How to Remove Algae & Moss from Your Patio – Complete Guide

If your patio has turned green and slippery, you’re not alone. Algae and moss are two of the most common problems UK homeowners face, especially in damp, shaded gardens. A green patio doesn’t just look bad — it can become a serious slip hazard, particularly after rain. Many people try quick fixes like scrubbing or pressure washing, only to see the problem return weeks later. In this guide, you’ll learn what causes green patios, how algae and moss differ from black spots, and how to remove them properly using the right methods. We’ll also show you how to prevent regrowth so your patio stays clean for longer.

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Why Patios Go Green

Green patios are caused by two main organisms: algae and moss. Although they often appear together, they behave differently and need to be understood properly to stop them coming back.

Algae form a slimy green layer called a biofilm. This develops when airborne spores land on damp surfaces and multiply rapidly. Algae thrive where there is constant moisture, little sunlight, and poor airflow — such as north-facing patios, shaded corners, or areas under trees. The layer spreads across the stone surface and creates a smooth film that becomes extremely slippery when wet.

Moss behaves differently. It tends to establish itself in joints, cracks, and uneven areas where soil and moisture collect. Over time, moss can spread underneath paving edges, sometimes lifting slabs slightly and allowing even more moisture to pool. This not only looks unsightly but can weaken jointing and cause movement in paving.

Unlike black spots (lichen), algae and moss sit closer to the surface and are easier to remove. However, they grow faster and will return quickly unless treated properly. Most importantly, green growth is a genuine slip hazard — especially on steps, slopes, and walkways.

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How to Remove Algae and Moss

There are several ways to treat algae and moss, depending on how severe the problem is. The most effective approach often combines physical removal with chemical treatment.

Physical removal (for thick moss)
Start by removing moss with a stiff brush or scraper. This immediately improves appearance and stops further moisture retention. It won’t kill spores, but it clears the bulk of growth.

Soapy water and scrubbing (for light algae)
A bucket of warm soapy water and a stiff brush works well on thin green growth. Scrub in sections and rinse thoroughly. This is safe and effective for routine cleaning, but algae usually returns quickly without further treatment.

Bleach solution (with caution)
A diluted bleach solution can kill algae and moss on contact. However, bleach is harmful to lawns and plants and may discolour joints or stone if misused. Never spray directly near borders, and rinse overspray immediately. Bleach cleans quickly but does not prevent regrowth.

Spray and leave cleaner (best long-term option)
A spray and leave cleaner penetrates the stone and kills algae and moss at the root. Rain reactivates the treatment, allowing cleaning to continue over several weeks. This method offers the longest-lasting results with minimal effort.

Pressure washing (fast but temporary)
A pressure wash removes visible growth instantly, but it also increases stone porosity. This allows algae to return faster unless followed by chemical treatment.

Best approach: combine methods
Remove thick moss manually, apply a cleaner to kill remaining growth, then use light pressure washing only if needed.

For effective algae and moss removal, see our Patio Guard product.

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Preventing Regrowth

Green patios keep coming back because conditions allow them to.

Reduce shade by trimming plants and trees where possible. Improve drainage so water doesn’t pool. Increase airflow around fencing and edges. Sweep frequently to remove debris and spores. Apply a preventative spray once or twice per year.

For long-term protection, consider sealing your patio after cleaning. A sealer reduces porosity and makes it harder for algae to re-establish.

With the right treatment and ongoing maintenance, green growth doesn’t have to be a permanent problem.