Quick answer: First identify your slab material, since limestone and some natural stone can be etched by acidic cleaners like vinegar. Sweep away loose debris, then wash with warm water and a mild detergent for routine grime. For algae and moss, a diluted patio cleaning solution or, on suitable materials, a vinegar and water mix works well. Use a pressure washer carefully and at a safe distance, since too much pressure can scar softer stone or blast out pointing.
A patio covered in green algae or dark moss isn’t just unsightly. It’s genuinely slippery underfoot, especially in the damp conditions the UK provides for a good chunk of the year. Cleaning it properly restores both the look and the safety of your outdoor space, but the right method depends heavily on what your slabs are actually made of.
Know Your Slab Material First
Around 90% of getting this right comes down to correctly identifying your material before choosing a cleaning method, since the wrong approach can leave permanent marks or damage.
- Concrete: durable but absorbs oil and grease readily.
- Sandstone: naturally porous, stains quickly, and holds moisture.
- Limestone: smooth and elegant, but very sensitive to acidic cleaners, which can cause etching and permanent colour loss.
- Porcelain: non-porous and easy to maintain, though it can get slippery when wet.
- Granite: tough and stable, resistant to most cleaning methods.
Limestone combined with an acidic cleaner is a common mistake that causes instant etching and colour loss, while sandstone subjected to too much pressure-washer force can end up scratched and visibly worn. Always test any new cleaning method on a small, discreet corner first.
What You Will Need
- A stiff outdoor broom
- A bucket, hose, and mild detergent
- White vinegar (for suitable materials only, not limestone)
- A scrubbing brush or weeding knife
- Plastic sheeting to protect nearby plants and lawn, if using chemical treatments
- A pressure washer, ideally with a rotating surface cleaner attachment
Step 1: Prepare the Area
- Move furniture, pots, and the barbecue off the patio entirely.
- Sweep thoroughly with a stiff broom to remove loose debris, leaves, and grit. This alone lifts a surprising amount of surface dirt and stops you grinding it in during the next steps.
- Remove any weeds growing between the slabs by hand or with a weeding knife.
- If pointing between slabs is cracked, loose, or missing, address that before or after cleaning, rather than blasting straight over it with a pressure washer, which can make the damage worse.
- Cover nearby borders, plants, and lawn areas with plastic sheeting, particularly if you plan to use any chemical treatment, since products like sodium hypochlorite will scorch foliage on contact.
Method 1: Soap and Water (Routine Maintenance)
This is the safest method for all patio types and is ideal for light dirt and regular monthly upkeep.
- Pre-wet the slabs with clean water to help reduce the chance of streaking or new staining.
- Mix a mild detergent solution in a bucket and apply it across the surface with a brush.
- Scrub in sections, working from the highest part of the patio downward.
- Rinse thoroughly with clean water before the soapy water has a chance to dry on the slabs.
This method won’t shift black spot lichen or deep, ingrained algae, but it’s ideal for keeping a patio looking fresh between deeper cleans.
Method 2: White Vinegar Solution (Algae and Stains)
A stronger DIY option for tackling light algae and stubborn surface staining on suitable materials.
- Mix white vinegar and water in a 50:50 ratio.
- Apply to the affected area and leave it to work for a few minutes.
- Scrub with a stiff brush, then rinse thoroughly.
- Never use vinegar on limestone or other acid-sensitive natural stone, since it will etch and permanently dull the surface.
- Avoid overspray onto nearby lawns or borders, since vinegar’s acidity will damage living plants. Rinse immediately if any overspray occurs.
Method 3: Pressure Washing (For Heavier Build-Up)
A pressure washer tackles ingrained dirt and moss effectively, but it carries real risk of damage if used carelessly.
- Use a fan-tip nozzle rather than a narrow jet, and keep the nozzle a reasonable distance from the surface, moving it steadily rather than holding it in one spot.
- A rotating surface cleaner attachment gives a more even result and reduces the risk of visible streaking or gouging compared with a handheld wand.
- Go easier on softer, more porous stone like sandstone, where excessive pressure can scar the surface, and be cautious around pointing, since high pressure can blast it out of the joints.
- Wear appropriate protective equipment, since pressure washers can cause high-pressure injection injuries if misused. The Health and Safety Executive publishes guidance on the safe use of water jetting equipment, worth reading if you’re new to using one.
Why Moss and Algae Keep Coming Back
Green algae, moss, and lichen thrive in the UK’s damp climate, particularly on north-facing patios or anywhere shaded by fences and trees. These organisms form a slick biofilm layer when wet, which is both a slip hazard and a genuine structural concern. Left unchecked, moss and weed roots can penetrate mortar joints between slabs, creating structural instability that eventually leads to loose slabs and a much more expensive repair than routine cleaning would have cost.
Preventing Regrowth
- Clean on a regular schedule, roughly monthly, rather than waiting for visible growth to return.
- Trim back overhanging trees or shrubs where possible to reduce shade and dampness on affected areas.
- Apply a patio sealant once the surface is fully clean and dry, which makes future cleaning considerably easier and slows how quickly algae and moss re-establish.
- Keep drainage clear, since standing water encourages both organic growth and slab movement over time.
Professional Cleaning Costs
If you’d rather have the job done professionally, expect to pay around £3 to £6 per square metre for a straightforward pressure wash and treatment in the UK, with many companies charging a minimum call-out fee of £120 to £200. As a rough guide, a typical 25 square metre patio might cost £150 to £300 cleaned and treated, more if resanding or sealing is included in the job.
Related Guides
Frequently Asked Questions
Can pressure washing damage patio slabs?
Yes, if used incorrectly. Excessive pressure or holding the nozzle too close can scar softer stone like sandstone and blast pointing out from between slabs. A fan-tip nozzle, a reasonable distance, and steady movement reduce this risk significantly.
Is vinegar safe to use on all types of patio stone?
No. Vinegar is safe for concrete, granite, and most porcelain, but should never be used on limestone or other acid-sensitive natural stone, where it can cause permanent etching and colour loss.
How much does professional patio cleaning cost in the UK?
Typically £3 to £6 per square metre, with most companies charging a minimum call-out fee of £120 to £200. A standard 25 square metre patio usually costs between £150 and £300 to clean and treat.
Why does moss keep coming back on my patio?
The UK’s damp climate, particularly on shaded or north-facing areas, provides ideal conditions for algae, moss, and lichen to regrow. Sealing the patio once clean, improving drainage, and reducing shade where possible all help slow regrowth.
Final Thoughts
Correctly identifying your slab material before choosing a cleaning method is the single most important step in cleaning patio slabs safely, since limestone, sandstone, and pressure washers each carry their own specific risks. A regular soap-and-water routine keeps most patios looking fresh, while pressure washing and sealing handle the deeper, longer-lasting clean when growth has built up.