Catching rain from your roof is the easiest and most cost-effective water source for many off-grid homes, but it only works well if every part of the system is planned together.
What rainwater can be used for
The book lists garden and crop irrigation, livestock watering, bathroom use, laundry, drinking water, a fire suppression reserve and topping up greywater. Water for drinking needs proper storage, filtration and purification first, which is covered in filtration vs purification.
How much can you collect?
The book gives an example: a 2,000 sq ft roof in a 30-inch annual rainfall zone can collect about 33,900 US gallons a year, which it says is enough for a family of four with proper storage. In metric terms that is roughly 128,000 litres from about 186 square metres of roof in about 760 mm of rain.
A simple rule for your own estimate: one millimetre of rain on one square metre of roof is one litre of water. Multiply roof area by annual rainfall to get a ceiling, then allow for losses, because you will never catch everything. Use your local rainfall figures, and remember that the rain does not fall evenly through the year.
The five parts of the system
- A metal roof. The book calls it the most cost-effective collecting surface: water runs off easily and the roof does not itself make the water dirty or toxic.
- Gutters and downspouts. The book suggests 4 to 6 inch gutters (about 100 to 150 mm) with a fall of 1/16 inch per foot (roughly 5 mm per metre) towards the downspouts.
- A first flush diverter. This sends away the first runoff of each rainfall, which carries most of the dirt, pollen and leaves, so cleaner water reaches the tank.
- Storage. The tank should be opaque and UV-resistant to discourage algae. Concrete or steel can be used if lined with a food-grade liner (see planning water storage).
- Distribution. Either raise the tank so water reaches the house by gravity, or store it underground and use a pump.
Check the rules first
The book stresses local law: some places encourage rainwater collection, while others restrict it under water rights rules. Check local regulations before you build, and read off-grid water rights basics.
Do not skip treatment
Collected rainwater is not automatically safe to drink. A first flush diverter improves the water, but filtration, purification and regular testing are still needed before drinking.
The full water chapter, from collection to purification and winter protection, is in the Off-Grid Water guide. To see how water fits with every other system on the property, read The Off-Grid Survival Manual.