Black Spots on Patio Slabs and Driveways: What They Are and Why They’re So Stubborn
If you've ever pressure washed a patio and stood back thinking it looks great apart from hundreds of little black dots, you're not alone.
I see this regularly on patios and driveways around Walsall.
The surface dirt disappears. The green growth comes off. The paving brightens up.
But the black spots are still there.
People often assume the pressure washer hasn't been powerful enough. Sometimes they move the nozzle closer and keep going over the same area trying to blast the marks away.
That's where damage can start.
Those stubborn black spots often aren't ordinary dirt at all. They're lichen, and once lichen becomes established on paving, pressure alone isn't always the answer.
What are the black spots on driveways and patio slabs?
Black spot is the name commonly given to dark lichen growth found on paving.
A lichen isn't simply algae or fungus. It's a partnership between a fungus and an alga or cyanobacterium.
You don't need to know the biology to clean a patio, but it does explain why black spot behaves differently from the green film you might see after a wet winter.
Lichen attaches itself extremely firmly to the surface it grows on. On porous paving, particularly some natural stone, it can become well established within the texture of the material.
That's why it can survive a pressure wash that removes almost everything around it.
Specialist paving guidance also identifies black spotting on paving as commonly being lichen and notes how difficult established colonies can be to remove mechanically.
Why doesn't pressure washing remove black spot?
Because more pressure isn't always what the job needs.
A pressure washer is excellent at shifting surface dirt, loose organic matter and years of general grime.
Established lichen is different.
If I clean black spots on patio slabs and they remain, I don't automatically keep increasing the pressure until they disappear.
Doing that can mean damaging the paving before you've dealt with the lichen.
I've been cleaning exterior surfaces for more than 20 years, and one of the things experience teaches you is when to back the pressure off.
The objective is to clean the paving, not win a fight with it.
Where appropriate, black spot needs the correct treatment alongside the cleaning process. Heavier or older growth may also need more than one treatment rather than disappearing instantly.
Is black spot worse on Indian sandstone?
Indian sandstone is one of the surfaces where black lichen can become particularly noticeable.
Natural sandstone has texture and porosity that give lichen somewhere to establish itself. Once you've got hundreds of mature black spots across a light-coloured sandstone patio, they can be very obvious.
That's also when I'd be particularly wary of random cleaning advice from social media.
I've seen recommendations ranging from extremely aggressive pressure washing to strong acids and all sorts of homemade mixtures.
Natural stone isn't something I'd experiment on.
Some acid-based cleaners can react badly with certain imported sandstones and potentially leave staining that is considerably worse than the original black spots.
If I'm dealing with sandstone, I want to know what I'm cleaning before deciding how I'm going to clean it.
What about black spots on concrete slabs?
Concrete paving can get lichen too.
Again, the condition of the paving matters.
An older weathered concrete slab isn't necessarily going to respond the same way as a relatively new one. Decorative surfaces and coloured concrete also need some thought before aggressive cleaning.
This is why I don't particularly like the idea of one cleaning method being described as suitable for every patio.
It isn't.
The surface comes first. The method comes second.
Can driveways get black spot as well?
Yes.
Black spot isn't just a patio problem.
I see lichen and other organic growth on block paved driveways, concrete and other hard surfaces as well.
Block paving has another consideration because pressure cleaning removes jointing sand from between the blocks.
Once the driveway has been thoroughly cleaned and allowed to dry, those joints need replenishing with the correct kiln-dried sand.
That's why re-sanding is included as standard with my block paving driveway cleaning.
It isn't an optional finishing touch.
It's part of completing the job properly.
Why does one part of a patio get covered while another stays fairly clean?
This is something customers ask me quite a lot.
You can have one patio where half of it looks pretty good and the other half is covered in green growth, moss and black spotting.
Usually, something about the surroundings is different.
Look at the worst section and it might be:
beside a fence or wall
underneath a tree or hedge
behind garden furniture
somewhere that gets very little sunlight
an area where water sits after rain
somewhere with poorer airflow
The RHS notes that algae and many other organic growths favour damp conditions, with shady areas often being more badly affected. It also points out that these growths can make patios, drives, paths and steps slippery.
I've seen exactly that pattern plenty of times.
