Black Spot on Your Patio: What It Is and How to Remove It

Black spot is caused by a type of dark-coloured algae or mould that grows on patio and path surfaces. Unlike green algae, which sits on the surface, black spot penetrates into the pores of the stone, making it much harder to remove. It appears as small dark dots or larger black patches and is particularly common on Indian sandstone, limestone, and other porous natural stone surfaces.

Removing black spot requires specific chemical treatment rather than pressure washing alone. A specialist black spot remover or strong biocidal solution is applied to the affected areas and allowed to dwell for an extended period — sometimes several hours. The chemical penetrates into the stone along with the algae, breaking down its cellular structure from within. After dwell time, the surface is pressure washed to remove the residue.

Preventing black spot from returning involves reducing the conditions in which it thrives. Applying a high-quality penetrating sealant after cleaning reduces the porosity of the stone, making it harder for the algae to take hold. Ensuring good drainage around the patio and removing sources of shade and moisture where possible also helps. Even with preventive measures, periodic retreatment is likely to be needed on porous natural stone.