How To Remove Algae And Black Spots From A Patio

A patio that was bright in summer can look tired by autumn. Green slime appears in the shade, and small dark dots spread across the slabs. If you want to remove black spots from patio slabs and clear algae at the same time, the right method depends on what is actually growing on the surface.

This guide explains what black spots and algae are, how black spot removal works, how to remove black spots on patio surfaces step by step, which homemade options are worth trying, and how to reduce the chance of regrowth. It is general guidance. Always follow the exact product label and safety data sheet (SDS) for the cleaner you choose.

Black Spots vs Algae: What Are You Dealing With?

Before you treat anything, identify the problem. They look different and behave differently.

In the UK's damp climate, algae problems tend to be most noticeable through the wetter months, roughly October to April. That makes autumn a sensible time to clean and plan your protection.

What Causes Black Spots On Patio Slabs?

Black spots on patio slabs are most commonly black lichen. Lichen is a partnership between a fungus and an alga. Unlike loose surface algae, it grows into the pores of the paving, which is why a quick wash often leaves the marks behind.

Several conditions make a black spot on patio surfaces more likely:

• Shade and poor airflow, so slabs stay damp for longer

• Poor drainage, where water sits on or under the paving

• Leaf litter and organic debris that feed growth

• Porous paving, such as natural stone and some concrete, which holds moisture

• Furniture and planters that trap damp underneath

Some dark staining is mould or mildew rather than lichen, but the approach is similar: treat the growth, rinse, and then reduce the moisture that fed it.

Does A Pressure Washer Remove Black Spots?

Not reliably. A pressure washer is good at clearing loose dirt and soft algae, but black lichen is anchored in the stone. Pointing a narrow, high-pressure jet straight at the spots can also damage the paving surface or wash out jointing material, and some specialists warn it can leave the underlying growth in place so the spots return.

A better approach is to use a pressure washer on a wide, low setting for general cleaning and rinsing, and to treat the black spots with a dedicated treatment first.

How To Remove Black Spots From Patio Slabs: What You Need

For a typical patio clean you will need:

• A suitable black spot and algae remover (see the products below)

• A watering can or low-pressure sprayer

• A stiff outdoor brush

• Gloves, goggles, long sleeves and waterproof footwear

• A hose or low-setting pressure washer for rinsing

• Clean water to pre-wet surrounding plants and lawn

Our Black Spot Remover Options

Extreme Clean Blackspot Remover is a concentrated sodium hypochlorite formula (14-15%) made for black lichen and algae on patios, driveways and stone. It can be diluted for routine cleaning or heavier staining, or used neat on stubborn spots, and its product page lists concrete, Indian sandstone, limestone and block paving among suitable surfaces, subject to a test patch.

Extreme High-Strength Moss & Algae Remover is a black spot and algae remover aimed at moss, green film and surface lichen on paving, driveways and paths. It suits patios where green growth is the main problem alongside a few dark spots.

Extreme Clean Patio Cleaner (Sodium Hypochlorite) is another high-strength option for dirt, algae and organic growth. You can compare the whole range on our patio cleaner collection.

These are different formulations, not interchangeable general cleaners. Check each product's surface list, dilution and contact time before you buy.

Removing Black Spots On Patio Surfaces: Step By Step

This method follows the directions on our Blackspot Remover product page. Always check the label on the product you actually use.

Step 1: Pick the right day

Choose a dry day. The product guidance says to avoid application if rain is forecast within 6 hours. Cooler, overcast conditions also help stop the treatment drying on the surface too quickly.

Step 2: Prepare the area

Sweep off leaves, soil and loose debris. Remove weeds from joints. Pre-wet surrounding grass, plants and borders with clean water so any splash or runoff does less harm. Move furniture and planters away.

Step 3: Do a test patch

Treat a small, inconspicuous area first and check for any change in colour or finish. This matters most on natural stone, which is more porous and varies from one slab to the next. Avoid using sodium hypochlorite products on polished stone, decorative metals or painted surfaces.

