Rainwater from the roof is soft, free and good for vegetables, balcony flowers and potted plants. But if you just leave the butt to its own devices, by midsummer you often have green, cloudy water, a rotten smell and small larvae at the surface. To clean a rainwater butt, you need neither chemicals nor much time – just the right moment, a brush and a well-fitting lid.
A clean, covered butt also helps against the Asian tiger mosquito, which is spreading in Switzerland. Five steps show how to keep it in good shape from spring to winter.
Why a rainwater butt gets dirty
Rain washes pollen, leaves, moss, bird droppings and dust from the roof into the butt. The organic material sinks and gradually forms a layer of sludge. When it breaks down without oxygen, the water starts to smell. Light and warmth also make algae grow, especially in pale or translucent containers.
Open, standing water is also an ideal breeding site for mosquitoes. Females lay their eggs on or just above the water surface, and larvae hatch within a few days.
| Sign | Cause | Fix |
|---|---|---|
| Green, cloudy water | Algae from light and warmth | Shady spot, lid, scrub the butt |
| Rotten smell | Sludge and leaves at the bottom | Empty, remove sludge, leaf filter |
| Wriggling larvae at the surface | Mosquitoes laying eggs in open water | Lid or mesh, empty the butt |
| Slimy film on the walls | Biofilm of dirt and algae | Scrub with a brush and water |
| Dripping or blocked tap | Sludge and leaves in the outlet | Unscrew the tap and flush it |
1. Choose the right moment
The easiest times to clean the butt are in spring, before the watering season starts, and again when the water starts to smell in summer. Plan the job after a dry spell, when the level is low – you need the water for the garden anyway.
Empty the butt before cleaning by watering beds, lawn and tubs, so nothing is wasted. Rainwater from a butt is, incidentally, not drinking water.
2. Empty it and remove the sludge
- Close the inflow from the downpipe or set the rain diverter to "winter" or "closed".
- Drain the water through the tap or scoop it out with a watering can and bucket. For large butts, a small submersible pump helps.
- Remove the sludge from the bottom with a trowel, a cup or a wet vacuum.
- Small butts can be tipped out at the end. A full butt, however, should never be lifted or tipped: one litre of water weighs one kilo.
The sludge is rich in nutrients. Spread it thinly on beds or under shrubs, or add it to the compost – never down a drain.
3. Clean a rainwater butt without chemicals
For the actual cleaning, clear water and a long-handled brush are enough. Scrub the walls and bottom thoroughly until the slimy film has gone, then rinse with the hose. For stubborn biofilm, some hot water from the kitchen helps. Also clean the lid, the tap and the leaf filter in the rain diverter.
Do without cleaning products, bleach or concentrated vinegar. Their residue would end up in your watering water – on your plants, in the soil and among beneficial insects. Pour the rinse water onto the lawn or a bed, not into the street drain. The cantonal environment offices point out that in separate sewer systems rainwater from roofs and terraces flows untreated into nature, and even readily biodegradable cleaners harm waterways.
While you are at it, check the inflow: if the gutter is full of leaves, the butt soon fills with dirt again. How to clean gutters safely is covered in our guide to cleaning gutters.
4. Lid and mesh against tiger mosquitoes
The tiger mosquito prefers to lay its eggs in small water containers around the house. The Federal Office for the Environment recommends that from May to September all containers that can hold water – including rain barrels – are covered, or emptied and cleaned at least once a week.
- Lid: it should close tightly, ideally with a single opening for the inflow.
- Mesh: where no lid fits, stretch a fine insect mesh tightly over the opening and secure it with a bungee cord.
- Check: look every week to make sure lid and mesh are still tight.
5. Overflow, location and winter break
So that the butt does not overflow in a downpour and soak the house wall, it needs an overflow. Many rain diverters on the downpipe stop the inflow automatically once the butt is full. A shady spot keeps the water cool and slows algae growth.
Before the first frost, get the butt ready for winter: empty it completely, leave the tap open, switch the diverter to winter mode and cover the butt or store it upside down. Freezing water expands and can split plastic butts and taps.
Common mistakes
- Leaving it uncovered: mosquito larvae develop within days.
- Disinfecting with bleach: harms plants, soil and waterways and is unnecessary.
- Tipping a full butt: risk of injury and a broken butt.
- Leaving it full in winter: frost splits butt and tap.
- Forgetting the gutter: leaves in the inflow dirty the freshly cleaned butt straight away.
For mosquitoes that get indoors, read our guide to mosquitoes in the flat.
Frequently asked questions
How often should you clean a rainwater butt?
At least once thoroughly in spring. In summer, clean it as soon as the water smells or larvae appear. From May to September the butt should be covered or emptied every week.
What helps against algae in a water butt?
A shady spot, a lightproof lid and regular scrubbing. Chemical algaecides and bleach do not belong in watering water.
How do I get rid of mosquito larvae in a water butt?
Empty the butt onto the garden, scrub the walls and then fit a tight lid or fine mesh. Without an open water surface, mosquitoes can no longer lay eggs.
Does a water butt need emptying in winter?
Yes. Freezing water expands and can damage the butt and tap. Empty it before the first frost, leave the tap open and switch the inflow to winter mode.
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