What is black spot on patios?

What it is

Black spot is lichen: a slow-growing mix of fungus and algae that roots into tiny pores in the stone. It starts as small dark dots and slowly spreads into patches. Because it's anchored into the surface, a brush or a jet wash goes straight over it.

Why it appears

• Natural stone. Sandstone and limestone are slightly porous, which gives lichen a grip.
• Shade and damp. Patios that rarely dry out are the worst affected.
• Time. It builds up over several years, so it's common on patios that have never had a deep clean.

How it's removed

1. Deep clean the patio to remove algae, moss and dirt.
2. Apply a black spot treatment suited to the stone. It kills the lichen rather than blasting at it.
3. Let it fade. Treated spots fade and break down over the following weeks as the weather works on them.

We'll always tell you from your photos what to expect. Very old, deep-set black spot can fade a lot without disappearing completely.

Keeping it away

A yearly clean with a treatment stops lichen getting established again. Re-pointing failed joints also helps, as it gives moss and weeds less to grow in. See our Indian sandstone cleaning page.

Recent patios

See more on our patio cleaning page.

Questions

Will pressure washing remove black spot?

Not on its own. Washing removes the green algae and dirt around it, but black spot is rooted into the stone, so it needs a treatment that kills it.

Does black spot damage the patio?

It's mainly cosmetic, but it spreads over time and makes a patio look old and dirty even straight after a clean.

Which patios get it?

Mostly natural stone like Indian sandstone and limestone, plus older concrete slabs. It's less common on porcelain, though porcelain can still pick up dark marks and staining.