Step 4: Dilute to the right strength

For Extreme Clean Blackspot Remover, the product page gives three options:

• 1:6 (one part product to six parts water) for routine cleaning

• 1:3 for heavier staining

• Neat for the most stubborn black lichen

Start at the weakest strength that suits the job and increase only if needed.

Step 5: Apply and keep the surface wet

Apply evenly with a watering can or low-pressure sprayer until the surface is saturated. Leave the treatment to work for the dwell time given on the label, and do not let it dry out. Lightly re-mist if needed. On deep-set spots, light brushing helps lift embedded growth. As the lichen breaks down, the spots may turn a brownish colour before rinsing.

Step 6: Rinse thoroughly

Rinse with a garden hose or a pressure washer on a wide, low setting. Rinse your tools and any splashes on nearby surfaces too.

Step 7: Repeat if necessary

Old or heavily established lichen may need a second application the following day. No product can guarantee black spot removal in a single pass, especially on porous or long-neglected paving.

Safety essentials

• Wear gloves, goggles, long sleeves and waterproof footwear.

• Keep children and pets away until the area is rinsed and fully dry.

• Never mix cleaners. Combining a chlorine-based product with acids, including vinegar, can release toxic chlorine gas.

• Keep runoff away from ponds, water features and watercourses.

Homemade Patio Black Spot Remover: Does It Work?

If you want to try a homemade patio black spot remover first, the most common options are a baking soda and white vinegar paste, or a solution of biological washing powder. Spread the paste or solution over the spots, scrub with a stiff brush, leave it briefly, and rinse well.

These methods can lift light surface marks mainly through scrubbing and mild chemical action. They are less likely to kill established lichen, which grows into the stone. Vinegar in particular may fade light algae but does not reach the roots of black lichen.

A few cautions:

• Acidic cleaners such as vinegar can etch limestone and some other natural stones, so test a small area first.

• Household bleach may lighten the spots temporarily, but it does not always kill lichen at the root and can leave uneven colouring.

• Never combine bleach with vinegar or any acid.

For deep black spots, a dedicated black spot remover is usually the more dependable route.

How To Remove Black Spots On Patio Types: Surface Notes

If you are unsure about your paving type or any previous treatment, send us the details through our contact page before choosing a cleaner.

After Cleaning: Seal The Patio To Reduce Regrowth

Once the patio is clean and completely dry, sealing helps limit how much moisture soaks into the paving, which is one of the conditions lichen and algae rely on. A sealer does not make a patio immune, but it makes regular maintenance easier.

Key points when sealing a freshly cleaned patio:

• Wait until the surface is fully dry. Trapped moisture can cause a milky or cloudy finish. If that happens on sandstone, see our guide on how to fix milky Indian sandstone after sealing.

• Match the sealer to the surface. Sandstone, block paving and concrete are not interchangeable. Our surface-by-surface sealer guide explains the differences.

• Choose a stone-appropriate product for natural paving. Our guide to the best Indian sandstone and stone sealers covers what to check.

You can browse our natural and sandstone sealers, block paving sealers and concrete sealers. Check the label for temperature and weather limits, as sealing needs a dry spell.

How To Stop Black Spots Coming Back

No treatment gives permanent protection, but these habits slow regrowth:

1. Sweep regularly to remove leaves and organic debris.

2. Improve drainage so water does not sit on or under the slabs.

3. Increase light and airflow. Trim back overhanging branches and cut back shrubs where practical.

4. Move planters and furniture now and then so damp does not build up underneath.

5. Re-treat early. Small isolated dark dots are easier to remove than widespread, established lichen.

6. Seal after cleaning with a sealer suited to your paving.

When To Call A Professional

Consider specialist help if the lichen is widespread and deeply embedded, the paving is fragile or heavily weathered, you are working near water features or at height, or you are unsure which cleaner is safe for your stone.

Ready To Clean Your Patio?

Compare our Blackspot Remover, Moss & Algae Remover and the full patio cleaner range. If you are not sure which one suits your paving, contact our team with the surface type and we will help you check suitability.

This guide is general information. Always follow the specific product label, SDS and manufacturer instructions, and carry out a test patch before treating a whole surface.