Roof water and cisterns

What roof water harvesting is

A roof is the most dependable catchment there is. It is hard, sloped and already built, and the water running off it is far cleaner than water that has crossed the ground. Roof water harvesting collects that runoff through gutters and downpipes, frees it of leaves and dust and stores it in a closed tank, the cistern, until it is needed. Where rain falls in winter and the dry season lasts for months, the cistern does what the soil cannot: it carries water from the wet half of the year into the dry half.

It is the counterpart to the earthworks described under rainwater harvesting. The soil stores water in the slope, unseen and shared by every root; the cistern keeps it in a container, measured and ready at the tap.

How much a roof yields

The arithmetic is simple, because one millimetre of rain on one square metre is exactly one litre. The yield in litres is roof area times rainfall in millimetres times a runoff coefficient. Roof area means the footprint seen from above: a steep roof catches no more rain than a flat one with the same plan. The coefficient covers what never reaches the tank, water that wets the surface, evaporates or splashes past the gutter. For hard roofs of metal or tile, guides usually calculate with about 0.8 to 0.9, for gravel or planted roofs with much less.

An example with assumed figures: 100 square metres of roof, 600 millimetres of rain a year, a coefficient of 0.8. That gives 100 × 600 × 0.8 = 48,000 litres, 48 cubic metres a year. Reality takes some away. Laboratory tests on ceramic tiles found coefficients above 0.9 in moderate and heavy rain, but far lower ones in light drizzle, which the tiles largely absorb. The first flush is diverted, and whatever falls on a full tank runs over. In the Mediterranean one more limit weighs heavily: nearly all the water arrives in the cool months, when the garden needs little. What is available in August depends less on the roof than on the size of the cistern.

From gutter to tank

The water passes four stations. First gutter and downpipe, with enough fall that nothing stands in them. Then a leaf guard, a screen or downpipe filter that keeps out leaves, twigs and needles. Third, the first flush diverter. The first rain after a dry spell rinses the roof of dust, pollen and droppings, and this first portion fills a small chamber before the cleaner water flows on. How much to divert is not fixed; the Texas rainwater manual names about 0.4 to 0.8 litres per square metre of roof, and more makes sense under trees. The chamber is emptied after each rain.

Fourth, the cistern. Concrete and masonry are heavy, durable and keep water cool; rainwater is naturally slightly acidic and very soft, and in such tanks it takes up some minerals from the walls. Polyethylene tanks are light and cheaper, steel tanks need a lining against corrosion. Whatever the material, the enemies are light and warmth: light feeds algae, warmth speeds up spoiling. A cistern is therefore opaque and closed, ideally underground or at least in shade. A calmed inlet keeps incoming water from stirring up the sediment on the floor, a floating intake draws from just below the surface, and vents and overflow are screened against mosquitoes and small animals.

The overflow belongs in the plan from the start. Instead of vanishing into a drain, the surplus can run into a planted basin, a bed under mulch or a level swale on the slope, where it soaks in like the rest of the rain, well away from foundations.

What the water serves

Roof water suits the garden: soft, free of chlorine and at hand when summer is longest. It goes furthest with methods that waste nothing, drip lines or ollas, clay pots buried in the bed and refilled from the tank. Animals can drink it if the tank is kept clean. In the house it can flush toilets through a separate, clearly marked pipe system that never meets the drinking water supply, and after that first use part of it can return to the land as greywater. Drinking it is another matter: untreated roof water can carry germs, so it needs filtration, disinfection and regular testing first.

Cisterns in Mediterranean history

Where rivers are few and summers long, cisterns are old. In the Punic city of Tharros on Sardinia's west coast, the excavated water system includes several cisterns of the type called "a bagnarola", long basins with rounded ends, a form typical of the Punic world that stayed in use in Roman Sardinia; the private ones collected rainwater for the household. In Malta, after the Great Siege of 1565, the builders of Valletta, on a peninsula without springs, required every house to have an underground rainwater tank; the Maltese still call these cisterns wells. Malta also shows the limit: a cholera epidemic in the 1860s exposed how many cisterns had been fouled by leaking sewers and cesspits. A cistern is only as clean as its roof, its filters and its care.