How Much Rainwater Can My Roof Catch?
How to work out how much water your roof can put in the tank, and whether it can keep up with what you use.
A tank only stores water. The roof is what fills it. If the roof is too small for your use, a bigger tank won't fix that.
1. The rule to remember
1 mm of rain on 1 square metre of roof = 1 litre of water.
That's before any losses. So 10 mm of rain on a 100 m² roof is 1,000 litres in theory.
2. Step 1: measure the roof
- Measure the footprint from above, meaning the area the roof covers on the ground, including the eaves. Don't measure up the slope. A steep roof doesn't catch more rain than a flat one with the same footprint.
- Only count the parts that drain to this tank. If only one side's gutters run to the tank, count only that side. The calculator has a "share of the roof" box for this. For example, 50% for one side of a gable roof.
- Add any shed or carport roofs that are, or could be, plumbed to the tank.
Area = length × width × the share that goes to the tank.
3. Step 2: find your rainfall
The Bureau of Meteorology's climate statistics pages (bom.gov.au) list the mean rainfall for a month and a year for towns across Australia. Find the station nearest you.
Averages hide bad years. Look at the lower figures in the same data too, because planning on an average year leaves you short in a dry one.
4. Step 3: allow for losses
Not every drop reaches the tank. Some splashes off, some evaporates, the first flush is diverted, and heavy rain can overflow the gutters. The calculator starts at 80% catch efficiency as a common starting point for a metal roof.
Light showers often produce very little runoff, because the roof has to wet first.
5. Worked example: a year of rain
A farmhouse roof measures 15 m × 10 m from above, and all of it drains to the tank. The town's mean rainfall is taken as 600 mm a year for this example. (That is a made-up figure: use your own town's.)
- Area: 15 × 10 = 150 m².
- Rain landing on it: 150 × 600 = 90,000 litres.
- Caught, at 80%: 90,000 × 0.8 = 72,000 litres a year. That's about 197 litres a day on average.
6. Will it keep up with your use?
Now compare that with what you use. The family in our water tank guide uses about 605.8 litres a day, which is about 221,000 litres a year.
The roof catches about 72,000 litres a year, so on these figures the house roof alone can't keep up. The gap can be closed in several ways:
- Plumb in more roof. A 20 m × 12 m shed adds 240 m². With the house, that's 390 m² × 600 mm × 0.8 = 187,200 litres a year. That's closer, but still short.
- Use less. Garden and washing-down water are usually the easiest to cut.
- Add another source, such as a dam or bore for the garden and stock.
- Plan for carting water in a dry spell.
The calculator does this comparison for you if you enter your daily use.
7. One storm
The same sum works for a single fall of rain. 25 mm on the 150 m² roof is 150 × 25 = 3,750 litres, or about 3,000 litres caught at 80%. That's handy for knowing what a good storm is worth to your tank.
8. Before you rely on it
- Real catch depends on gutter and downpipe size, how heavy the rain is, how full the tank already is, and the roof material.
- Rainfall varies a lot from year to year.
- Drinking water: ask your local council or state health department about roof materials, first-flush diverters and treatment.
9. Common mistakes
- Measuring up the slope of the roof instead of the footprint.
- Counting roof areas whose gutters don't run to the tank.
- Planning on the average year only.
- Expecting light showers to fill the tank.
- Buying a bigger tank when the real problem is too little roof.
10. Quick summary
- 1 mm on 1 m² = 1 litre.
- Use the footprint area that actually drains to the tank.
- Allow for losses (about 80% is a common starting point).
- Compare a year's catch with a year's use.
- Short? Add roof, use less, or add another source.
Try it with your own roof: Roof Rainwater Catch Calculator.
General information only. Ask your council about drinking water and plumbing approvals.