Two sections of the same patio. Same slabs, same age, same house.
Completely different levels of growth because one stays wetter than the other.
Is every black mark on a patio black spot?
No, and this is an important one.
I wouldn't look at every dark mark on paving and immediately tell a customer it's lichen.
Natural stone has its own variations and markings.
Trees can leave organic staining.
Rust can produce marks.
Some paving can even show staining associated with the bedding underneath it.
So if somebody sends me photographs of a patio, I'm looking at the pattern, the material, the age of the paving and where the marks are appearing.
Sometimes the answer is obvious.
Sometimes it isn't.
Treating something before you've worked out what it is can create a bigger problem than the one you started with.
Can black spot lichen be completely removed?
Often, very significant improvements are possible.
But I'm careful about promising 100% removal before I've seen the paving.
A patio with relatively recent spotting is very different from sandstone that's been heavily colonised for years.
Some established lichen can need repeated treatment. In severe cases, there may also be permanent changes or weathering to the surface underneath where the growth has been sitting.
That's why I'd rather look at the patio first and give somebody a realistic idea of what I think can be achieved.
I don't need to tell every customer their paving will look brand new.
It needs to look properly cleaned.
There's a difference.
Can you remove black spot permanently?
You can remove existing lichen, but nobody can sensibly promise that outdoor paving will never get it again.
It's outside.
Spores are present in the environment, rain will continue to fall and shaded areas will continue to stay damp.
If the conditions remain suitable, organic growth can return. The RHS makes the same point with algae, lichens and mosses on hard surfaces: removing the growth doesn't change the environment that allowed it to establish in the first place.
Regular maintenance can help.
Keeping leaves and debris off the paving helps.
Good drainage helps.
Dealing with new growth before it becomes heavily established helps.
But I'd be suspicious of anyone promising that a single clean will permanently prevent black spot from ever appearing again.
Does sealing stop black spot?
This is another one where I'd avoid making sweeping promises.
A suitable sealer on the right surface can make maintenance easier and provide protection, but sealing doesn't mean you can forget about the patio for the next ten years.
Dirt can still settle on top.
Leaves can still collect.
Organic matter can still build up.
And sealers themselves don't last forever.
Whether I'd recommend sealing depends on the paving, its condition and what the customer actually wants from it.
I don't automatically recommend it after every clean.
Can I remove black spots myself?
For lighter contamination, homeowners may be able to make improvements themselves with suitable products and careful cleaning.
The part I'd be cautious about is escalating things when the first attempt doesn't work.
More pressure.
Stronger chemical.
Longer dwell time.
Another chemical on top.
That's where a straightforward cleaning job can turn into damaged or discoloured paving.
Whatever product you're considering, check that it's suitable for the exact material you're treating and follow the manufacturer's instructions.
And never assume that because something worked on somebody else's concrete patio it'll be safe on your sandstone, porcelain or other paving.
When is professional driveway/patio cleaning worthwhile?
You don't need me to clean a driveway or patio just because you've found three black dots on it.
If the paving is generally clean and you're comfortable maintaining it yourself, that's fine.
Professional cleaning starts to make more sense when you're dealing with widespread black spot, heavy algae, moss, ingrained dirt or several years of build-up.
That's particularly true when you're unsure what material you're dealing with or you've already tried pressure washing and the stubborn marks haven't moved.
That's where experience makes a difference.
Not because I've got a magic pressure washer.
Because I know pressure isn't always the answer.
Black spot removal and driveway and patio cleaning in Walsall
I'm Dean from DMS Driveway and Patio Cleaning, based in Bloxwich.
I carry out professional patio cleaning and driveway cleaning throughout Walsall and surrounding areas including Aldridge, Brownhills, Pelsall, Cannock, Sutton Coldfield, Great Barr, Great Wyrley and Cheslyn Hay.
Every job is carried out by me, and I've got more than 20 years' experience cleaning exterior surfaces.
If you've got black spots covering your patio or driveway and you're not sure whether they're lichen, send me a few photos on WhatsApp.
I'll have a look and tell you what I think you're dealing with, whether professional cleaning is worthwhile and what I'd realistically expect from the job.
No sales. No gimmicks. Just straightforward advice and fair prices.
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