Why Pressure Washing Alone Won't Remove Black Spot Patio Stains

If you've jet washed your patio and the black spots came straight back within a few weeks, you're not alone, and you're not doing anything wrong. Those dark, tar-like marks on block paving, sandstone and concrete slabs are almost always lichen, and lichen doesn't behave like ordinary dirt. It grows into the surface of the stone rather than sitting on top of it, which is exactly why a pressure washer can blast away the surface layer one week and leave the problem staring back at you the next.

In this guide we explain what black spot actually is, why pressure washing on its own can't remove it for good, and what actually works instead.

What Is Black Spot on a Patio, Really?

Black spot is the common name most homeowners use for lichen, a slow-growing organism made up of algae and fungus living together. Unlike green algae or moss, lichen doesn't just sit on the surface, it forms a crust that bonds chemically and physically with porous stone such as Indian sandstone, York stone, concrete flags and some types of block paving.

You'll usually see it as small, tightly packed black or dark grey circles, often clustered in shaded, north-facing or damp corners of a patio where the surface stays wet longer after rain.

Why Pressure Washing Doesn't Get Rid of It

A pressure washer is brilliant at shifting loose dirt, algae, moss and general surface grime. It's the wrong tool for lichen, and here's why:

·         Lichen has root-like structures (rhizines) that anchor it into the pores of the stone, not just on top of it.

·         High-pressure water strips the visible black colour but leaves the organism's structure alive beneath the surface.

·         Blasting porous stone with too much pressure can actually open up the surface, creating more texture for spores to recolonise.

·         Without a biocidal treatment, any spores left behind will simply regrow, often within 4–8 weeks.

This is the single biggest reason homeowners tell us they've paid for a jet wash two or three times and the black spots keep coming back. The visible stain is treated, but the organism causing it is never actually killed.

Editor's note: Regrowth timelines vary depending on the patio's aspect, drainage and the local climate. We've given a typical range above based on jobs we see across the UK, but always ask your installer for a site-specific assessment.

What Actually Removes Black Spot for Good

A proper patio black spot removal process treats the cause, not just the stain. That means a biocidal wash formulated to kill lichen spores at the root, applied with a controlled dwell time so it has a chance to work into the stone before any rinsing takes place.

A typical professional process looks like this:

·         Site assessment to confirm the surface material and the extent of the lichen growth.

·         Application of a specialist biocide or algaecide designed for the specific stone type.

·         An appropriate dwell period, sometimes several days, to let the treatment kill the organism fully.

·         A low or controlled-pressure rinse to lift the dead material without damaging the surface.

·         A breathable sealant applied once the patio is fully dry, to slow future regrowth and protect the stone.

This is where working with a specialist patio black spot removal company makes the real difference. It's not about bigger equipment or higher pressure, it's about using the right chemical treatment, the right dwell time, and the right sealing product for your specific stone.

Why Sealing Matters as Much as the Treatment

Killing the lichen is only half the job. Porous, unsealed stone will keep absorbing moisture, and damp, shaded patios are exactly where black spot thrives. A quality breathable sealant reduces water absorption, makes the surface easier to clean, and significantly slows how quickly lichen and algae can re-establish themselves.

We generally recommend a maintenance clean and reseal every couple of years for UK patios, more often in heavily shaded gardens or where the patio sits under trees.

Can You Treat Black Spot Yourself?

Some DIY biocide products are available and can offer a temporary improvement on light growth. The results tend to fall short of a professional black spot patio removal service for a few reasons: consumer-grade biocides are typically weaker, dwell times are often too short if you're keen to get the patio usable again, and shop-bought products rarely include a compatible sealant step. For heavier or long-standing lichen, particularly on natural stone, professional treatment is usually the more reliable route to a lasting result.

How to Tell If You Need a Professional

Consider bringing in a specialist if:

·         Pressure washing has removed the black spots before but they've returned within a couple of months.

·         The lichen covers a large area or has spread across several patio sections.

·         You have natural stone (sandstone, York stone, limestone) rather than concrete, which is more prone to pitting under high pressure.

·         You want the patio sealed as well as cleaned, so the problem doesn't simply come back next season.

Quick summary

Black spot is lichen, not surface dirt, and it grows into the stone itself.
Pressure washing alone removes the visible stain but not the organism, so it regrows.
A biocidal treatment with proper dwell time is what actually kills the lichen.
Sealing afterwards is what stops it coming straight back.

Final Thoughts

If your patio keeps growing black spots no matter how often you jet wash it, the problem isn't your cleaning routine, it's the tool. Pressure alone was never designed to kill a living organism embedded in porous stone. A proper black spot removal treatment followed by sealing addresses the actual cause, and it's the only approach that tends to keep patios clear for the long term rather than for a few weeks at a time.