How do you get rid of black spots on a patio?

What are the black spots on my patio?

Black spots on a patio are usually lichen. Lichen is a fungus and an alga living together. It grows as flat, crusty spots and sends tiny root-like threads into the pores of the stone, which is why it holds on so firmly.

Sandstone, including Indian sandstone, is the most affected, because it’s porous. Dense surfaces such as porcelain and pressed concrete resist lichen much better.

Not every dark mark is lichen, though. Some are mould, mildew or algae, which sit more on the surface and clean off more easily. Algae usually shows as a thin green film rather than a crust. Others are stains left by fallen leaves, fruit or berries, which need a different treatment.

Why doesn’t jet washing get rid of black spots?

A jet wash cleans by force, and lichen isn’t only on the surface. The jet takes off the top of each spot but leaves the part rooted in the stone, so the spots stay visible or grow back. High pressure can even drive spores deeper into the pores.

Too much pressure also damages soft stone. Sandstone and limestone can be etched for good, slate can flake, and the sand or mortar in the joints can be washed out. A rougher, more open surface then gives moss and lichen more to grip.

What gets rid of black spots on a patio?

What works is a treatment that kills the lichen, given time to work:

• A biocide or specialist lichen treatment, applied to the dry stone and left to soak in, so it reaches the roots.
• Time. The dead lichen then fades as it weathers, or is rinsed off with a gentle wash.
• A second treatment for stubborn spots, which often need more than one.
• A gentle clean to lift the dead growth and dirt, at a pressure the stone can take.

Always read the label of any product, test it on a hidden corner first, and don’t apply it if rain is due in the next few hours.

What shouldn’t you use on a natural stone patio?

Some cleaners do more harm than the spots themselves:

• Acid-based cleaners, including vinegar. Acid dissolves limestone and marble. On sandstone it can bring out orange or rust-coloured stains, which are sometimes permanent.
• Bleach without a patch test. Some lichen treatments are bleach-based, but bleach can discolour some stone. Test first, and keep it away from plants and lawns.
• High pressure close up. A narrow jet held close to sandstone or limestone can etch the surface for good.

If you don’t know what your patio is made of, treat it as natural stone until you do.

How long does it take for black spots to go?

Most treatments take a few days to start working, and they work best in a spell of dry weather. Some specialist products are left on for a couple of hours and then rinsed off, while others are left to work and the dead lichen fades as it weathers.

Stubborn spots that have been growing for years often need more than one treatment before they clear.

Is black spot treatment safe for pets, plants and drains?

It can be, if it’s used with care. Lichen treatments are biocides, made to kill living things, so:

• Keep children and pets off the patio until it is completely dry, which can take several hours.
• Keep the run-off out of drains, ponds and water features. Patio biocides can be very toxic to fish and other water life.
• Protect plants and lawns. Wet nearby beds and grass first, and rinse off any splashes, because some treatments kill plants.
• Wear gloves and eye protection, and never mix products or make up your own mixtures.

How do you stop black spots coming back?

Lichen spores are everywhere, so no treatment keeps a patio clear for ever. You can slow them down, though:

• Seal porous stone. While a sealant is intact, lichen has nowhere to root. Sealing can change the colour and feel of the stone, and it needs reapplying every few years.
• Let in sun and air. Cut back overhanging trees and shrubs so the patio dries out faster.
• Help it drain. Patios that hold puddles grow more. A slight slope and good drainage along the edges help water run off.
• Sweep regularly. Fallen leaves and dirt hold damp and feed new growth.

Most patios need a clean every 12 to 18 months, and shaded ones about once a year.

When should you call a professional?

Call a professional if the spots cover a large area, if you’re not sure what your stone is, or if a treatment hasn’t worked. A professional clean also gets the rest of the patio back to colour and, for block paving, puts fresh sand back in the joints.

Washmates cleans patios back to colour and re-sands block paving with kiln-dried sand. Black spot and lichen treatment is part of every Washmates patio clean, not an extra. We use a surface cleaner or a soft wash to suit the surface, and a biocide only where the surface needs it. A professional patio clean typically costs £190 to £400, including VAT; see how much patio cleaning costs. Tell us about your patio and we’ll send you a fixed price, usually without a site visit.