How to Remove Black Spots From Patios

What are the black spots on my patio?

The black spots you see on patios, driveways and paving slabs are almost always lichen — a combination of algae and fungi that grows together as a single organism. They attach themselves firmly to the surface of the stone or concrete and are significantly harder to remove than regular dirt or algae alone.

Lichen is common across the North West of England where we get plenty of damp conditions, and it tends to appear first in shaded spots — under trees, near fences, or on north-facing patios that don't get much direct sun.

Why won't a regular jet wash remove them?

This is the most common frustration we hear. A standard pressure washer will remove surface dirt, green algae, and moss fairly easily — but lichen is a different challenge.

Lichen attaches to the surface at a microscopic level and has a waxy outer layer that water simply bounces off. You can blast it with high pressure and it either stays put or gets temporarily dislodged, only to regrow quickly from the remaining root-like structures (called rhizines) left behind.

Pressure alone won't fully shift it. You need the right chemistry.

What actually removes black spot lichen?

The effective approach is a two-stage process:

1. Pre-treatment with a biocide or specialist lichen remover — applied to the surface and left to dwell. This breaks down the waxy layer and kills the lichen at the root level.
2. Pressure washing after treatment — once the chemical has worked (usually 15–30 minutes), the surface is pressure washed to remove the now-dead lichen and any remaining contamination.

On severe cases, a second treatment pass or hot water pressure washing gives even better results.

Will they come back after treatment?

Lichen and algae will always try to return — especially in shaded or damp areas. The key to keeping them at bay longer is applying a post-treatment biocide after cleaning. This leaves a protective residue on the surface that inhibits regrowth for 12–24 months in most conditions.

Without a post-treatment, black spots in shaded areas can start returning within 6–12 months. With it, most customers see significantly longer-lasting results.

Can all patio surfaces be treated?

Yes — we treat block paving, concrete slabs, natural stone, porcelain, sandstone and Indian slate. Different surfaces require slightly different pressures and dwell times to avoid damage, which is why a professional approach gets better results than DIY.

Porcelain in particular needs careful handling — the wrong pressure or chemical can damage the surface finish. We always assess the surface before starting.