Common problems ／ 01

Soiled exterior walls and black rain streaks

Black streaks run vertically down from under windows or copings. Even after washing, they come back to the same place after a while.

Key points

The black streaks under windows and copings are airborne dirt and oils that run down with rainwater and soak into the same path. Washing only the surface leaves them inside, and they come back. We check the cause and the material, wash, and apply a treatment that makes it harder for dirt to soak in.

Black vertical streaks from rain running down a stone-clad exterior wall.
The photograph is one example. We decide on the cause after checking on site.

01

Possible causes

  • Exhaust fumes, dust and oil in the air run down with rainwater and are left along the same path
  • Dirt soaks into the fine pores of the material and remains inside even when the surface is washed
  • Oil seeps out of the sealant around windows (sealant staining)

02

What we check on site

  • The material (stone, tile, plaster, concrete) and its surface finish
  • Whether the dirt sits on the surface or has soaked in
  • The path rainwater takes, and the shape of drip edges and copings
  • The height, and whether scaffolding is needed

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

03

How we deal with it

  • Remove the existing dirt with washing and stain removal
  • After confirming the surface is dry, apply anti-soiling treatment that makes it harder for dirt to soak in
  • Record the date of work, extent, details and photographs

04

What it cannot do

  • It does not fix deterioration of painted surfaces or sealant that has reached the end of its life. It does not replace exterior repainting
  • The path the rain streaks follow does not itself change. It does not stop dirt from settling; it makes dirt harder to soak in and easier to remove

05

Before you try it yourself

For tile or stone exterior walls, start with a neutral detergent and a soft brush. Strong high-pressure washing can damage joints and surfaces. Black streaks that will not come off may be dirt that has soaked in.

06

What affects cost and time

It varies with the area of the wall, the height and scaffolding, how deeply the dirt has soaked in, and the type of material.

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.