How to Clean Green Algae and Black Mould from Brick, Render and Concrete in the UK Climate

How to Clean Exterior Green Algae and Black Mould for Good (Without Damaging the Surface)

If your exterior brick, render or concrete is covered in green slime or black patches, you’re not alone. The damp, mild UK climate is perfect for green algae, lichen and black mould to grow on shaded, north-facing or frequently wet surfaces. The good news is that most growths are easy to treat with the right products and a bit of know-how.

In this guide, we’ll look at why green algae and black mould appear on brick walls, rendered surfaces and concrete, how to clean them off safely, and what you can do to slow regrowth so you’re not scrubbing the same wall every year.

Why Green Algae and Black Mould Grow on Outside Walls

Algae and mould need three things to thrive: moisture, shade and a food source. Exterior surfaces in the UK often give them all three:

• Moisture: frequent rain, dripping gutters, leaking downpipes, splashback from paving, or walls that stay damp for long periods.
• Shade: north-facing elevations, high fences, trees, overhanging eaves and narrow side passages that never really dry out in the sun.
• Food: airborne dirt, dust, organic debris and pollution particles that settle on the surface of brick, render and concrete.

Green algae usually appears as a green film or slimy growth, particularly on shady, damp areas. Black mould or black spotting can be more stubborn and can work slightly deeper into some finishes, especially textured render and porous concrete.

Is It Just Cosmetic or a Bigger Problem?

On most exterior surfaces, green algae and light black staining are largely cosmetic. They make walls and paths look tired, and they can become slippery underfoot, but they don’t usually cause serious structural damage on their own.

However, persistent growth is often a sign of other issues:

• Blocked or leaking gutters and downpipes keeping walls wet for longer than they should be.
• Bad drainage or pooling water at the base of walls or on concrete surfaces.
• Walls that can’t dry out properly because of shade, poor airflow or overgrown vegetation.

So while cleaning the surface will quickly improve the appearance, it’s also worth looking at how water is getting onto the wall or slab and how quickly it can dry off afterwards.

Safety, Tools and Preparation

Before you start cleaning algae and mould from brick, render or concrete, take a few minutes to set up properly. Most treatments are straightforward, but you’re still dealing with slippery surfaces and often with chemical cleaners.

Personal protective equipment (PPE)

At a minimum, you should use:

• Gloves (suitable for chemical use if you’re using a biocidal wash or cleaner)
• Eye protection
• Old clothing or overalls
• A suitable mask if using sprays or strong cleaners

Tools and materials

• Stiff brush or deck scrub (plastic or natural fibre, not wire)
• Bucket and clean water
• Low-pressure hose or watering can for rinsing
• Exterior algae and mould cleaner/biocide suitable for masonry and concrete
• Dust sheets or plastic sheeting to protect nearby plants and delicate finishes

Always read the product label before you start. Some treatments are “apply and leave”, while others need scrubbing and rinsing. Protect plants and lawns from overspray and run-off where possible.

How to Clean Green Algae and Black Mould from Brick Walls

Brick is usually robust, but you still need to avoid unnecessarily harsh methods that can damage the surface or mortar joints.

1. Dry brush and basic clean

Start with the simple stuff:

• Use a stiff brush to remove loose growth, cobwebs and dirt from the brick surface.
• Brush from top to bottom so debris falls away from cleaned areas.
• Clear any leaves, soil build-up or moss at the base of the wall.

This makes your cleaner more effective and sometimes removes much of the problem on its own if the growth is light.

2. Apply an exterior algae and mould cleaner

Next, apply a dedicated exterior algae and mould treatment or biocidal wash suitable for brickwork:

• Check the product is safe for use on masonry and follow the dilution instructions.
• On a dry day, apply the solution evenly to the affected brick with a sprayer, brush or watering can and rose.
• Work it into the surface with a brush if recommended, paying attention to heavily stained areas and mortar joints.

Allow the solution to dwell for the time stated on the label. Some products are designed to break down growth which then gradually washes away with the weather, others are intended to be rinsed off sooner.

3. Rinse carefully (if required)

If the product needs rinsing:

• Use a low-pressure hose or watering can rather than a high-powered jet.
• Avoid blasting directly into mortar joints, weep holes or cracks, as this can drive water into the wall.
• Let the wall dry fully and check whether a second application is needed for stubborn areas.

For very heavy black staining or long-neglected walls, a second treatment is common. In most cases, the wall will continue to brighten over the next few days and weeks as dead growth is washed away by rain.

How to Clean Green Algae and Black Mould from Rendered Walls

Render (especially textured or coloured monocouche and thin-coat systems) can be more delicate than plain brick. Over-aggressive cleaning risks scarring the surface or exposing the aggregate.

1. Gentle brushing first

Use a soft to medium brush to remove loose dirt and organic material. Avoid very stiff brushes or wire brushes that could roughen the render and make it more prone to future staining.

2. Use a render-safe biocidal wash

Choose a cleaner specifically described as safe for rendered surfaces:

• Apply on a dry render surface, ideally on a dry, overcast day to prevent rapid evaporation.
• Work from the top down to minimise streaking.
• Use a low-pressure sprayer for even coverage, avoiding over-application.

Many modern biocidal washes for render are “no rinse” and continue working over time, with green and black staining fading gradually as weathering takes place. Follow the product guidance carefully; over-rinsing too soon can reduce effectiveness.

