The one number that matters
One millimetre of rain on one square metre of roof equals one litre of water. That is the whole foundation of rainwater harvesting.
It is not a rule of thumb. The metric system makes it exact. A millimetre across a square metre is one thousandth of a cubic metre, which is one litre.
From there the formula is simple. Roof area times annual rainfall times a runoff coefficient gives litres per year.
A 120 m² roof, 900 mm of rain, coefficient of 0.85: 91,800 litres. Enough to irrigate a serious garden through a dry season, or flush toilets all year.
Run the same math month by month and the seasons reveal themselves.
Measuring the roof
The number you want is the horizontal footprint, not the sloped surface. A steep roof and a flat roof of the same plan area catch the same rain. Rainfall is measured vertically.
For a simple rectangle, measure length and width at the eaves, including overhang. A 12 by 10 metre house with 0.5 m eaves gives you 13 by 11, which is 143 m².
Complex roofs. Break them into rectangles and triangles. Sum the pieces. For L-shapes, sketch the footprint on graph paper, or measure off a satellite image.
Count only the sections plumbed into your tank. A detached garage is a separate catchment unless gutters connect it. If you collect off the 72 m² north slope only, use 72. Not 143.
Runoff coefficients
No surface delivers everything that lands on it. Some water splashes off the edge. Some evaporates on hot metal before it reaches the gutter. Joints leak. The coefficient captures all of that.
- Metal roof (corrugated iron, steel, aluminium): 0.90 to 0.95. Smooth, impervious, fast.
- Concrete or clay tile: 0.80 to 0.90. A little porosity, gaps between tiles.
- Asphalt shingle: 0.75 to 0.85. The granular surface holds water back.
Then come the system losses. A first-flush diverter deliberately dumps the first 1 to 2 litres per square metre of each storm to wash off bird droppings, dust, and leaf grit.
If you get 80 storms a year and dump 1.5 L/m² each time, that is 120 L/m². On a 100 m² roof, 12,000 litres gone before the tank sees a drop.
Add a few percent for gutter overflow in heavy storms. A realistic whole-system efficiency lands between 0.75 and 0.85.
Use the conservative number. An inflated estimate buys you an undersized tank and a disappointing March.
Build a monthly water budget
Annual totals lie. Climates are seasonal. The useful exercise is twelve rows on a page.
Down the left, list the months. Pull monthly rainfall from the nearest weather station and use the median, not the mean. Medians describe a normal year. Means get distorted by one freak deluge.
Multiply each month's rainfall by your effective catchment and coefficient. That is supply.
Next to it, write demand. Garden irrigation, which spikes in summer. Toilets. Laundry. Whatever the tank feeds.
The running difference shows the tank filling in the wet months and drawing down in the dry. The deepest drawdown is your minimum storage volume.
Tank full at 15,000 litres going into a five month dry season, drawing 2,500 litres a month: you need 12,500 litres of live storage. Add 20% for bad years. Call it 15,000.
When the numbers tell you something
The budget will tell you which side is the bottleneck.
If the roof delivers 60,000 litres and you need 20,000, storage is cheap because the tank refills every few weeks.
If the roof delivers 25,000 and you need 20,000, you have no margin. Expand the catchment. Plumb in the shed roof, add a carport, or run a surface catchment line from a driveway into the same tank.
The most expensive mistake in rainwater is buying the tank before doing the arithmetic. Either it never fills, or it empties in August. Both are avoidable.
See also
- Rainwater Harvesting Basics
- First-Flush Diverter
- Tank Sizing
- Surface Catchment
- Tank Placement
- Overflow Management