Pressure washing guide
How to remove algae from a patio
Algae turns patio surfaces green, slimy and dangerous when wet. London's damp climate and shaded gardens make algae growth almost inevitable. Here is how to remove it effectively and stop it coming back.
Algae is one of the most common complaints from London homeowners about their patios. It colonises paving surfaces rapidly in damp and shaded conditions, creating a thin green or black film that becomes extremely slippery when wet. Beyond the slip hazard, algae left untreated penetrates porous surfaces and causes long-term staining that becomes harder and harder to shift. The good news is that algae responds well to several cleaning methods, from simple pressure washing through to specialist biocidal treatment for particularly stubborn or established growth.
4 to 6 weeks
algae can visibly colonise paving in London conditions following a sustained wet period
Pressure washing
the fastest method for algae removal on hard surfaces but does not prevent spores from re-establishing
Biocidal treatment
kills algae at root level and prevents regrowth for months, lasting far longer than pressure washing alone
Why London patios are particularly prone to algae
London's annual rainfall of around 600mm, mild winter temperatures and the prevalence of shaded gardens created by surrounding buildings, walls and fencing combine to create near-perfect conditions for algae year-round. Algae needs moisture and minimal light to thrive, and London patios typically have both in abundance. Shaded north-facing patios can develop visible algae coverage within weeks of a wet period, while even south-facing patios in London's built-up environment will sustain algae growth through autumn and winter.
Removal methodsMethods for removing algae from a patio
Pressure washing
Fast and effective for removing surface algae on hard paving. Use a 25 or 40-degree nozzle at medium pressure. Works immediately but does not kill spores, so algae returns faster than with biocidal treatment.
Biocidal patio cleaner
Applied directly to the algae, allowed to dwell for 30 to 45 minutes, then rinsed off. Kills algae at root level and prevents regrowth for several months. Best used on its own or combined with pressure washing.
Soft washing
Professional method combining low-pressure water and specialist biocidal solutions. Kills algae at the source, extends the interval between cleans to 12 to 18 months and is safer for delicate surfaces like sandstone.
Manual scrubbing
Stiff brush with warm soapy water or a dilute biodegradable cleaner. Suitable for small areas and light algae. Labour-intensive on larger patios and less effective on established growth.
Which method works for your patio surface
PreventionHow to stop algae coming back
Removing algae is only part of the solution. Without addressing the conditions that allow it to thrive, it will return within weeks or months depending on how shaded and damp the patio is. The following measures all help to slow the return of algae growth after cleaning.
Seal the patio
Applying a quality patio sealant after cleaning creates a barrier that resists moisture penetration and makes future algae less able to take hold. This can extend the clean period from months to 2 to 3 years on most surfaces.
Improve drainage
Standing water feeds algae growth. Improving the slope of the patio or clearing drainage channels keeps the surface drier for longer after rain.
Increase sunlight exposure
Trimming back overhanging plants, hedges and fences that shade the patio allows more direct sunlight, which naturally inhibits algae growth.
Annual biocidal treatment
Applying a dilute biocidal spray annually prevents spores from establishing. This is a low-effort, low-cost maintenance step that significantly extends the intervals between deep cleans.
Professional pressure washing London
Algae removal and patio cleaning across London
Cloud Nine removes algae from patios across London using the right method for each surface type. We use biodegradable biocidal treatments and commercial pressure washing to deliver a thorough clean that lasts. Contact us for a free quote.
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Pressure washing help and guidance
Everything you need to know about pressure washing for London homes and businesses, from surface types to costs and what to expect from a professional clean.
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