Is the collected water drinkable?

Not as it arrives. Roof runoff carries whatever is on the roof — bird droppings, dust, moss, particles from the air and, on some older roofs, material from the covering itself. Making it safe to drink requires filtration and disinfection, a suitable roof covering, and in most countries compliance with rules that also govern how the pipework is kept separate from the mains. For watering a garden, flushing a lavatory or washing a car, none of that applies and the water is entirely suitable as it is.

How do I measure the footprint of an awkward roof?

From above rather than on the roof. A satellite view with a measuring tool gives the outline in a couple of minutes and is accurate enough for a tank decision; the plans of the building give it exactly if they exist. Where a roof drains to several downpipes, measure each catchment separately — only the part that reaches the tank counts, and half a roof feeding a different side of the house is simply not in the calculation. Include the overhang, since rain falling on it still runs to the gutter.

A Steeper Roof Does Not Catch More Rain

Published 10/2/2026 · 3 min read · Everyday calculators

Marco Bianchi — Home, DIY & motoring writer at OneKitly

Renovation · Materials

Checked against 2 sources

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In short

A millimetre of rain on a square metre is a litre, so 100 m² under 50 mm would be 5,000 litres if everything arrived in the tank. The calculation applies a runoff coefficient of 0.9 and returns 4,500, the missing tenth being what wets the tiles, evaporates before it runs off, overshoots the gutter in heavy rain or leaves through the first-flush diverter. The area to enter is the one the roof covers on the ground, not the surface you would tile. A roof pitched at thirty degrees has about 115 m² of tile over a 100 m² footprint, and entering 115 would overstate the yield by fifteen per cent — rain falls vertically, so what a roof catches is decided by the shadow it casts, not by how much material it took to build.

100 m² of roof under 50 mm of rain yields 4,500 litres. The area to enter is the footprint, not the sloping surface — and the 10 % missing is water the roof never delivers.

The tank is sized by the dry spell, not the annual total

Seventy-two thousand litres a year sounds like far more than a garden needs, and it is — but almost none of it falls when the garden wants it. What decides the tank is the longest stretch between useful rainfalls and the demand across that stretch: a garden drawing 200 litres a day through a hundred dry days needs 20,000 litres of storage to cover it, and no annual total changes that. Two places with the same 800 mm need very different tanks if one spreads it evenly and the other delivers it in four months. Take the monthly distribution from a local record rather than the yearly figure, and size against the gap.

Nine tenths is a good roof, not every roof

The 0.9 in this calculation suits a smooth pitched roof in metal, slate or glazed tile, where water leaves quickly and little is absorbed. A rough or porous surface holds more; a flat roof with gravel ballast holds a great deal more and releases it slowly; a planted roof is designed to retain water and can give back less than half. Gutters that are undersized for a downpour lose the peak of every heavy storm regardless of the roof, which is a separate loss the coefficient does not model. If the roof is not smooth and pitched, treat 4,500 litres as an upper bound and reduce it deliberately.

A 100 m² footprint, at 0.9 runoff
Rainfall	Collected
10 mm (one wet day)	900 L
50 mm (a wet month)	4,500 L
800 mm (a year)	72,000 L

Worked with our own calculator

Rainwater harvesting calculator

Given

Roof area (m²): 110
Rainfall (mm): 55

Result

Water collected (litres): 5,445

These figures are produced by the calculator below, not typed in by hand — they are recomputed whenever the tool changes.

Run it on your own figures →

Frequently asked questions

Is the collected water drinkable?: Not as it arrives. Roof runoff carries whatever is on the roof — bird droppings, dust, moss, particles from the air and, on some older roofs, material from the covering itself. Making it safe to drink requires filtration and disinfection, a suitable roof covering, and in most countries compliance with rules that also govern how the pipework is kept separate from the mains. For watering a garden, flushing a lavatory or washing a car, none of that applies and the water is entirely suitable as it is.
How do I measure the footprint of an awkward roof?: From above rather than on the roof. A satellite view with a measuring tool gives the outline in a couple of minutes and is accurate enough for a tank decision; the plans of the building give it exactly if they exist. Where a roof drains to several downpipes, measure each catchment separately — only the part that reaches the tank counts, and half a roof feeding a different side of the house is simply not in the calculation. Include the overhang, since rain falling on it still runs to the gutter.

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Sources

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