Green algae on stone and granite: how to remove it without damage
Green algae is the green layer of algae and moss that grows on stone in the shade and damp: granite walls, pavement, outdoor steps, pool edges. Besides looking bad, it makes the stone slippery in winter.
Why it grows on stone
Natural stone is porous: it holds water and dust, and that is where algae takes hold. In Greater Porto, with high humidity from October to April, one spot without sun is enough for the green to come back every year.
What you can do at home
- A stiff-bristle brush and water, on small areas
- A pH-neutral anti-moss product, left for the time stated on the label
- Rinse well and let it dry in the sun
What never to do
- Muriatic acid or «cement cleaner» on granite and pavement: it burns the stone and leaves white stains (we explain it in why Portuguese pavement is never washed with acid)
- Neat bleach on porous stone: it discolours and does not stop the green from coming back
- Maximum pressure with the nozzle close up: it rips out joints and opens the pores
When to call a professional
On large areas, steps and high walls, a wash with pressure set for the type of stone and a neutral product removes the algae without damaging the surface. See the care for each material in natural stone and Portuguese pavement, and the same problem on walls in green algae on the facade.
How much it costs
Patio and paving washing costs from €4.5/m², with a €89 minimum per visit; walls and fences from €6.5/m². The guide price shows in the site calculator before you book, with no prepayment.