The catchment area for rainwater is the roof’s footprint on the ground, measured as length times width of the building including the eaves, not the length of the sloping surface. Add up every roof that drains to your gutters: 5 metal-roofed buildings totalling 200 m² at 800 mm of annual rain, with a runoff coefficient of 0.9, collect 144,000 L a year.
Part of Cisterns and Rainwater Storage
Catchment area is the surface that collects rain and delivers it to the gutters and tank. Its size, together with rainfall and the roof material, fixes how much water a cistern can ever receive.
Rain Falls Straight Down, So The Footprint Counts
Rain arrives vertically in still air, so the water a roof intercepts is set by the area it covers when seen from directly above. A steep roof is longer along its slope than the room beneath it, but it catches no more rain than a flat roof over the same footprint. Measure the ground-plan rectangle out to the edge of the eaves.
Wind-driven rain can add water to a wall-facing roof slope. Treat that as a margin, not a figure to plan on.
What You Need
- A tape or measuring rod — see Measuring Rod
- A sketch plan — each roof, with its length and width marked
- A record of which roofs drain to which gutter — only gutters leading to the tank count
- The roof material for each — the runoff coefficient depends on it
Method: Footprint, Then Filter, Then Total
Step 1: Measure each roof from eave edge to eave edge along both sides, as a ground rectangle. A 10 m by 5 m building gives 50 m².
Step 2: Subtract what does not drain to the tank. A roof that sheds into a valley you cannot gutter, a section of thatch near a cooking fire, or a roof that leaks into a wall does not count.
Step 3: Rule out unsafe surfaces. Rain collected from a roof carries whatever is on it. Roofs with lead flashing, old asbestos-cement sheets, or treated timber shingles are poor sources for drinking water, and a roof dusted with chimney soot or bird droppings needs the first-flush diverter and a good screen.
Step 4: Total the usable roof area in m².
Step 5: Apply the formula with your local rainfall and the coefficient for each roof’s material: litres = area × rainfall × coefficient.
Worked Example
Five buildings with metal roofs totalling 200 m², 800 mm of rain a year, coefficient 0.9 for metal:
200 m² × 800 mm × 0.9 = 144,000 L per year.
If the same group wants to add a thatch roof of 40 m² (coefficient 0.5), its contribution is 40 × 800 × 0.5 = 16,000 L per year, only 11% of the metal total (16,000 ÷ 144,000), and with worse water quality. The cheaper way to add water is usually to extend gutters on an existing metal or tile roof. The ground surface can also serve as a catchment: the cisterns page gives 0.8 for concrete or plastic ground surface, but ground runoff picks up far more dirt than a roof.
Doubling the catchment doubles the yield, and the tank must be sized for the larger inflow as well. See Dry Spell Bridging.
Common Mistakes
- Measuring the slope length. The yield is overestimated.
- Counting roofs with no gutter to the tank. They deliver nothing.
- Forgetting overhang. The eave edge, not the wall line, is the boundary.
- Ignoring the roof’s condition. A rusted, flaking or lead-contaminated roof ruins the water however large it is.
What’s Next
- Learn next: Roof Coefficient — how much of the rain each material actually delivers
- Combine with: Rainfall Data — the other input to the formula