Short answer
Black spots on patio slabs are usually lichen: small, flat growths that cling tightly to the stone and do not come off with ordinary scrubbing or pressure washing. They are removed with a treatment designed for black spot or lichen, given time to work, then scrubbed and rinsed. Test any product on a hidden area first, particularly on natural stone.
What are the black spots?
Lichen is a partnership between a fungus and an alga. On paving it often appears as small, round, black or dark grey spots, sometimes with paler grey, white or yellow patches alongside. It grows slowly and attaches itself firmly to the surface of the stone, which is why it survives brushing and pressure washing.
Not every black mark is lichen. Spots that wipe away are probably surface dirt or mould. Scattered dots that feel slightly raised and will not scrub off are much more likely to be lichen. Rings or patches under pots, metal furniture or a barbecue may be staining rather than growth, and need a different approach.
Why it is difficult to shift
Lichen anchors itself into the tiny pores in the surface. Removing the visible part with pressure often leaves the anchor behind, so the spot reappears or remains as a faint mark. Turning the pressure up to compensate can damage the stone, especially sandstone, before the lichen gives up.
Treatments work by killing the lichen so that it loosens its grip and can be scrubbed or weathered away. That is why patience is part of the process.
How to remove black spots from a patio
- Sweep the patio and give it a general clean so the treatment can reach the spots. Our guide on how to clean patio slabs covers this.
- Choose a product labelled for black spot or lichen removal, and check the label says it suits your paving, whether natural stone, porcelain or concrete.
- Test it on a hidden slab and see how the stone reacts before treating the whole patio.
- Apply it in dry weather as instructed, and keep children and pets off the area while it works.
- Leave it for the time the manufacturer recommends. Some products work quickly with scrubbing; others are left on and work gradually.
- Scrub the spots with a stiff synthetic brush and rinse thoroughly.
- Repeat if needed. Stubborn spots may need more than one treatment.
Safety and what to avoid
Read and follow the product label in full, wear gloves and eye protection, and never mix products, as some combinations can give off harmful fumes. Keep treatments away from plants, lawns and ponds, and do not let run-off go into drains.
Avoid acid-based cleaners on natural stone and some concrete, as they can etch the surface. Avoid bleach on sandstone, where it can discolour the stone. And avoid trying to blast the spots off with very high pressure, which tends to scar the stone without removing the lichen completely.
Sandstone and porcelain
Indian sandstone is particularly prone to lichen because of its porous, textured surface, and it is also the most easily damaged by harsh cleaning. Our guide to cleaning Indian sandstone covers suitable methods. Porcelain is far less porous, so lichen struggles to take hold on the tiles, but marks can still appear in the joints or on textured finishes; see our guide to cleaning a porcelain patio.
If green slime and moss are the bigger problem, our guide to moss and algae on patios is a better starting point.
Stopping it coming back
Lichen settles on damp, undisturbed surfaces. Regular sweeping, prompt removal of leaves and occasional washing all make it harder for new growth to establish. Some treatments are designed to keep working after application; follow the manufacturer's guidance on repeat use.
When to call a professional
Lichen removal can take time and repeated effort, and the wrong product or too much pressure can leave permanent marks, especially on natural stone. Consider a professional if the spots cover a large area, if earlier treatments have not worked, or if your patio is sandstone.
When we clean patios, we treat delicate stone with care, adjust the pressure for the material and can apply a biocide treatment after cleaning to help slow regrowth. We will tell you honestly what is likely to come off and what may remain.
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Sandstone, porcelain, slabs and decking brought back to life. Family-run by Jason and Lucas Prior, covering south Essex.
Frequently asked questions
Why won't the black spots on my patio come off?
They are most likely lichen, which grips the stone firmly and survives scrubbing and pressure washing. It needs a treatment designed to kill it before it can be removed.
Can I pressure wash black spots off a patio?
Pressure washing alone usually removes the surface but leaves the lichen anchored in the stone, and high pressure can damage softer paving. Treat the spots first, then scrub and rinse.
Is lichen on a patio a problem?
Lichen is mainly a cosmetic problem, as it spoils the look of the paving. Slippery patches are more often caused by moss and green algae, which are worth dealing with for safety.
Related guides
- How to clean patio slabs: the general method for most patios, when a pressure washer helps and when it does more harm than good.
- How to get rid of moss and green algae on a patio: clearing slippery green growth from paving and joints, and the shade, damp and debris that let it return.
- How to clean Indian sandstone: gentle cleaning for soft, porous sandstone, the products that can stain it and how to use a pressure washer safely.
- How to clean a porcelain patio: everyday cleaning for dense porcelain tiles, with most of the attention on the joints where dirt and growth collect.