see rock

The rock patio alongside the house in the courtyard is sheltered from the
sun. Consequently, it cultures a thin film of mildew over time that, when
the rains persist, becomes slippery.
Powerwashing restores a bright, non-slip surface. The stairs also acquire a
slippery film that I don't like to introduce to visitors. Now they have a
more secure grip to them when taking that "flight".