Roof Lichen: Why It’s Not Dirt and Why Jet Washing Fails

Mark Cave

Picture it. Monday morning, quarter past eight, phone goes. It's a homeowner, and they are not happy. They paid you nine hundred pounds for a roof clean, and two days later there are still chalky white circles stuck across every single concrete tile. They want a full refund. So, um, who's in the wrong there?

Mark Cave

And here's the thing, it might be nobody. Yet. But only if you understand what those white circles actually are. Because lichen is not dirt. There's a SoftWash UK guide on roof lichen that puts it plainly: lichen is not just surface dirt. It is a living organism that anchors itself tightly to roof tiles.

Mark Cave

Living organism, and actually it's two of them. A fungus and an algae, living together, sort of a partnership. And the fungus side sends out these tiny root like structures, rhizines, down into the pits and pores of the tile. On concrete, it sits right in the textured face. So you're not wiping a stain off. You're, you're, you're pulling a plant out of the ground.

Mark Cave

Now, you'll hear somebody say, just blast it, jet wash, job done. Think about a deep taproot in dry clay. You can yank and yank, and what comes up first? The top of the soil. Same on a tile. The pressure shears off the protective cementitious slurry on the surface and exposes raw aggregate, and that happens long before the lichen lets go. So the roof looks cleaner for a week, and it's been wrecked, and it'll regrow faster on the rough surface you just made.

Mark Cave

Which leads to the other panic move. Early on in this industry, I saw it more than once, cleaners ringing round in a flap, and they'd dump raw, unbuffered ten percent sodium hypochlorite on the roof. The idea was, melt the white stuff, fast. And yes, it can lighten it. But it ran off onto lead flashing and scorched it. Border shrubs, dead. And on the tiles, stubborn ghost patches, burned into the face, permanently.

Mark Cave

So brute force with water fails, and brute force with chemistry fails. Fair enough. But then what do you actually tell that angry customer on Monday morning?

Mark Cave

You tell them about time. A professional biocide, say a fifty percent DDAC concentrate like Soft Wash Pro 50, used at a two percent working solution. That's twenty five to one, and the product page lists it as the guide rate for first time cleans on heavier contamination, roofs and render where suitable. Now, I'd say the biological kill can be done in around 48 hours. I'm giving you that with a bit of caution, because weather and tile matter. But dead and gone are two different things.

Mark Cave

Dead crustose lichen is still stuck there. It needs rain, and freeze thaw, to loosen it and break it down. Eight to twenty four weeks, is what I'd tell people. The guide I mentioned says weeks or months, so, so it's the same ballpark. So that white circle your customer is staring at on Monday? It's a corpse, not a failure. Not the nicest way to put it to a homeowner, mind.

Mark Cave

And one catch, because somebody will ask. Heavy moss is a separate problem. That product page is clear it isn't a moss killer, so thick moss gets physically removed first, then you treat.

Mark Cave

Next question: how do you get it on the roof? Not by walking it. A treated roof pitch is slick, tiles are fragile at the overlaps, and, um, a fall costs more than any clean ever earns. So you stay on the ground. Water fed pole, a dedicated softwash fan nozzle, low pressure, even coverage. The aim is coverage, not blasting.

Mark Cave

But the real fix, the one that stops that Monday call, happens before you ever pick up the pole. At the survey. Show them before and after photos of weathering on a similar roof. Put the timeline in writing. Eight to twenty four weeks, no instant new roof. People are fine with slow when they've been told slow. It's the surprise they hate.

Mark Cave

And that honesty pays. Because once a customer understands the roof is a biological problem, not a cosmetic one, the annual biocide maintenance agreement is an easy conversation. One off clean becomes recurring work. Everyone wins.

Mark Cave

This is why I bang on about training. Not for a certificate to hang up, but because knowing the chemistry and biology stops you reaching for the jet wash or the strong bleach in a panic. The SoftWash UK CPD Certified Course, and Approved Softwash Applicator status, are there to put you in the trusted expert bracket, not the splash and dash one.

Mark Cave

So that angry Monday call? Two days in, the lichen is dead but still sitting there, and the tile is untouched. Say that, with the photos and the written timeline, and a refund demand turns into a thank you in about twenty weeks. Alright, that's me done.