This Indian sandstone patio in Reigate, Surrey had heavy black algae. Some slabs had gone almost completely dark, and the customer’s own treatments hadn’t shifted it.

Before. Black algae across the Indian sandstone.

What we did

Put a treatment on the blackened slabs to break down the algae before washing
Gave the worst slabs longer for the treatment to work
Cleaned the whole patio with a rotary surface cleaner at a pressure that suits sandstone
Rinsed the area and checked the slabs were even before we left

After

The sandstone is back to its natural warm colour across the patio. We treat black algae before washing so it lifts out of the stone instead of being spread around.