How much rainwater can I actually collect from my roof?

Water butt sizing is usually done by eye and by what fits, and the result is a butt that overflows in October and is empty in July. The arithmetic is trivial and it usually reveals that far more water is available than is being caught.

Step by step

1. Measure the plan area of the catchment, not the sloping area.Look at the roof from above — its footprint. Rain falls vertically, so a steep roof catches no more than a flat one with the same footprint. For a simple pitched roof draining both sides, each downpipe takes roughly half the footprint.
2. Include the catchments you forgot.The shed, the greenhouse, the garage, a lean-to, a porch. Small roofs collect more than people expect, and a greenhouse roof supplies water that is already at ambient temperature and free of mains treatment.
3. Apply the conversion.Rainfall in millimetres multiplied by plan area in square metres gives litres. Ten millimetres of rain — an ordinary wet day — on a modest shed roof already fills a substantial part of a standard butt.
4. Take off the losses.The first few millimetres wet the roof and the gutter and never arrive. More is lost to splash out of the gutter, overshoot in heavy rain and evaporation. A practical yield of around four fifths of the arithmetic is a reasonable working assumption.
5. Compare against your demand.Work out roughly what you use in a dry week — count watering-can loads. Then compare that against your storage. If a fortnight of dry weather empties everything you have, more storage is worth having.
6. Look at your own local rainfall, not a national average.Rainfall varies enormously across the UK, and what governs your storage need is not the annual total but the longest dry run you get. A wet region with long dry summer spells needs as much storage as a drier one with steady rain.
7. Decide whether to store more.If your butt overflows in every heavy shower, you are throwing away the water you will want in August. Linking butts, adding a larger tank or fitting a second downpipe diverter are all cheaper than the alternative.

Tips

• Fit the diverter so the butt overflows back into the downpipe rather than onto the ground, or you will create a damp patch against a wall.
• Raise the butt as high as you practically can. All the pressure at the tap comes from the height of the water above it, and a butt on the ground gives a dribble.
• Cover any stored water. A lid keeps out light, leaves and mosquito larvae, and it is also a safety matter around small children.
• A large tank hidden behind a shed stores far more per pound than several decorative butts, if appearance is not the constraint.
• Rainwater is better than tap water for ericaceous plants, for seedlings and for anything in a container in a hard-water area.

Common mistakes

• Using the sloping roof area in the calculation — It overestimates the catchment, sometimes substantially on a steep roof. Rain falls vertically, so the footprint is what intercepts it.
• Sizing storage from the annual rainfall figure — The annual total is irrelevant. What matters is how much falls between the dry spells and how long the dry spells are, because that is what the storage has to bridge.
• Fitting one butt and considering the job done — A single standard butt is filled by one decent shower off a house roof. Everything after that goes down the drain, in the season you are trying to store for.

Questions people ask