Algae treatment for paving and patios
When spring arrives, we often hear from homeowners with green paving and slippery patios. The grey-green growth is unsightly, can make surfaces slippery and keeps returning without proper treatment. This guide explains algae treatment for paving and patios: what grows, why it appears and which methods work over the longer term.
What is growing on the paving?
Green paving rarely has algae alone. It usually carries a mix of algae, moss and lichen, each suited to slightly different conditions. Knowing the difference helps explain where the problem is worst and how to treat it.
- Algae form the thin green or black layer on the surface. They grow quickly in damp conditions and make paving slippery.
- Moss consists of small green cushions that particularly settle in joints and edges. Moss loves moisture and shade and retains water exceptionally well once established.
- Lichen forms stubborn grey, yellow or white patches with a crust-like surface. It can thrive in sunlight and is the hardest of the three to remove.
Common to all three is their need for moisture and light to grow. Therefore, both treatment and prevention fundamentally involve making conditions less damp.
Why does it keep coming back?
Algae, moss and lichen are living organisms that can spread and settle on outdoor surfaces. Damp, shaded spots are often worst affected, including north- and east-facing areas, beneath trees and shrubs, along hedges and on patios that see little use over winter.
Patios often develop growth quickly because water can drain slowly and buildings, fences and plants provide shade. Annual treatment is therefore common. If cleaning removes only the visible layer while growth remains in the joints and pores, the surface may turn green again within a season or two.
Methods for algae treatment: how they work
There are several paths to a clean patio, and they each work differently. Some provide immediate results, while others work over time but last longer. Here are the most common ones.
Mechanical cleaning: brushing, boiling water and pressure washing
The simplest method is to remove the growth physically. Brown soft soap in lukewarm water and a stiff brush work well on light green growth while being gentle on the joints. Boiling water poured onto moss and algae kills some of it immediately without cleaning chemicals, but is practical only for small patches.
A pressure washer removes visible growth quickly. Excessive pressure can dislodge jointing sand and roughen softer paving, making it easier for algae to take hold next time. Use moderate pressure, keep the nozzle at a consistent distance and top up the jointing sand afterwards.
Common to mechanical methods is that they remove what you can see but don't tackle the spores embedded in the surface. Therefore, they are good for maintenance, but you should expect to repeat them.
Chemical treatment with an algae remover
A liquid algae remover is usually applied at low pressure so it can penetrate the surface. It acts on the algae, causing the growth to dry out and die. This treats the living growth as well as the visible surface.
However, it requires patience. The result isn't immediate but develops over weeks and months as rain, wind, and wear wash away the dead growth. Most algae disappear within a few weeks, while stubborn lichen can take several months to detach, and in some cases, up to a year. Many products also leave a residual effect that keeps the surface clean for some time to come.
Most algicides on the market are based on quaternary ammonium compounds, which break down biological growth. In Denmark, algae control products must be approved by the Danish Environmental Protection Agency, and one should avoid products with copper and chlorine, where the risk of harm to the environment and material is often greater than the benefit. There are biodegradable products where a large part of the product is broken down by natural bacteria within the first few days. Always follow the dosage on the container and allow the specified working time.
Combined cleaning and algae treatment
The method that yields the best and most durable results is to combine the two: first a gentle cleaning to remove the thick layer, followed by an algae treatment on top to inhibit new growth. This way, you get both a clean surface here and now and a longer period before it returns. This is exactly how we work when we perform professional paving cleaning and algae treatment, using a method suited to your paving and a follow-up treatment that protects the joints.
Methods compared
The four approaches are compared below.
| Method | Effect | How long it lasts | Effort | Best for |
|---|---|---|---|---|
| Brown soft soap and brush | Immediate on visible growth | Short, needs repetition | Hard physical work | Light green growth on small areas |
| Boiling water | Kills some immediately | Short, temporary | Low, but only for small areas | Spots, joints, steps |
| Pressure washer | Fast and visible | Short, can damage joints | Moderate, requires care | Robust paving with heavy growth |
| Algae remover | Over weeks and months | Long, inhibits new growth | Low, but requires patience | Entire patio, prevention |
For more on the cost of each method and the factors that affect it, read our guide to paving cleaning prices and methods. For help with the growth itself, read how to remove lichen and algae from paving.
