Rainwater Harvesting

Catch it, store it, use it wisely

5 minBeginnerUpdated 2026-10-23 Rainwater Harvesting

You will learn

Estimate how much rain your roof can collect
Set up a simple, safe barrel system
Know what rainwater is safe for and what it is not

⚠️ Rainwater from roofs is not drinking water. Use it for gardens, cleaning and toilets only.

⚠️ Never cross-connect a rain tank to mains plumbing; indoor use needs a plumber and approval.

⚠️ Screen all openings against mosquitoes and keep tanks lidded and childproof.

How much water is on your roof

The sum is simple: roof area in m² times rainfall in mm gives litres. A 50 m² roof in a place with 600 mm of rain a year catches about 30,000 litres. Knock off 20 percent for splash, evaporation and overflow and you still have 24,000 litres, enough to water a vegetable patch through a dry summer if you can store it. The problem is timing. In a Mediterranean climate, 80 percent of that rain falls between October and April, and almost none falls when the garden needs it most. Storage capacity, not roof size, is the real limit.

💡 Look up your town’s average monthly rainfall. It tells you when your tanks fill and how long they must last.

The basic barrel setup

Start with one downpipe. Fit a rain diverter (a small kit that cuts into the pipe) leading to a 200 to 300 litre barrel on a raised stand of concrete blocks so you can get a watering can under the tap. Add a fine mesh screen where water enters to keep out leaves and mosquitoes, a sealed lid, and an overflow pipe that sends excess back to the drain or away from your foundations. A first-flush diverter, a short vertical pipe that fills with the dirtiest first litres before clean water passes on, improves quality a lot. Link two or three barrels with a hose at the base to multiply storage.

💡 A full 300 litre barrel weighs 300 kg. Build the stand to take it.

What rainwater is safe for

Roof water carries bird droppings, dust, moss, pollen and whatever the roof is made of. Treat it as non-potable: fine for watering plants, washing tools, flushing toilets, cleaning the car, and topping up a pond. Water vegetables at the soil, not over the leaves, and stop a couple of days before harvest for salad crops. Do not drink it, cook with it, or give it to animals as their only water without proper filtration and disinfection, which is a separate, serious project. Never connect a rain tank to your house plumbing; a cross-connection can contaminate your mains supply and is illegal in most places.

💡 Label every rain tap 'Not drinking water' so guests and children know.

Keeping it clean

Mosquitoes breed in open water within a week, so every inlet and overflow needs mesh finer than 1 mm. Empty and rinse barrels once a year, ideally at the end of summer before the autumn rains. Clean gutters in autumn after leaf fall. Keep the barrel out of direct sun or paint it a dark colour, because light grows algae. If the water smells, it has gone anaerobic: drain it onto the garden and start fresh. Avoid collecting from roofs treated with moss killer, from old asbestos-cement roofs, or from lead flashing runs if the water will go anywhere near food crops.

💡 A handful of barley straw in a mesh bag slows algae growth in an open tank.

Scaling up and the rules

Once barrels feel small, the next step is a 1,000 litre IBC tote (cheap second-hand, but only ever food-grade ones) or a buried cistern of 5,000 litres and up. Traditional Portuguese and Spanish farms ran on cisterns for centuries, filled from roofs and courtyards. Larger systems may need planning consent, especially buried tanks, and using rainwater indoors (toilets, washing machine) requires a separate labelled pipe network and usually a plumber and building approval. Collecting rain for your garden is legal almost everywhere; check with your municipality before digging or plumbing indoors.

💡 An IBC in full sun turns green in weeks. Wrap it in shade cloth or build a simple box round it.

Check yourself

A 40 m² roof gets 500 mm of rain a year. Roughly how much water is that?

Area in m² times rainfall in mm gives litres: 40 × 500 = 20,000 litres, before losses.

What is a first-flush diverter for?

The first rain washes dust and droppings off the roof. The diverter catches that dirty water so cleaner water goes to the tank.

Which use is appropriate for untreated roof water?

Untreated rainwater is non-potable. It is excellent for gardens, cleaning and toilets, never for drinking without proper treatment.

Why must a rain tank never be connected to house plumbing without a separate system?

A cross-connection can push non-potable water back into the drinking supply. Indoor use needs a separate labelled network and approval.

Try it for real: Calculate and catch

Measure or estimate your roof area and find your annual rainfall
Work out your potential litres and how many barrels you would need for a dry month
Set up one barrel or bucket under a downpipe with mesh over the top
After the next rain, measure what you caught and compare it with your estimate