How to clean render and pebbledash walls

Render and pebbledash walls are magnets for green algae, black streaks and general grime — especially on north-facing elevations and in the damp Lincolnshire climate. Knowing how to clean render and pebbledash walls properly means you can shift years of build-up without cracking the surface or making the problem worse.

The short answer: a low-pressure wash combined with a specialist biocide treatment is almost always the safest, most effective approach. High-pressure blasting sounds tempting but regularly causes more harm than good. Read on and we'll walk you through the full process.

Why render and pebbledash get so dirty

Both surfaces are porous and textured, which makes them brilliant at trapping moisture, dirt and organic spores. Algae and lichen take hold in the damp patches first — usually the areas that never fully dry out, such as shaded north-facing walls or anywhere sheltered by an overhanging roof. Once algae establishes itself, it darkens over time, turning from green to black or brown and making the whole front of a house look neglected.

In places like Nettleham and Welton, where older rendered semis are common, we see this constantly — walls that were perfectly clean a couple of years ago are suddenly looking tired and stained.

The tools you need

Before you start, gather the following:

• A garden pump sprayer or a low-pressure garden hose with a fan attachment
• A soft-bristle brush (a long-handled deck brush is ideal for reach)
• A proprietary render or masonry biocide (look for sodium hypochlorite-based products sold for exterior use)
• A bucket and warm water for rinsing
• Safety glasses and gloves — biocides are strong

Avoid wire brushes entirely. They scratch render and dislodge pebbles from pebbledash, leaving bare patches that absorb moisture even faster.

How to clean render and pebbledash walls step by step

1. Wet the wall down first. A gentle rinse with a garden hose stops the biocide from drying out too fast and helps it penetrate evenly.
2. Mix and apply the biocide. Follow the manufacturer's dilution instructions. Apply generously from the bottom up using your pump sprayer — this prevents dirty run-off from streaking areas you've already treated.
3. Leave it to dwell. Most biocides need 15–30 minutes to kill the algae and lichen properly. Don't let it dry on the wall in direct sun.
4. Scrub gently. Use your soft brush in small circular motions on any stubborn patches. You'll often see the green lift away almost immediately.
5. Rinse thoroughly from the top down. Work downward so you're always rinsing clean water over already-treated areas. A low-pressure hose is fine — you don't need a pressure washer for this stage.
6. Repeat if needed. Heavy build-up, especially dark lichen, may need a second application after 24 hours.

What about a pressure washer?

A pressure washer can be used on render and pebbledash, but only on a low-pressure setting (typically under 80 bar) and with a wide-angle fan nozzle held at least 30 cm from the surface. Getting too close, or using a narrow jet, can chip render, blast out the pebbles in pebbledash and force water behind the coating — which is far more damaging than a bit of algae.

If you're not confident with the settings, it's genuinely better to rely on the biocide and brush method and leave the pressure washer out of it. We've repaired or re-rendered walls in Cherry Willingham and Branston after enthusiastic DIY pressure washing did serious damage — it happens more than people realise.

Dealing with stubborn black lichen

Black or dark brown lichen is tougher to shift than green algae because it anchors itself into porous surfaces more deeply. For heavy lichen, apply a stronger dilution of biocide, cover the treated area loosely with plastic sheeting to stop it drying out, and leave it overnight. By morning, most lichen will have released its grip and can be brushed or gently rinsed away.

Should you seal the wall afterwards?

This is a question we get asked a lot. A breathable masonry sealant can reduce the rate at which water and spores penetrate the surface, which does slow regrowth. However, render and pebbledash must be completely dry before you apply any sealant — applying it to damp masonry traps moisture inside and can cause the coating to crack or bubble. Wait at least two dry weeks after cleaning before considering this step.

For more on whether sealing outdoor surfaces is worth the investment, take a look at our patio cleaning guides and advice, which covers the key questions in detail.

When it's worth calling in the professionals

DIY cleaning works well on a single-storey wall with moderate staining. But it gets complicated quickly if:

• The wall is above first-floor height (working from a ladder with a sprayer and brush is tiring and risky)
• There are cracks or loose sections in the render that could let water in
• The staining is severe and has been building up for years
• You're getting ready to sell your property and first impressions count

We cover homes and businesses across Lincolnshire — from Lincoln and Saxilby through to Heighington, Waddington and beyond. You can see the full range of areas we work in on our areas page. We use professional-grade equipment and treatments that are simply more effective than what's available off the shelf, and we can often transform a wall in a single visit.

We also handle connected outdoor surfaces — patios, paths, driveways and more — so if you want everything done at once, see our full services list.

Keep it clean for longer

A few simple habits make a real difference:

• Trim back any shrubs or trees that press against the wall and trap moisture
• Clear gutters and downpipes regularly so water doesn't overflow and run down the render
• Apply a diluted biocide spray once a year as a maintenance treatment — it stops algae establishing before it gets a hold

Done consistently, this can more than double the time between full cleans.

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If your render or pebbledash has got to the point where a brush and hose isn't going to cut it, we're happy to take a look. Get a free, no-obligation quote and we'll let you know exactly what we can do and what it'll cost — no surprises.

Frequently asked questions

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Need a hand with yours? We cover Lincolnshire for patios, driveways, paths and more — see the areas we cover or browse our services. Get a free quote or call 07874 010739.