Black Spots on Your Patio: What They Are & How to Remove Them

What black spots on a patio actually are

Black lichen is a slow-growing organism, part fungus, part algae, that bonds physically into the microscopic pores of concrete, sandstone and porcelain paving. Unlike surface algae or dirt, it doesn't sit on top of the stone, it grows into it, sending out root-like structures (rhizines) that anchor it firmly. This is exactly why scrubbing or a standard jet wash often just knocks the top off the spot while the black colour remains visible underneath.

It thrives in damp, shaded spots, north-facing patios, areas under trees, and paving that doesn't dry out quickly after rain are the classic locations you'll find it worst.

Why jet washing alone doesn't remove it

Pressure washing is genuinely useful for lifting general grime, moss and loose algae off paving, but lichen's anchoring structure means the black colour often persists even after a thorough jet wash, because the rhizines are still embedded in the stone's pores. Worse, aggressive pressure on softer paving like sandstone or block paving can etch the surface or blow out jointing sand, causing more problems than it solves.

This is the most common reason people end up frustrated after a DIY jet wash: the patio looks generally cleaner but the black spots are still stubbornly there, sometimes looking almost the same as before.

The treatment process that actually works

Professional black spot removal uses a biocide or specialist lichen treatment applied to the affected areas, which kills the organism at the root level rather than just knocking off the visible surface. This is left to dwell, sometimes for several days to a couple of weeks depending on the product and severity, before the dead lichen is much more easily removed with a gentle wash.

For heavy, long-established infestations, a second treatment cycle is sometimes needed. It's a process that takes patience rather than raw power, which is exactly why it's often mis-sold as a quick jet-wash job when it isn't one.

What it costs and what to expect

A full patio clean including lichen treatment typically runs £3-£6 per square metre depending on paving type, severity and whether re-sanding of joints is needed afterwards. Treatment-only visits for lichen-heavy areas without a full clean are sometimes priced separately and lower.

Be wary of any quote promising instant, complete removal of established black lichen in a single pressure wash, it's usually a sign the black spots will be back, or never really gone, within weeks.

It's also worth asking whether the quote includes a re-sand of joints on block paving or gaps around slabs, since a proper clean often disturbs jointing material and leaving it out reduces the finish and can invite regrowth sooner than expected.