Clean Patio Slabs with Soda Crystals and Washing-Up Liquid

Many people instinctively reach for a pressure washer or harsh products from the cleaning cupboard. Yet this build-up can often be removed with a straightforward combination of two inexpensive household products – without noise, vinegar or damaging the joints.

Why patios look so bad after winter

After months of rain, frost and limited sunshine, patio slabs quickly develop a mixture of algae, moss, lichens and surface grime. Rust marks from chair legs, plant pots or fire bowls can add to the problem. What barely registers during the dull winter months becomes painfully obvious in the first spring light.

The usual response is to spend hours blasting the area with a pressure washer or adding powerful cleaners to water. Both approaches have drawbacks. A forceful jet of water washes out jointing material, roughens the stone surface and can break pieces away from older slabs. Aggressive cleaning products attack joint mortar, discolour natural stone and can run into flowerbeds and lawns with the wastewater.

Many people also swear by vinegar because it is considered “natural”. However, it is still an acid. On calcareous natural stone, it can etch the surface, leave dull patches and alter the structure of the stone over time. If larger quantities reach the soil, they can put stress on micro-organisms and plant roots.

“Anyone who uses a pressure washer and harsh chemicals every year makes their patio more vulnerable and prone to getting dirty in the long term.”

The simple two-product method for clean patio slabs

A British DIY expert demonstrates a much gentler routine. Rather than using vinegar, strong chlorine cleaner or specialist chemicals, he relies on two cupboard staples: soda crystals and washing-up liquid.

What soda crystals can do

Soda crystals, also known as washing soda, have traditionally been used for laundry. They break down grease, soften dried-on dirt and help lift organic deposits from hard surfaces. Unlike drain cleaner based on caustic soda, they are considerably milder and suitable for typical patio build-up when correctly dosed.

They are stocked in the laundry or cleaning aisle of many supermarkets, usually as a powder or coarse crystals. Anyone with sensitive skin should ideally wear thin gloves when handling them, as soda has a mild degreasing effect on skin.

Why ordinary washing-up liquid is enough

Washing-up liquid helps loosen grease and oil residues, such as those left by barbecues, soot or traffic dust. It also helps spread the soda solution evenly. A basic colourless product without bleaching additives is sufficient. Those wishing to minimise the impact on their garden can choose an eco-friendly version.

Step by step: how to clean your patio slabs

The method is deliberately simple. You need only a few items:

• Soda crystals (washing soda, not drain cleaner)
• Ordinary washing-up liquid
• A stiff outdoor broom or scrubbing brush
• A small trowel or joint scraper
• A garden hose or watering can filled with water
• Optional: a bucket and warm water

Preparation: remove loose dirt

Start by clearing away leaves, twigs and loose debris. Weeds and moss can be removed from the joints using a joint scraper or narrow trowel. Then sweep the entire area thoroughly. The cleaner this stage is, the better the mixture can work.

Next, wet the patio using a hose or watering can. The slabs should be damp but not standing in water. This moisture helps the soda crystals dissolve directly on the surface.

Apply the mixture and work it in

• Scatter soda crystals thinly over the damp surface. There is no need for a thick layer – an even covering is enough.
• Drizzle washing-up liquid over the slabs in zigzags, as if soaping up a sink.
• Scrub firmly with the broom. Work in strips so that every section receives some of the mixture.

If you prefer working with a bucket, dissolve two to three tablespoons of soda crystals in hot water, add a squirt of washing-up liquid and spread the solution across the area with a scrubbing brush.

“The trick does not rely on force, but on time: the mixture needs time to soften algae and dirt.”

Leave it to work rather than rinsing straight away

In the example shown, the mixture remains on the surface for up to 48 hours. Ideally, conditions should stay dry during that period, as even a brief shower dilutes the effect. If you are concerned about sensitive plants, soak beds and lawns with clean water beforehand so the soil is already saturated.

Once the waiting time has passed, a thorough rinse with the hose is enough. On many patios, the slabs will look noticeably lighter afterwards, while the green coating will have disappeared or been significantly reduced. Stubborn areas can be treated a second time.

Cleaning patio slabs safely: dosage, materials and common mistakes

How much soda should you use?

Because soda is alkaline, thick layers are unnecessary. For normally dirty slabs, a thin, even scattering or a slightly milky bucket solution is enough. Using too much is not only wasteful; it can also leave streaks on sensitive stone.

Which materials need attention?

• Concrete slabs: Usually unproblematic, particularly if they have been laid for several years.
• Clay pavers and bricks: Also generally suitable; rinse them well so no residue remains in their pores.
• Light natural stone (such as limestone or sandstone): Test first in an inconspicuous spot and use a soft brush.
• Polished slabs or delicate surfaces: Scrub especially carefully to avoid damaging the glossy layer.

As a general rule, children and pets should not walk on the area while it is damp with the mixture. Once it has been thoroughly rinsed, the patio can be used as normal again.

What can undo the benefits of this method

Anyone who turns immediately to bleach, acidic limescale removers or metal brushes after gentle cleaning risks causing fresh damage. Bleach can fade stones and attack joints. Very stiff brushes or wire brushes leave scratches. These rough areas then attract even more dirt and algae in future, creating a vicious circle.

A pressure washer can still be useful, for example where driveways are covered in thick soil. On older patios with loose slabs, however, it blows away jointing material and can cause stones to shift. For ordinary spring cleaning, the combination of soda crystals, washing-up liquid, water and a little elbow grease is often entirely sufficient.

How to keep your patio clean for longer

Following a thorough initial clean, the aim is to preserve the fresh appearance for as long as possible. That does not mean hours of cleaning, only a few small routines:

• Sweep briefly every one to two weeks to remove leaves and soil.
• Avoid puddles by placing saucers or pot feet beneath plant pots.
• Treat barbecue grease or drink spills immediately with warm water and a little washing-up liquid.
• Give the area a light clean with a diluted soda-crystal and washing-up-liquid solution in autumn, before winter arrives.

If green build-up repeatedly appears in shaded, damp areas, consider thinning out plants slightly or moving pots. Every extra ray of sunshine helps slow algae growth.

What the terms mean and where the limits are

Soda, bicarbonate of soda and caustic soda are often confused. The product intended for patio cleaning is soda crystals, or washing soda. Bicarbonate of soda is milder and more suited to the kitchen and household, while caustic soda is found in powerful drain cleaners and is far too aggressive for outdoor stone surfaces.

The method described has clear advantages: inexpensive products, little equipment and a moderate impact on the garden and soil. However, it is not a substitute for professional restoration. Heavily weathered slabs, crumbling joints or deeply set rust and oil stains can only be improved to a limited extent. In these cases, it is worth seeking professional advice before spending too much time on repeated cleaning attempts.

For a typical family patio with a light covering of moss, the combination of soda crystals and washing-up liquid is usually more than adequate. Once you have seen the grey film lift away, you may be more likely to pick up a scrubbing brush than a pressure washer next spring – protecting not only the slabs, but also your nerves, your neighbours and the garden soil.