Pressure washing guide
How to remove black spots from paving
Black spots on paving are lichen, a completely different organism to algae, and one of the most stubborn problems on outdoor surfaces. Pressure washing alone cannot shift them. This guide explains why and what actually works.
Black spots on paving are one of the most common and frustrating problems on London driveways and patios. They appear as flat, dark grey to black marks of varying sizes and are notoriously resistant to cleaning. Many homeowners discover this the hard way after pressure washing their patio only to find the black marks remain completely unchanged. The reason is that black spots are lichen, not algae, and lichen is a fundamentally different organism that requires a specific approach to remove.
Lichen
black spots are lichen, a symbiotic organism that anchors itself deep into the pores of paving with microscopic filaments
2 to 4 hours
minimum dwell time needed for a specialist black spot remover to kill lichen before rinsing
Turns white
when lichen has been killed the black marks turn white — only rinse off once you see the colour change
Why black spots are not the same as algae
Lichen is a combined organism made up of a fungus and bacteria or algae living together in a relationship that makes each component more resilient than it would be alone. It travels as airborne spores, landing on paving surfaces and sending microscopic filaments called hyphae down into the pores of the stone or concrete to anchor itself and extract nutrients. These filaments spread through the surface layer over time, which is why lichen on sandstone tends to be most difficult to shift — sandstone has large, open pores that allow deep penetration. Standard algae removers have no meaningful effect on lichen. Weed killers can actually feed lichen. Pressure washing removes surface matter but leaves the root filaments intact, meaning the lichen returns rapidly.
Which surfaces are worst affectedWhich paving surfaces are most affected by black spot
How to remove itHow to remove black spots from paving: the correct process
The only effective approach to black spot removal is a sodium hypochlorite-based specialist lichen remover applied at full strength with sufficient dwell time. General patio cleaners and algae removers do not work on lichen.
Step 1: Clear the surface and pre-wet
Remove all furniture and sweep the paving. Pre-wet the surface with clean water before applying the treatment. This prevents the solution from drying out too quickly and ensures even penetration.
Step 2: Apply specialist black spot remover neat
Apply the black spot remover undiluted using a pump sprayer or watering can. Cover all affected areas generously. Protect any nearby grass, plants and soft surfaces from the solution as it will kill plant life on contact.
Step 3: Allow 2 to 4 hours dwell time
Leave the solution to work for a minimum of 2 to 4 hours. For severe infestations, leave overnight. Keep the surface moist by misting with water if it begins to dry out. The key indicator is the colour change — when the black spots turn white, the lichen has been killed.
Step 4: Agitate with a stiff brush
Once the spots have turned white, scrub the surface with a stiff brush to loosen the dead lichen from the pores of the stone before rinsing.
Step 5: Pressure wash or rinse thoroughly
Rinse the surface thoroughly with clean water or pressure wash to remove all dead lichen residue and solution. Multiple rinse passes ensure all product is cleared.
Step 6: Re-treat stubborn spots
Heavily infested areas or deep-rooted lichen in sandstone may need a second application a few days after the first. With each treatment, the remaining lichen weakens further until fully eliminated.
Safety and surface warnings
Sodium hypochlorite-based black spot removers are significantly stronger than household bleach. Always wear chemical-resistant gloves, eye protection and old clothing when applying. Do not apply on warm sunny days or when rain is expected within 24 hours. Test in a discreet area first on any surface you are uncertain about, particularly on coloured block paving. Do not use on tarmac, resin-bound driveways, decking or any surface that could be affected by bleach.
Professional pressure washing London
Black spot removal by Cloud Nine across London
Cloud Nine removes black spot lichen from all paving types across London using specialist treatments and professional equipment. We assess each surface and use the correct product and dwell time to produce a lasting result. Contact us for a free quote.
Part of our guide
Pressure washing help and guidance
Everything you need to know about pressure washing for London homes and businesses.
Back to the guide