Your catchment area is the part of your roof that drains to the tank, measured as a footprint — the area it covers when seen from above.
Why footprint, not sloped area
Rain falls vertically. A steeply pitched roof presents more surface, but it occupies the same ground, and the same volume of rain lands on it. A 10 m × 15 m house has a catchment of 150 m² whether the roof is nearly flat or steeply gabled.
This is the single most common mistake in tank sizing, and it always errs optimistically: measuring along the slope inflates the figure, so the tank looks better supplied than it is.
Measuring it
For a simple rectangular house, multiply length by width in metres. Include the eaves — they catch rain and it goes into the same gutters.
For an L-shaped or more complex roof, split it into rectangles and add them up.
What to leave out
Count only the roof that actually drains to your tank:
- Sections plumbed to a different downpipe that discharges to ground
- Sheds and garages, unless they are connected
- Any area where the first-flush diverter sends water away
If half your roof drains to the tank and half to a soakpit, your catchment is half the footprint.