Remove Moss and Algae from Patios with the Washing-Up Liquid Trick

Many homeowners do not realise that neither a pressure washer nor harsh chemicals are needed to remove this slippery coating. Gardening professionals recommend a simple kitchen staple that tackles moss and algae at the root – and keeps them away for months.

Why patios keep developing algae

Moss and algae thrive in shade, cool conditions and constant damp. These are precisely the conditions found on many patios: a north-facing position, paving that dries poorly, and puddles left after every shower. Following a wet winter in particular, the surface can quickly become a green slip hazard.

This is more than an appearance issue. Anyone who slips on slimy steps risks bruises or broken bones. There is also potential damage to the surface itself: moss holds moisture against it like a sponge. Concrete and natural stone can eventually develop fine cracks, while timber can start to rot if it never dries properly.

Many homeowners instinctively reach for a pressure washer – only to find another green film appearing a few weeks later.

Why pressure washing often causes more harm than good

Garden designers have warned for years against using excessive pressure on patio paving. Although the water jet removes the visible coating, it also erodes the material's upper layer. Concrete blocks and softer natural stones develop rough microscopic areas where fresh dirt can adhere even more easily.

The joints are the second problem. This is where the most resilient moss and algae spores remain. The jet generally cleans only the middle of each slab, leaving green growth around the edges – exactly where moisture lingers longest. The result is the familiar pattern of pale centres and dark borders.

For this reason, many people turn to specialist moss and algae cleaners. However, these often contain biocides that can affect aquatic organisms and soil life. Label warnings advising users to wear protective gloves and keep products away from drains are a reminder of this. That is hardly reassuring for an area where children play and pets run about.

“A gentler approach: washing-up liquid in the right dilution loosens the coating, reaches spores in the joints and is kind to both the paving and the garden.”

The washing-up liquid trick: how this patio method works

The key to this trick is the chemistry of washing-up liquid. It contains substances known as surfactants. These compounds break down the protective, waxy outer layer of moss and algae. The plants lose moisture, dry out and die. At the same time, the soapy solution works into tiny cracks and joints where spores are hidden.

For patios, choose a mild, biodegradable washing-up liquid with no bleach, ammonia or strong perfumes. This allows the paving to be treated without unnecessarily affecting nearby borders, lawns or soil organisms.

Many amateur gardeners also use vinegar or bicarbonate of soda. These can work in isolated spots, but they have drawbacks:

• Vinegar can etch calcareous natural stone, such as limestone, and cause permanent discolouration.
• Bicarbonate of soda and washing soda often leave white residues and require plenty of water to rinse away.
• Washing-up liquid dissolves more readily, works across the whole surface and can simply be swept away and rinsed off.

Step by step: tackling the green growth with a bucket

Only a few items are needed, most of which are already available in a typical household.

What you will need

• A bucket containing around 4 litres of warm water
• 3 tablespoons of mild, biodegradable washing-up liquid
• A sturdy outdoor broom or scrubbing brush with synthetic or natural bristles
• Work gloves
• Optionally, a watering can or garden sprayer for larger areas

How homeowners can treat the patio

• Dry clean first: Sweep away leaves, soil and loose debris before starting. This ensures the solution can reach the paving surface.
• Mix the soap solution: Fill the bucket with warm water and stir in the washing-up liquid until thoroughly combined.
• Wet the area: Spread the solution evenly over the affected sections using a watering can or bucket, paying particular attention to joints and shaded corners.
• Allow time to work: Choose a period with several dry hours. During this time, the green film loosens and the moss turns brown and crumbly.
• Scrub thoroughly: Work firmly across the slabs with the broom, then sweep up and dispose of the loosened residue.
• Rinse lightly: Pour or spray clean water over the area so that no pools of soapy water run into beds or across the lawn.

For stubborn patches, repeat the process selectively after a few days. By the second treatment, firmly attached edges should come away noticeably more easily.

How to keep the patio clean for longer

Even the best cleaning method has limited effect if the surface is constantly wet. To keep moss and algae at bay over the long term, make a few simple adjustments around the patio.

Common weak points and quick solutions

Problem	Simple approach
Standing water after rain	Check the guttering and the slope; where possible, add a drainage channel or gravel strip
Permanent shade	Trim back overhanging branches and thin hedges to allow more light and air onto the surface
Lots of leaves and soil in the joints	Sweep roughly at least once a month, and more often after storms or during autumn
First green traces in spring	Apply diluted soapy solution straight away, before a thick layer can form

“Those who carry out light cleaning regularly avoid the major scrubbing job – moss and algae then never get the chance to build up in layers.”

Considerations for pets, children and sensitive plants

Compared with many specialist cleaners, a mild soap solution is considerably more benign. Even so, it is worth considering a few possible risks.

• Pets: Animals should not drink puddles containing concentrated solution. Ideally, treat the area while dogs or cats are indoors, and let them back out only once the surface has mostly dried.
• Young children: It is best not to let them run barefoot across the patio during the hours when the solution is working. Once rinsed, there is no particular risk.
• Plants: Do not allow strong soapy water to run directly into plant pots or vegetable beds. Rinse the edges carefully with clean water.

When other methods may be appropriate

For severely algae-covered and weathered surfaces, a bucket mixture may occasionally be insufficient. Homeowners can then combine methods: first clean roughly with plain water and a brush, then use the soap solution to target the spores. With highly porous stones, check the manufacturer's product information before adopting any method for regular use.

Anyone planning a new patio can account for the issue from the outset: incorporate a gentle slope away from the house, free-draining joints, lighter rather than very dark paving, and avoid dense planting directly along the edge. The faster water drains away and the better the air circulation, the fewer opportunities moss and algae have to establish themselves.

Ultimately, a safe, well-kept patio surface does not require a chemical onslaught or hours of noisy pressure washing. A bucket of warm water, a little washing-up liquid, a robust broom and a few sensible habits are enough to keep the paving dry, grippy and visually appealing for months.