Calculate What Your Roof Can Actually Catch

How the harvest formula works

One millimetre of rain on one square metre of roof yields one litre of water, before any losses. That simple identity is the whole maths. Multiply your roof's plan area by your yearly rainfall in millimetres, and you have the theoretical catch in litres. A sixty square metre roof under eight hundred millimetres of rain could, on paper, shed forty eight thousand litres in a year. In practice, a share of that never reaches the tank. Rough tile holds more water than smooth metal. Wind blows spray off an open edge.

Capture efficiency folds all of that into one number. A smooth metal roof with a good first flush diverter can reach eighty five to ninety percent. A rough tiled roof with a basic screen often sits nearer sixty five to seventy five percent. The default here is eighty percent, a reasonable middle ground for planning. Change it only if you know your roof type and gear well enough to justify it. A screen and a diverter trade a little volume for much cleaner water, which is usually the right trade.

• Measure the plan area, the footprint the roof covers, not the sloped surface.
• Smooth metal sheds best. Rough tile or shingle holds more water back.
• A first flush diverter costs volume and buys water quality.

Worked example: sizing a tank for a real dry spell

Take a roof of eighty square metres in a place that sees fourteen hundred millimetres of rain a year. The theoretical catch is one hundred and twelve thousand litres. At eighty percent capture, about ninety thousand litres can realistically reach a tank. That sounds generous until it is spread across twelve months. Divide by three hundred and sixty five and the average daily supply is around two hundred and forty five litres. A garden and laundry household might use more than that on a hot week, and much less in winter. Average is not the same as available when you need it.

Storage solves that mismatch. If the driest stretch in your area runs about thirty days with little useful rain, and the household uses one hundred and fifty litres a day from the tank, the minimum working size is four thousand five hundred litres. Anything smaller forces a backup plan, like a mains top up or a delivered water fill, part way through the dry spell. A bigger rainfall does not mean a big tank is unnecessary. It means the tank fills and empties on a schedule set by your climate, not your average.

What this tool does not model

This is an annual figure, not a monthly one. Real rainfall arrives in seasons, and a dry month in the middle of a wet year can empty a tank faster than the average suggests. It does not model roof temperature, wind loss or the exact behaviour of your first flush diverter either. It also assumes every drop that reaches your catchment is worth collecting. In very polluted or industrial air, the first flush matters more than usual, and a professional filter check is worth the cost.

Tank size is only part of the plumbing question. Pump pressure, overflow routing and screen mesh size all affect how much of the modelled water you actually use. Water quality is a separate subject again. Rainwater is not automatically drinking water, even off a clean roof, and treatment needs depend on what you intend to do with it. Use this tool for volume planning. Use your local building code and water guidance for what is safe to store, and how.

Getting more from the same roof

Once the maths is set, three practical changes usually raise usable supply more than buying a bigger tank would. First, match gutter and downpipe size to the roof area. An undersized downpipe spills water at exactly the moment a storm carries the most of it. Second, keep the diverter and screens clean. A blocked first flush can quietly send a wet season's worth of water to waste. Third, split the load. A garden on a small barrel need not draw from the same tank as the household laundry.

Where the annual harvest still falls short of demand, the honest options are to cut use or to add catchment area. A simple meter reading usually shows that outdoor irrigation drives most of a household's rainwater demand. Fixing that demand, with drip lines or mulch, often closes the gap cheaper than a second tank. Only after that is a bigger roof surface or a larger storage vessel likely worth the money.

Accuracy, assumptions and limitations

This tool assumes your rainfall figure is a long term annual average, not a record wet year or a single season's observation. It assumes your roof drains fully to the catchment you have measured, with no part of it feeding a soakaway or a neighbour's downpipe. It assumes a constant capture efficiency across the year, which is not quite true. The first flush after a long dry spell carries off more dirt than one after a light shower, so real quality varies more than real volume does.

Treat the harvest figure as a planning band, not a promise. If the answer says you cover most of your demand, that is still an average across twelve months. It says nothing about whether the tank is full on the specific week you need it. Keep a margin on the tank size. Check your local water restrictions before designing a system that assumes rain will always be there when the calendar says it should be.