Algae

In detail

Algae is a green film or powdery deposit on paving and stonework, and the paving trade distinguishes it from two neighbours people lump in with it. MOSS forms coarse tufts or cushions. LICHEN is crust-like, leafy or scurfy and comes in silver-grey, grey-green, yellow or orange. All three are organic growth and none is mould, which matters because the indoor mould evidence in this catalogue does not transfer to them. The useful part is where it appears: heavily shaded areas, ground under trees, and paving not laid to an adequate fall. That makes recurrence a drainage and light problem rather than a cleaning failure, which is the same shape as the indoor damp answer reached from a different direction. Treatment behaves unlike a stain. Proprietary treatments take some DAYS rather than minutes and work best during a dry spell; thick growth is scraped off first; and dead growth is left to weather away rather than scrubbed. Residual protection is unlikely to outlast two or three years. One product warning is specific and easily walked into: preparations containing FERROUS SULPHATE react with concrete and leave a brown stain of their own.

What works

• Mild abrasion is effective against Algae Supported

  Scrape the thick growth off first, and brush the wash in well. Dead growth is then left to weather away rather than scrubbed.

Why

• Algae is explained by Moisture control and recurrence

  Shade, trees and a surface that does not drain. The same shape as the indoor damp answer, reached outdoors from a paving manufacturer instead of a health authority.