Those small black spots on your patio are often black spot lichen.
They are not usually just dirt sitting on the surface. That is why they can still be there after sweeping, rinsing, or even pressure washing.
Black spot lichen is a form of organic growth that can cling tightly to paving, stone and concrete. It often appears as small, round, dark marks across the surface. Some patios only have a few spots. Others become heavily speckled, especially in shaded, damp or weather-exposed areas.
The good news is that black spots can often be improved. The honest answer is that they can be stubborn, and not every mark will disappear completely.
What causes black spots on patio slabs?
Black spots usually appear when organic growth has settled into or onto the surface of the paving.
Patios live outside all year. Rain, shade, leaves, soil, poor drainage and damp corners all create conditions where organic growth can take hold.
Lichen tends to grow slowly, so many people do not notice it at first. It may start as a few small dark marks, then become more obvious over time. Patios under trees, beside borders, near damp walls or in shaded gardens can be more prone to it because the surface stays wet for longer.
The type of paving matters too. Porous stone, older slabs, weathered surfaces and uneven joints can all give dirt and organic growth more places to settle.
Is a black spot the same as moss or algae?
No. Moss, algae and lichen are all outdoor growths, but they do not behave in the same way.
| What you can see | What it may be | How it usually behaves |
| Small dark spots that do not brush away easily | Black spot lichen | Often stubborn and tightly attached to the surface |
| Green film or slippery-looking coating | Algae | Often appears in damp, shaded areas |
| Soft green growth between slabs or on edges | Moss | Usually raised and easier to see |
| General dullness, mud or weathering | Surface dirt and grime | Often improves well with cleaning |
| White marks or pale powdery areas | May be mineral staining or efflorescence | Needs different advice from lichen |
This is why a patio can look much cleaner after pressure washing, but still have black marks left behind. The general dirt may have gone, while the lichen remains.
Why does black spot lichen feel so hard to remove?
Black spot lichen is stubborn because it is not just loose dirt.
It can cling firmly to the paving and can be harder to shift than moss or surface grime. On some patios, pressure washing improves the overall appearance but does not fully remove the black spots.
This does not mean the patio has been cleaned badly. It often means the mark needs a different level of treatment, or that the surface condition is limiting what can be achieved safely.
Will pressure washing remove black spots?
Pressure washing can remove a lot of dirt, algae and loose organic matter, but it may not remove every black spot.
A deep clean can make a patio look much fresher. It can also remove slippery-looking build-up and brighten the surface.
Black spot lichen is different. On some surfaces, pressure alone may not fully remove it. On others, using too much pressure can risk damage, especially if the slabs are delicate, the pointing is weak, or the surface is already weathered.
A good professional should check the patio before deciding how to clean it. The surface type, age, drainage, access, pointing and any loose or damaged areas all matter.
Can I remove black spots myself?
Be careful before applying strong patio products or household chemicals.
Some homeowners try patio cleaners, bleach-based products or other household methods before asking for help. The risk is that patios are not all made from the same material. A product that seems fine on one surface may mark, lighten or damage another.
Natural stone, older slabs, delicate paving, damaged pointing, and areas close to plants or lawns all need care.
If you are unsure, the safest first step is to stop before adding more products. Take a few photos and ask for advice. It is much easier to assess the patio before several different products have been used on it.
What can I do about black spot lichen?
There are patio cleaning products sold for black spot lichen, but they need care.
Some well-known products include Tile Master Algo, Patio Black Spot Remover and Smartseal Patio Clean Xtreme. These types of products often contain sodium hypochlorite, which is a strong bleach-based chemical used in some outdoor cleaning treatments. Safety sheets for these products show sodium hypochlorite as a hazardous ingredient, with warnings around skin, eye and environmental risks.
This matters because run-off can affect nearby grass, plants, ponds and borders if the product is not controlled properly. It can also bleach clothing and may mark or damage some surfaces if used too strongly, left too long or applied to the wrong material.
If you decide to use a patio black spot product yourself, read the label carefully, protect nearby plants and avoid guessing with dilution or dwell time. If the patio is natural stone, old, delicate, damaged, close to planting or already has loose pointing, it is safer to ask a professional before treating it.
A professional will assess the surface first, choose a suitable method and work to reduce the risk of damage to your patio, garden edges and surrounding features.
What should a professional check first?
The first check should be the patio itself, not just the black spots.
A professional should look at:
- the surface type
- the condition of the slabs or blocks
- loose or damaged areas
- the condition of the joints
- drainage and runoff
- access to equipment
- whether sanding, sealing or re-pointing may be needed separately
For block paving, jointing sand may need attention after cleaning. For some patios, sealing may be worth discussing. For older patios, damaged pointing may need separate repair.
These extras should not be assumed as part of a basic clean unless they have been clearly agreed.
Will my patio look new again?
A patio clean can often make the surface look much fresher, but it should not be sold as a guaranteed “like new” result.
The result depends on the patio’s age, material, condition, staining, drainage and previous treatment. Some black spots may improve but not fully disappear. Some surfaces may be too fragile for aggressive cleaning. Some patios need more than cleaning if the joints, pointing or surface have deteriorated.
A realistic aim is a cleaner, fresher and more usable patio, with clear expectations before the work starts.
Can black spot lichen come back?
Yes. Outdoor growth can return if the conditions are still right for it.
A shaded patio that stays damp, collects leaves, or drains poorly will usually be more prone to moss, algae and lichen returning.
Regular sweeping, clearing leaves, improving airflow where possible and dealing with drainage issues can all help slow the return.
Cleaning can improve the surface, but it cannot change the weather, shade or the way water moves across the patio.
When should I ask for help?
Ask for advice if the black spots are widespread, if pressure washing has not worked, or if you are worried about damaging the surface.
It is also worth asking before using strong products, especially if your patio is natural stone, old, delicate, recently laid, close to planting or has loose joints.
At Art of Clean, we can look at the surface, explain what is realistic and let you know whether a deep clean is suitable. Where needed, we can also talk through whether sealing, re-sanding or re-pointing should be considered separately.
A simple next step
Send us a few clear photos before you try another product.
Try to include:
- a close-up of the black spots
- a wider photo of the patio
- any loose joints, damaged areas or drainage concerns
- the type of paving, if you know it
We can then give you a clearer idea of what may be possible before any work is booked.
🎥 And for a quick explainer, check out this short video where our very own Pierre De Wet shares more about these mysterious black spots and how we tackle them professionally.
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