Rainwater Harvesting from Roofs

Rainwater harvesting from roofs means collecting the water that runs off a roof, usually through gutters and downspouts, and storing it in tanks or cisterns for later use. It is practised worldwide, from rural households that depend on it for drinking water to urban buildings that use it for irrigation and toilet flushing.

How much water a roof yields

In theory, 1 mm of rain on 1 m² of roof yields about 1 litre. The Texas Water Development Board's rainwater manual gives the equivalent of about 0.62 US gallons per square foot of roof per inch of rain, and notes that installers typically assume 75-90 percent collection efficiency to account for splash, evaporation and first-flush diversion.

Roof material suitability

The roof surface is the first stage of water treatment, so its material matters, especially where water will be used for drinking. The Texas manual's guidance includes:

• Metal: recommended; plain galvanized roofing or metal with epoxy or latex paint is suitable for potable systems, and Galvalume is commonly used. Copper flashings can discolour fixtures.
• Clay and concrete tile: usable for potable or non-potable systems but porous, losing roughly 10 percent of water; a suitable sealant or paint helps prevent bacterial growth.
• Slate: ideal for potable use if no toxic sealant is applied, though cost often rules it out.
• Asphalt or composite shingles: not advised for potable systems because compounds can leach into the water; acceptable for irrigation.
• Wood shingles and shakes: usually suitable only for irrigation because of leaching.

Coatings, sealants, moss treatments and lead flashings can also affect water quality.

First flush and filtration

The first rain after a dry spell carries the most dust, bird droppings, leaves and other contaminants. First-flush diverters discard this water. One rule of thumb in the Texas manual is to divert at least about 38 litres per 93 m² (10 US gallons per 1,000 sq ft) of roof. Gutter screens, prefilters and drawing water from a floating outlet improve quality. Water for drinking needs treatment, such as filtration and disinfection.

Where it is promoted or required

Tamil Nadu in India made rooftop rainwater harvesting compulsory for buildings in 2001, and Mumbai required it for large new buildings from 2002. Bermuda requires it for new construction, it is widespread in rural New Zealand and Australia, roughly 40 percent of Thailand's rural population relies on it, and Santa Fe, New Mexico requires it for new dwellings.

Maintenance

Gutters and screens need regular cleaning, tanks should be sealed against mosquitoes and light, and roofs should be kept free of overhanging branches.