How to prevent it from coming back
Once your patio is clean, there’s a lot you can do yourself to keep algae at bay. It's all about removing what they thrive on: moisture, shade, and nutrients.
- Ensure proper drainage. Standing water after rain encourages algae. Check that the paving slopes towards the drains and that water can run away.
- Prune bushes and branches. More sun and air for the paving means it dries out faster, making it harder for both algae and moss to establish themselves.
- Sweep away leaves and seeds regularly. Clearing dead leaves and other organic debris keeps the surface easier to maintain.
- Seal after cleaning. Sealing reduces open surface pores so paving absorbs less moisture and algae find it harder to take hold. This helps extend the time between treatments.
When is the best time of year to treat the patio?
The season matters more than most people think. Algae treatment works best when the temperature is above approximately 10 degrees Celsius, allowing the product to penetrate and work before it washes away. In practice, treatment can be done from about March to November.
Early spring is often a good time to treat the patio, getting ahead of seasonal growth so you can enjoy it in summer. Autumn offers another opportunity to clean up before winter damp sets in. Choose mild, relatively dry weather so the product has time to work.
Safety while you work
Take care when using algae remover or boiling water. Protect plants you want to keep and avoid rinsing the product into beds or onto lawns. Check where drains lead, and keep pets indoors during the work and immediately afterwards. Use a product approved by the Danish Environmental Protection Agency, preferably biodegradable, and follow its instructions.
Clean it yourself or hire a professional?
You can maintain a small patio with a brush, soap and some patience. Professional help makes sense for large areas, recurring growth, or old and delicate paving that is easily damaged by harsh cleaning.
Professional treatment combines a cleaning method suited to your paving with a follow-up treatment to slow new growth without damaging the joints. It can mean less work for you and a longer-lasting result. We provide a fixed price with no surprises, and we always get back to you within 24 hours, so you quickly know the cost of the job for your specific patio.
Cleaning paving on patios and driveways may qualify for Denmark's household services deduction. In 2026, eligible adults can claim up to 18.300 kr in qualifying labour costs, giving a tax saving of approximately 26% of the amount claimed. Materials and travel do not qualify, and payment and documentation conditions apply. See our tax deduction information and check the current Danish Tax Agency guidance for your work.
Cleaning a wooden deck needs a different approach, as wood cannot withstand the same treatment as paving. We cover this in our guide to sanding balconies and wooden decks, and you can also read our general guide to maintaining your balcony and deck.
Frequently asked questions
What is the difference between algae, moss, and lichen?
Algae form a thin green or black layer on the surface. Moss grows in small green cushions, especially in joints and along edges. Lichen forms stubborn grey or yellow patches. Algae and moss thrive in damp shade; lichen can also grow in sunlight and is harder to remove. All three need moisture.
How long does an algae remover take to work?
An algae remover works over time. Results develop over weeks and months as rain and wind wash away dead growth. Most algae disappear within a few weeks, while stubborn lichen can take several months to come away, sometimes up to a year.
When should you treat your patio for algae during the year?
Best when the temperature is above approximately 10 degrees Celsius, which in practice is from March to November. Early spring is often the most obvious, as you'll then have a clean patio all summer and get ahead of the season's growth. Autumn is the other good window. Choose a period with mild and relatively dry weather.
Should a patio be treated for algae every year?
Often, yes. Patios can drain slowly and tend to have more shade, so algae develop quickly. Cleaning, algae treatment and sealing can extend the time between treatments, but most patios benefit from an annual inspection.
Can I claim tax relief for algae removal and paving cleaning?
The Danish Tax Agency lists cleaning paving on patios, driveways and similar areas as an eligible household service in 2026. This is not a blanket allowance for every algae treatment. Eligible adults may claim up to 18.300 kr in qualifying labour costs, with a tax saving of approximately 26%, provided the payment, documentation and other conditions are met.