How to Clean a Facade: Moss, Algae and Dirt

Where green deposits and streaks come from

Green and black deposits are algae, moss and fungi that develop on damp, shaded walls. You see them most often on the north side and near the ground, where the wall stays wet longer. Streaks, in turn, are dirt carried by rainwater, often from under sills, gutters and flashings.

Simply rinsing with water is usually not enough. You have to remove the microorganisms and then limit the moisture that lets them return. That is why it is best to treat cleaning and protection as two stages of one job, not two separate matters.

Facade types and the right way to clean

Before you start, note what your facade is made of, because not every surface tolerates the same treatment.

• Thin coat render, acrylic or silicone, is the most common finish over external insulation. It is relatively delicate, so it needs moderate pressure and gentle handling.
• Mineral and traditional render is often harder, but it too dislikes aggressive scrubbing.
• Facades of brick, clinker and stone are more durable, yet the joints still collect dirt and green deposits.
• Painted surfaces require a check that the product does not attack the paint layer.

Choosing the product for the type of dirt

• Green and black deposits, that is algae and moss, you remove with a facade cleaner that acts on microorganisms.
• Dust and general dirt come off when washing with a cleaning product.
• Greasy streaks need a product matched to the type of dirt and the render.

What to prepare before cleaning

• A facade cleaner matched to the type of dirt.
• A brush or soft broom on a telescopic pole for higher areas.
• Water, a garden hose or a pressure washer on a moderate setting.
• Film and tape to cover windows, doors and plants by the wall.
• A stable and safe access to higher spots, in line with the rules for working at height.
• Gloves, goggles and other protective equipment stated on the product data sheet.

Cleaning a facade step by step

1. 1
   Protect the surroundings. Cover plants, windows and parts that should not get any product. Set up a safe access to the higher parts of the wall.
2. 2
   Apply the product from bottom to top. Applying from the bottom reduces streaks on the still dry part of the wall. Let the product dwell for the time stated on the label.
3. 3
   Rinse from top to bottom. Rinsing in the other direction washes the dissolved dirt evenly downward. On thin coat render use a moderate jet.
4. 4
   Assess the result and repeat if needed. Heavily overgrown spots sometimes need a second pass. Better to repeat the treatment than to raise the washer pressure beyond a reasonable level.

Protecting the facade after cleaning

A clean facade is the best moment for impregnation. A water repellent impregnation limits the wall absorbing water, slows the return of growth and makes cleaning easier. Good products of this type keep the render breathable, so the wall can still release moisture.

The protection goes onto a dry facade, under the conditions on the product label. As with pavers, it is worth doing a test in an inconspicuous spot first. How to choose and apply it we describe in the guide on impregnating a facade.

How much product you need

Coverage depends on the absorbency and texture of the render. A rough wall takes more than a smooth one. On the data sheet of each product we state the coverage in square metres, and the calculator by the product converts the wall area into litres.

The most common mistakes in facade cleaning

• Too high pressure on thin coat render, which damages the top layer.
• Applying the product from the top, which creates streaks on the dry wall.
• Working in full sun, when the product dries before it acts.
• Skipping protection after cleaning, so algae return quickly.
• Underestimating safety when working at height.

When to leave the cleaning to a professional

Small, low areas of a facade can usually be cleaned yourself. It is different with large areas and work at height, where safety matters most, but also an even result across the whole wall.

If the facade is high, heavily overgrown or needs scaffolding, a contractor is the sensible choice. A well chosen product still helps, because it does most of the work, and professional equipment reaches difficult spots safely.