3. Avoid harsh pressure washing

High-pressure washing is rarely a good idea on render. It can:

• Strip away the surface layer and expose the aggregate.
• Drive water into the render and substrate, contributing to damp problems.
• Cause streaking or patchy, uneven areas of colour.

If heavy staining doesn’t respond to careful cleaning with suitable products, it may be time to consult a specialist, especially on insulated or proprietary render systems.

Read our guide on Softwashing, so you don't damage your walls. 

How to Clean Green Algae and Black Mould from Concrete and Paving

Concrete slabs, driveways and paths are common places for algae and mould to grow, particularly in shaded corners and under dripping gutters. As well as looking unsightly, they can become very slippery.

1. Scrub and rinse

For light growth:

• Brush the surface with a stiff broom to remove loose growth and debris.
• Apply a mixture of cleaner (or dedicated paving wash) and water as directed.
• Scrub with a deck brush, working the solution into the pores of the concrete.
• Rinse off with a hose or watering can, directing run-off away from drains if required by the product label.

2. Treat with an exterior biocide

To slow regrowth, follow up with an algae and mould treatment designed for hard surfaces:

• Apply evenly according to the instructions.
• Pay attention to heavily shaded or always-damp patches.
• Allow it to dry and work over time – many products are left on the surface.

3. Pressure washing: use with care

Pressure washers can be useful on concrete and some paving, but they’re not risk-free:

• Too much pressure can etch the surface or remove the finer cement paste, leaving a rough finish.
• Jointing sand between paving slabs can be blown out, leaving joints open to weed growth and movement.
• Using water alone kills very little – the surface may look clean but growth can quickly return.

If you do use a pressure washer, keep the lance moving, avoid getting too close, and ideally combine it with an appropriate treatment so you’re not repeating the job every few months.

Choosing the Right Cleaner: Biocide, Cleaner or Pressure Washer?

There are three broad approaches people use to tackle outdoor algae and mould: physical cleaning, chemical cleaners and biocidal treatments.

Physical cleaning

Brushing and low-pressure rinsing are the safest starting points and are often enough for light growth. They’re also a good first step before any chemical treatment, removing the worst of the dirt so products can penetrate properly.

Chemical cleaners

These are designed to remove dirt and staining quickly. Some are mildly alkaline, others contain surfactants to break down grime. They’re useful when you want fast cosmetic improvement, but they may not give long-term protection against regrowth on their own.

Biocidal washes and mould/algae treatments

These target the micro-organisms themselves – algae, mould, lichen and spores. They often work more slowly, but they help keep surfaces cleaner for longer. Look for:

• Products clearly labelled for exterior algae and mould on masonry, render and concrete.
• Guidance about whether to rinse off or leave to weather naturally.
• Compatibility with any existing coatings or sealers.

Always avoid mixing different cleaners or combining bleach with other chemicals, as this can create hazardous fumes and damage certain surfaces.

How to Prevent Algae and Mould Coming Back

No treatment can stop algae and mould forever, especially in the UK climate, but you can stretch out the time between cleans by tackling the conditions they like.

Improve drainage and fix leaks

• Repair dripping or overflowing gutters and downpipes.
• Check that paving and concrete fall away from the building and don’t hold standing water.
• Clear moss, soil and debris from the base of walls and from joints in paving.

Increase light and airflow where possible

• Trim back dense shrubs, hedges and low branches close to walls and paths.
• Open up narrow side returns where practical, for example by removing clutter stored against walls.

Consider protective treatments

On some surfaces, a breathable water-repellent treatment or suitable sealer can:

• Reduce how much water soaks into brick, render or concrete.
• Help surfaces dry out more quickly after rain.
• Make it harder for dirt and growth to stick to the surface.

Choose products that are vapour-permeable on walls, so moisture from inside the building can still escape, and always follow the manufacturer’s preparation and application instructions.

FAQs

Is green algae on walls harmful?

Green algae on exterior walls is usually more of a visual and slip hazard than a direct health risk, especially outdoors. However, it indicates that surfaces are staying damp, which is worth investigating. Removing it and dealing with the underlying moisture will improve both appearance and safety.

Can I just use bleach to clean black mould and algae outside?

Bleach can sometimes make growth look like it has disappeared, but it is not always the best or safest option. It can damage some surfaces, harm plants and doesn’t always give long-term control. Dedicated exterior algae and mould treatments are usually more effective and more suitable for masonry, render and concrete.

How often will I need to clean algae and mould from outside surfaces?

It depends on how damp and shaded the area is and which products you use. In a typical UK setting, a treated surface might stay clean for one to three years. Very shaded or constantly wet areas may need more frequent attention, while sunny, well-drained surfaces will stay cleaner for longer.

Summary

Green algae and black mould on brick, render and concrete are part of living in a damp UK climate, but they don’t have to be permanent.

• Start with brushing and basic cleaning to remove loose growth.
• Use a suitable exterior algae and mould treatment matched to the surface – brick, render or concrete.
• Rinse carefully where required and avoid overly aggressive pressure washing, especially on render and brick.
• Fix leaks, improve drainage and consider breathable protective treatments to slow regrowth.

With the right approach, you can safely clean exterior walls and hard surfaces, reduce slipperiness, and keep your property looking fresher for much longer between maintenance visits.