What the staining actually is
The dark marks on a rendered wall are rarely dirt on their own. They are living growth — algae, lichen and moss — with airborne soiling collecting on top of it. That is why the colour ranges from green through grey to near black on different elevations of the same house.
Render is a good host for it. The surface is textured, slightly porous and holds moisture, so once growth is established it spreads across the face of the wall rather than staying in one spot.
Why one elevation is always worse
Reading the pattern is useful. A defined vertical stripe usually points at a gutter or joint above it. A uniform haze across a whole elevation is normally shade and exposure. A dark band along the base is almost always splash-back or ground moisture.
- North-facing and shaded walls dry slowly and stay damp for longer after rain.
- Overhanging trees and tall hedges cut light and airflow and drop organic debris.
- Leaking gutter joints, blocked outlets and overflow pipes wet the same strip of wall repeatedly.
- Splash-back from paving, planting beds and downpipe discharge darkens the bottom metre.
- Ground levels bridging the render, or paving laid too high, keep the base of the wall wet.
Why pressure alone is the wrong answer on render
The instinct with a stubborn mark is to get closer with the nozzle. On render that is where the damage happens. High pressure can break the surface texture, expose the coat beneath, lift painted and coloured coatings, and leave a striped finish that is more obvious than the staining was.
It also drives water into the wall. Render is a weather layer, and forcing water past it — particularly through an existing crack or around a failed bell cast — can cause damp problems that were not there before.
Removing growth from the surface without treating it also tends to give a short-lived result, because what is left behind re-establishes.
How render is cleaned properly
The wall is assessed first. We look for cracks, hollow or loose sections, failed or flaking coatings, previous repairs, open joints around windows, and anything that would let water in. If we find something that needs attention before cleaning, we tell you.
The clean itself is a low-pressure soft wash: an appropriate treatment is applied to the surface, allowed to work on the growth, and then rinsed at a pressure the coating can take. Windows, doors, planting and paving are protected where practicable and run-off is controlled as far as the site allows.
Results are usually most obvious once the wall is fully dry. Wet render looks darker than dry render, so it is worth waiting before judging the finish.
Where cleaning cannot help
Cleaning improves the appearance of a sound wall. It does not fix a defective one. Cracked, blown or debonded render, a failed coating, damp coming through from inside, or a persistent leak above the affected area are all separate issues, and a clean will not resolve any of them.
Where a previous coating has failed unevenly, cleaning can make the patchiness more visible rather than less, because the dirt sitting over it is removed. We flag that before starting so there are no surprises.
Sorting the cause is what changes the long-term picture: clearing the gutter, fixing the overflow, cutting the hedge back, improving drainage at the base of the wall.
Getting a quotation
Photographs of each affected elevation, ideally taken in dry conditions, plus your postcode and any known history — when it was rendered, whether it has been painted, whether there is a leak — give us enough for an initial view.
Where the render needs assessing in person, we arrange a site survey before confirming the method and the figure. Every quotation is written and free.