Pressure washing or algae treatment: which do you need?
When a patio turns green and slippery, should you use a pressure washer or apply an algae treatment? We are often asked this. The methods work differently, and the best result often comes from combining them. This guide explains when to use each one, its strengths and its limitations.
How the methods differ
Pressure washing removes visible dirt and growth immediately, leaving paving looking clean within minutes. An algae treatment acts on the living growth over time, slowing its return. It does not give the same immediate visual result.
Pressure washing gives a quick improvement that may need repeating sooner. Algae treatment takes longer to show its full effect but can keep the paving clear for longer. The right choice depends on your priorities and the surface.
How a pressure washer works
A pressure washer removes growth mechanically. The water jet lifts algae, moss and dirt from the surface, giving an immediate result without cleaning chemicals.
Advantages of pressure washing
- Fast and visible. You see the difference as you work, and a patio can be clean in an afternoon.
- No cleaning chemicals. It uses water under pressure, so there is no chemical dosage to measure.
- Good for heavy overgrowth. It removes thick, visible growth quickly.
Disadvantages of pressure washing
- Jointing sand is removed. Too much pressure blasts the sand out of the joints, which then need to be refilled, otherwise the joints remain open and collect water.
- Roughened paving. Excessive pressure can roughen soft or porous paving, making it easier for algae to take hold next time.
- It comes back. Pressure washing removes visible growth, but growth within pores and joints can return within a season or two.
Use moderate pressure, keep the nozzle at a consistent distance and a shallow angle to the paving, then top up the jointing sand. For more on the technique and cost, read our guide to paving cleaning prices and methods.
How an algae treatment works
A liquid algae remover is applied at low pressure so it can penetrate the surface and act on the living growth. The algae dry out and die, then rain, wind and foot traffic gradually remove the dead material. This treats more than the visible surface.
Advantages of algae treatment
- Durable results. Treating the living growth helps the patio stay clean for longer than mechanical cleaning alone.
- Gentle on the surface. Low-pressure application avoids the forceful water jet that can roughen paving or dislodge jointing sand.
- Slows new growth. Many treatments leave an effect that keeps the surface clean for some time to come.
Limitations of algae treatment
- Time to work. The result does not appear overnight. Most algae disappear within a few weeks, while stubborn lichen can take several months to disappear, and in some cases up to a year.
- It must be approved. In Denmark, anti-algae products must be approved by the Danish Environmental Protection Agency, and you should avoid products containing copper and chlorine.
- It does not immediately remove a thick layer. If the growth is really heavy, it will still look green until the dead growth is washed away.
Most anti-algae products on the market are based on quaternary ammonium compounds, which break down biological growth. Choose a Danish Environmental Protection Agency-approved and preferably biodegradable product, follow the dosage on the container, and allow the working time indicated. If you want to know more about the growth itself and why it keeps returning, you can read our guide to algae treatment for paving and patios.
Pressure washing and algae treatment compared
Here the two methods are compared side by side, so you can quickly see what each is suitable for.
| Pressure washer | Algae treatment | |
|---|---|---|
| Effect | Immediate on visible growth | Over weeks and months, acting on the growth |
| How long it lasts | Short, must be repeated | Long, inhibits new growth |
| Risk to surface | Can roughen paving and dislodge jointing sand | Gentle, no harsh jet |
| Effort | Hard physical work now | Low, but requires patience |
| Best for | Thick growth on robust paving | Prevention and durable results |
The methods complement each other: one removes visible growth, while the other slows its return. This is why combining them often works best.
The answer is often both
Gentle cleaning first removes the thick, visible growth. An algae treatment then acts on what remains and slows regrowth. This gives you a clean surface straight away and a longer period before it turns green again.
Clean first so the product can reach the surface beneath the build-up. Light green growth may need only an algae treatment; heavy growth usually benefits from cleaning beforehand. The aim is a clean surface that stays clear for longer.
Which method suits your surface?
The choice depends not only on how green the patio is, but largely on what it's made of. The wrong choice can result in a damaged surface.
Paving and concrete
Robust concrete slabs and paving blocks can be pressure-washed at moderate pressure, with jointing sand topped up afterwards. Cleaning followed by algae treatment often works well. Older, soft or porous paving needs more care because the jet can roughen it.
Soft and porous surfaces
Natural stone, clay paving and other soft or porous materials need a gentle approach. An algae treatment may be suitable, sometimes combined with brown soft soap and brushing. Avoid excessive pressure that roughens the surface and makes it easier for growth to return.
Wooden decking
Wood needs a gentler approach. Excessive pressure raises wood fibres and can leave uneven, pale streaks. Use low pressure or a gentle algae remover intended for wood, then oil it for protection. If the deck needs sanding as well, see our guide to sanding balconies and wooden decks.
When is it a job for professionals?
You can maintain a small patio with a brush, soap and some patience. Professional help is useful for large areas, recurring growth, or older and delicate surfaces that are easily damaged.
A professional chooses the method for your surface and combines cleaning and algae treatment to protect the joints and achieve a lasting result. This is how we approach professional paving cleaning and algae treatment. You get a fixed price before we start, so there are no surprises, and we always get back to you within 24 hours, so you quickly know what the job will cost for your specific patio. If you would like a no-obligation quote, you are also welcome to call 20 98 88 11.
Frequently asked questions
Which is better for green paving: pressure washing or algae treatment?
Pressure washing gives an immediate result, but growth left in pores and joints can return. Algae treatment acts more slowly and keeps the surface clear for longer. Gentle cleaning followed by an algae treatment often gives the best combined result.
Can a pressure washer damage paving?
Yes. Excessive pressure can dislodge jointing sand and roughen soft or porous paving, making it easier for algae to take hold. Use moderate pressure, keep a consistent distance and top up the sand afterwards. Wood and natural stone need particular care and may be unsuitable for pressure washing.
How long does an algae treatment take to work?
An anti-algae treatment works over time. The result does not appear overnight, but develops over weeks and months as rain and wind wash away the dead growth. Most algae disappear within a few weeks, while stubborn lichen can take several months to disappear, and in some cases up to a year.
Can I combine pressure washing and algae treatment, and in what order?
Yes. Clean first to remove heavy growth and let the surface dry. Then apply the algae treatment so it can reach the clean surface and act on remaining growth. This gives an immediate improvement and helps delay its return.
What does patio cleaning and algae treatment cost?
The price depends on the area, how overgrown the surface is, and what type of surface it is. We provide a fixed price before we start, so you know the amount in advance and avoid surprises. Call 20 98 88 11 or request a no-obligation quote, and we will get back to you within 24 hours with the price for your specific patio.