Tackling Black Spots on Patios: The Biology and Softwashing Workflow

Mark Cave

You finish a patio. It's soaking wet, it looks spotless, and the customer is delighted. Then it dries. Pale rings appear where the black spots were. Some of the specks haven't moved at all. And a few weeks later, or a few months if you're lucky, the spotting is back. So what happened?

Mark Cave

I'm Mark Cave from SoftWash UK, and today I want to talk about black spots on patios. The short version is this. You weren't cleaning dirt. You were dealing with a living organism, and that changes how you should approach the job.

Mark Cave

If you've listened to our earlier episodes on why patios turn green again, and on protecting mortar joints, this is the tougher cousin of that story. Green algae sits on the surface. It's a film. It's relatively easy to shift, and it's the reason a patio goes green again. Black spot is different. It's lichen, and lichen goes in.

Mark Cave

A lichen is a partnership. A fungus living together with algae or cyanobacteria. The fungus gives it structure and grip, and the partner makes food from light. On paving you'll see small black, grey, white or dark green circular marks. They show up on Indian sandstone, limestone, concrete flags, older block paving, and even porcelain grout lines.

Mark Cave

Here's the key point. Those spots are anchored into the microscopic pores, pits and tiny capillaries of the stone or concrete. What you see on top is only part of the organism. It also produces dark pigment, which helps protect it from UV and harsh weather. It can go dormant in dry spells and wake up again when moisture returns. That's one reason a patio can look better in summer and then slide backwards through autumn and winter.

Mark Cave

Now think about what a pressure washer actually does. It shears off the top of the growth. The roots, the hyphae, stay put in the surface. So you get a patio that looks better for a short while, often with a pale ring or a stubborn dark speck left behind. And the colony simply regrows from what's still in there, plus spores landing from nearby trees, walls, borders and shaded corners.

Mark Cave

This is where contractors get themselves into trouble. The spot doesn't shift, so the pressure goes up. Then you're chasing individual spots with a turbo lance. In the field, we often see the results. Etched lines and wand marks on sandstone and limestone. Exposed aggregate on concrete flags. Jointing compound and mortar blown out. Water driven under loose slabs. And a patchy finish, because the operator concentrated on spots instead of treating the whole surface.

Mark Cave

And here's the cruel part. Scouring the stone opens up fresh voids and leaves a rougher surface. That gives future algae, moss and lichen even more places to take hold. It's the same lesson as with mortar joints. You chew the surface up, you create micro pores, and you speed up the next colonisation. So the job that was meant to fix the problem has made regrowth faster.

Mark Cave

Recurring black spot is a biology problem as much as a cleaning problem. That's the mindset shift. Stop asking how hard you can hit it, and start asking how you kill it where it lives.

Mark Cave

So let's walk through a proper workflow, step by step.

Mark Cave

First, survey and risk assess. Identify the paving. Is it sandstone, limestone, concrete, a sealed or coated surface? Is the stone acid sensitive? Limestone is the classic one, so harsh acids are not your answer there. Check the joints, are they loose, cracked or resin bound? And look at where the runoff will go. Ponds, lawns, planted borders, surface water drains. Plan that before you open a tap. Test a small, inconspicuous area first.

Mark Cave

Second, dry prep. Sweep the patio properly. Moss clumps, silt, leaves, soil. If you put an oxidiser onto an unswept patio, a chunk of your chemical gets used up on surface organic matter instead of reaching the lichen embedded in the stone. If there's heavy moss in the joints, scrape or brush it out first. A controlled pressure wash to expose the spots can be fine, as long as you use sensible pressure. The aim is to uncover the problem, not damage the stone.

Mark Cave

Third, chemistry. For black spot, professional softwashing commonly uses a controlled sodium hypochlorite solution, because it oxidises and breaks down the biological growth so it can release from the surface. Pair it with a bleach stable surfactant, something like Clever Wash Softwash Surfactant. Why? Because patio slabs are uneven and porous. Without a surfactant, the solution tends to bead up and run off. A surfactant breaks the surface tension, so the solution wets the surface evenly, clings, and gets into the pore structure. And it isn't about making the mix stronger. It's about keeping the right strength in contact for long enough.

Mark Cave

Fourth, and this is the one that gets rushed, dwell time. The solution has to stay wet and active to do its work. If it dries out, the breakdown stalls and you can be left with stubborn rings. On a warm or windy day, work in manageable sections, and if you need to, mist on light reapplications. Black spots rarely vanish instantly, and sometimes the answer is repeated light applications rather than one heavy hit.

Mark Cave

Fifth, gentle agitation. While the chemistry is working, use a stiff bristle broom to break open the softened lichen. Please, no wire brushes on natural stone. They scratch the surface and can leave metallic debris behind. And if a spot still won't release, don't just crank up the pressure or the chemical strength. Stop, reassess the surface, and repeat the process safely if it's appropriate.

Mark Cave

Sixth, rinse. Use clean water at low pressure, and rinse thoroughly. You're removing spent oxidiser and loosened organic debris, so watch the low points, drainage channels, thresholds and joints. Don't blast the jointing while you do it. Then come back and inspect once it's dry, because that's when your customer will judge it. Wet patios lie to you. Moisture hides the contrast.

Mark Cave

And then there's the part that protects your result. No cleaning process permanently stops nature. Spores will keep arriving from nearby trees, flower beds and shaded borders. So think about a longer term biocide treatment, such as Soft Wash Pro 50, which is a DDAC based product. It's slower acting, but it's used for ongoing biological control. Hypochlorite does the fast visual clean. The biocide helps hold the line. Wrap that into a maintenance plan, and the customer gets a better result while you get repeat work.

Mark Cave

Now, a podcast can show you the shape of the process. But professional softwashing means knowing why. Correct chemical selection, surface identification, equipment setup, safety, runoff control and pricing the work properly. Knowledge comes before products. That's why we built the SoftWash UK CPD Certified Softwash Training Course. It's online, you learn at your own pace, and it comes from years of real world experience, to help you avoid expensive mistakes. Complete it and you can become an Approved Softwash Applicator, with a directory listing and a place in the National Association of Softwash community.

Mark Cave

You can find all of that at softwashing.uk, along with chemicals, equipment and resources. Next time you're staring at a stubborn black spot, put the lance down and think like a biologist first. Thanks for listening, and I'll catch you in the next one.