Mold, moss and algae on exterior walls and grounds

Green or black growth spreads on north-facing walls, fences, and floors that get no sun.

Key points

Mold, moss and algae on exterior walls and grounds spread in shaded places that are slow to dry, using dirt left on the surface as a foothold. We wash them off and apply a treatment that makes it harder for dirt to settle, so a foothold is less likely to form. We cannot say that they will stop growing. We also suggest measures such as improving drainage.

The photograph is one example. We decide on the cause after checking on site.

Possible causes

Shaded, and slow to dry after rain
Dirt and moisture left on the surface give mold and moss a foothold

What we check on site

Sunlight, drainage, and where rainwater collects
The type of material, and how far the growth has spread

We do not decide on the cause, or whether the work can be done, from photographs alone.

How we deal with it

Remove the existing moss and mold by washing
Apply anti-soiling treatment that makes dirt less likely to settle, so a foothold is less likely to form
Also suggest measures against the source of moisture, such as drainage

What it cannot do

We cannot say that mold and moss will stop growing. We think that, as dirt becomes less likely to settle, a foothold is less likely to form, but the most we can say is that if they do grow, they are easier to remove
The anti-soiling treatment is not a biocide

Before you try it yourself

Chlorine-based chemicals can change the color of the material or harm plants. Test them in an inconspicuous spot before use.

What affects cost and time

It varies with the extent, the height and scaffolding, the material, and how far the growth has spread.

The on-site survey and estimate are free. Where there are costs, such as for distant locations or when scaffolding is needed, the installer in charge will tell you in advance.