Roof Rain Water Capture

November 2014

Ensuring the quality of water collected on roofs

More than 10% of New Zealand’s population depends on rainwater collection to supply their drinking water needs. Most of these people are on farms or lifestyle blocks. The predominant public perception is that roof water is pure, soft, free from chlorine and tastes good. It is usually most of these things, but it may not be as pure as people like to think, according to Stan Abbott, director of Massey University’s Roof Water Research Centre.

“We looked at the microbiological quality of roof-collected rainwater from 560 rural homes and found that at least half exceeded the acceptable standards for drinking water and 40% showed heavy fecal contamination,” says Stan.

“The main causes were the lack of even simple measures for preventing contaminants from getting into the storage tanks and the lack of maintenance of the collection system and tanks.”

Micro-organisms found in rainwater tanks can include salmonella, e. coli, campylobacter, aeromonas, giardia, cryptosporidium and legionella. However, direct links between contaminated water and illness are seldom found because problems are not reported unless there is a substantial outbreak. Individuals may put their symptoms down to a digestive upset and will not seek medical help unless the illness is severe or lasts a long time.

Also, many people develop immunity to these organisms over time. However, problems can arise for visitors to the farm, young children, the elderly or those with compromised immune systems.

Research has shown that the most common infections are from salmonella and campylobacter. In one study people on rainwater systems were found to be three times more likely to become infected with campylobacter than those on city treated water. Another study showed that children under five years old were particularly susceptible to giardia infections from rainwater.

The good news is that the vast majority of problems can be prevented by relatively simple measures to keep contaminants out of tanks. However, Stan says that in their large-scale 2006 study of rural collection systems, hardly any had even the most basic preventative measures in place.

“The problem is that as soon as it rains, people think it’s fantastic that their tank is filling up, but all our studies at Massey show that every time it rains, the pollutants off the roof flow into your tank unless you have those preventative installations in place,” he says.

“There can be dust, leaves, rotting vegetation, dead birds, and faeces from birds, rats, possums etc., but these can be excluded with just a little effort. And if that is done properly then the risk of family or visitors becoming ill after drinking rain water is very low.”

Stan recommends the following:

  • Install self-cleaning screened downpipe rainheads (1)
  • Install a first flush diverter (2) so that the first 200 litres or so of rainfall goes automatically to waste and so prevents bird and animal droppings, dust, ash and airborne chemical residues from entering the storage tank.
  • Install a floating value draw-off pipe (4) so that clean water is extracted from the top of the tank.
  • At the same time, install a “calmed inlet” pipe (3) that takes incoming rainwater towards the bottom of the tank but incorporates a U-bend to avoid disturbing any sediment.
  • Regular cleaning and maintenance of the roof, gutters, filters and pipes
  • Use a swimming pool vacuum pump or similar built-in device (5) when necessary to siphon out sediment.

More detailed information and other measures for special circumstances can be found on the Roof Water Research Centre website :

It goes without saying that overhanging trees should be kept clear of roofs so that animal droppings and other debris will not accumulate. Plastic mesh strips can be used to keep leaves, twigs and other debris blown onto roofs from entering the gutters.

These measures are necessary because most established farmhouses are surrounded by shrubs and trees. However, one Wairarapa farmer has avoided this problem by not collecting roof water from his house. Instead he removed trees from near the woolshed and collects water from that.

Roger Fairbrother who farms at Blairlogie near Masterton, was one of the farmers surveyed by Stan Abbott and colleagues back in the early 2000’s. Following that he made changes to his water supply system.

“Like most houses in the country ours is surrounded by trees so we get a lot of rubbish in the spouting, so we have completely eliminated using water from the house and we have tanks at the woolshed,” says Roger. “We cut down all the trees around the woolshed so there is very little contamination of the water collected there and we have first flush diverters on the downpipes, which eliminates any dirt and debris.”

Roger’s system doesn’t require pumps because the woolshed is at a higher elevation and so water is gravity fed down to the house.

“We have three 23,000 litre tanks connected in series. When they are full we close the tap on one of them, so it acts as a reserve in case pipes break or a tap is left on. It means for example, if there is an earthquake and pipes burst we don’t lose all the water,” he says.

The limited amount of contamination means that Roger’s tanks will not need to be cleaned out very often, which is an advantage. Cleaning can be expensive if water has to be bought to refill a tank and the cleaning process can be dangerous, as Stan explains.

“In some of the old tanks there is a big build-up of sediment in the bottom and that produces methane. People can asphyxiate when they are inside the drained tank, so great care needs to be taken,” he says. “It is far better that people have these basic installations and then the tanks don’t need to be cleaned out very often at all.”

Stan also recommends that people in urban environments install rainwater tanks as part of preparation for emergencies. “Even a 250 litre rainwater barrel means that if earthquakes or other disasters strike, people will have some emergency water. The Wellington Regional Emergency Management Office has sold more than 3500 small tanks that are quick and easy to install,” he says.

“Some councils now insist as part of a building consent that you can’t build a new house or do alterations unless you put in a 10,000 litre rainwater tank which gets used to flush the toilets.”

“Our mantra is ‘Save the rain, don’t let it drain’. And of course use the basic physical measures to prevent microbial contamination so that you can provide your family with safe, clean water.”