If your pebble dash has turned green, grey or speckled with black, you are not alone. The combination of frequent rain, mild temperatures and a rough, porous surface makes dash an ideal home for algae, lichen and moss. North-facing walls, gables under trees and homes near the coast tend to suffer most.
The temptation is to hire a pressure washer and blast it off. It looks satisfying, but on pebble dash it can do real harm. This guide explains why, and what to do instead.
Why high pressure is risky on pebble dash
Pebble dash is stones bedded into a thin top coat of render. On an older wall, that bond may already be weakened by weathering. A powerful jet can:
- Knock stones out, leaving bald patches that are hard to match
- Force water into hairline cracks and behind hollow render
- Leave the wall saturated just before a cold night, when frost can do its damage
- Strip surface growth without killing the roots, so it returns quickly
We regularly see walls where well-meant jet washing has caused more problems than the algae ever did.
How soft-washing works
Soft-washing turns the approach on its head. Instead of relying on force, it uses a cleaning solution suited to organic growth, applied at low pressure and left to dwell. The solution breaks down and kills the algae, spores and lichen. The wall is then rinsed gently. Some growth continues to fade over the following days and weeks as the weather completes the job.
Because the pressure is low, stones stay in place and very little water is driven into the wall. Plants, glass and surfaces nearby need protecting, and run-off has to be managed carefully, which is one reason many people prefer to have it done professionally.
If you want to tackle it yourself
For small areas, you can do a reasonable job with care:
- Choose a product intended for masonry and organic growth, and follow the label closely, including safety advice
- Wet surrounding plants first and cover anything delicate
- Apply with a low-pressure sprayer, not a jet
- Let it work for the recommended time before rinsing gently with a hose
- Work from ladders only if you are confident and someone is with you
Avoid scrubbing hard with wire brushes, which can dislodge stones, and do not mix cleaning products.
Look at the wall while you are at it
Cleaning shows up what the growth was hiding. Look for hairline cracks, loose stones and areas that sound hollow when tapped. It is far better to find these now than after a winter of water getting in. Small problems can often be dealt with by a simple crack repair or patch repair.
Slowing the return of algae
No treatment stops growth for ever in a wet climate, but you can make a big difference:
- Fix the water sources. Leaking gutters and overflows create the damp streaks algae loves.
- Let light and air in. Trim trees and shrubs that shade the wall or drip onto it.
- Keep the base clear. Soil and planting against the wall keep the lower render damp.
- Consider a periodic treatment. A light maintenance application on vulnerable walls can extend the time between full cleans.
Painted pebble dash
If your dash has been painted, the same soft-wash principles apply, but the method needs adjusting so sound paint is not lifted. If the paint is already flaking, cleaning will make that obvious, and repainting with a breathable coating may be worth considering. Our painting page covers that in more detail.
How often should you clean?
There is no fixed schedule. A sunny, sheltered wall in town might go many years without needing attention, while a shaded gable under trees on a damp hillside can turn green again within a couple of seasons. A good rule is to look at the walls each spring. If growth is starting to spread from the bottom of the wall or from under the sills, a light treatment then is easier and cheaper than waiting until the whole elevation has darkened. Cleaning before any planned painting or repairs also makes sense, since both work best on a clean surface.
Getting help
We provide pebble dash cleaning across South and West Wales, for homes and commercial buildings. As part of the job we check the render and let you know about anything that needs attention. Call 07565 147 170 or get in touch for a quote.
Last updated 22 July 2026.
