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Rainwater harvesting for your garden: tanks, sizes and rules

Every square metre of roof collects about a litre of water for every millimetre of rain. A small roof in a good storm can fill a tank. Collect the water, screen it, cover it, and use it on the garden.

Is it legal?

Collecting rain from your own roof for use at home is generally allowed without a licence. Municipal by-laws may still set rules: telling the municipality about your alternative water supply, signs on rainwater taps, the size and placing of tanks, and protection against backflow into the municipal pipes.

These rules differ between municipalities. Ask your water department before you install a tank, and ask your landlord first if you rent.

How much rain can you collect?

litres collected = roof area (m²) × rainfall (mm) × 0.8

Measure the roof area as if you were looking down on the house: the length times the width of the footprint, including the overhang. The 0.8 allows for splashing, evaporation and the first dirty flush.

For example, 50 m² of roof in a 20 mm storm gives 50 × 20 × 0.8 = 800 litres.

The parts of a simple system

1. Roof and gutters. Keep the gutters clean and sloping towards the downpipe. Avoid roofs with flaking paint, old asbestos sheeting or thatch.
2. Leaf screen. A mesh over the gutter or at the top of the downpipe.
3. First flush diverter. A section of pipe that catches the first dirty water off the roof before clean water passes to the tank. Empty it after each rain.
4. Tank. Choose a dark, opaque tank, because light makes algae grow. It needs a tight lid and fine mesh over every opening.
5. Tap, near the bottom, high enough for a watering can.
6. Overflow, leading water away from the house foundations, ideally to a garden bed.

Choosing a size

A tank that is too small overflows in every storm. One that is too large may never fill. Think about how much your roof collects and how much you use in the dry season.

In summer-rainfall areas a tank fills in summer storms and runs empty in the dry winter. In winter-rainfall areas it fills in winter and is used in summer. A tank helps most in the dry spells between rains. It rarely carries a garden through a whole dry season.

Many households start with one tank of 1 000 to 2 500 litres on the downpipe nearest the vegetables, and add more if it overflows often.

The base matters

Water is heavy. One litre weighs one kilogram, so a full 2 000-litre tank weighs two tonnes. The tank must stand on a firm, level base built for that weight: compacted, level ground with a layer of sand, a concrete slab, or a properly built stand.

A tank on a weak or uneven base can crack, tip or collapse, and it can kill someone. Never put a tank on a wall, a roof, a wooden deck or a carport that was not designed for it.

Where to put it

• close to a downpipe and to the plants that need the most water;
• on the shady side of the house if possible;
• with room to reach the tap, lid and first flush diverter;
• away from septic tanks and drains;
• where the overflow will not wet the foundations.

Keeping the water clean and safe

• Clean the gutters, screens and first flush diverter, especially after leaves fall.
• Keep the lid on and the mesh whole. That also keeps mosquitoes out.
• Do not drink rainwater or cook with it unless it has been tested and treated.
• Keep lids secured. Children can drown in a few centimetres of water.
• Never climb into a tank.

What you may do yourself

You can place a stand-alone tank on a proper base, connect it to the downpipe, fit screens and an overflow, and use the water by can, bucket or hose.

Call a registered plumber for any connection between a tank and the house plumbing or the municipal supply, and for pumps, large tanks and raised stands.

This guide is based on Chapter 7 of Water-Wise Gardening in South Africa.