paint

Specify only paint that the manufacturer recommends as suitable for collection of rainwater for drinking. Do not use collected water for drinking from roofs coated with lead-based paints or bitumen-based paints. Also do not use collected water from roofs coated with acrylic paint until it has been washed by a good rainfall.

Gutters and downpipes

Materials suitable for use for gutters and downpipes where potable water is being collected include extruded PVC, factory-coated zinc/aluminium alloy-coated steel, galvanised steel, copper (which may cause staining if water has a low pH), aluminium, and polyethylene/polypropylene.

Again, while the first consideration is health and safety, you may also want to take account of sustainability considerations such as embodied energy. See material use for more.

Leaf screens and guards

Leaf screens and guards help keep plant matter and other debris out of the rainwater storage tank.

Leaf screens and guards

Leaf screens located on each downpipe keep larger debris out of the rainwater tank.

Diverters

These devices divert the first rain away from the water collection tank, washing dust, leaves and other debris off the roof before water is collected.

One form of diverter has a float that rises as the rainwater flows in. When the floater reaches the top of the diverter pipe, it seals it off allowing the rainwater to flow into the tank.

Generally, the more water that is diverted, the better the quality of the collected water. To calculate the amount of water to divert, consider the level of the pollutants on the roof:

• for minimal pollution, allow 20 litres/100 m² (0.2 litre/m²) of roof area
• for heavy pollution, allow 50 litres/100 m² (0.5 litre/m²) of roof area.