How To Kill Moss On Render & Exterior Walls

Few things make a house look more tired than green-stained render. That moss and algae growth creeping up your walls is visible from the street, and it’s getting worse every year.

The instinct is to blast it off with a pressure washer – but that’s often the worst thing you can do. Render is more delicate than it looks, and the wrong cleaning approach can cause expensive damage.

Here’s how to kill moss on rendered walls properly, without destroying the surface in the process.

Why Render Is So Prone to Moss Growth

Rendered walls seem like an unlikely place for moss to thrive, but they’re actually ideal growing environments:

Textured surface. Most render has a slightly rough texture that gives moss rhizoids plenty of anchor points. Understanding how moss spreads and reproduces explains why textured surfaces are so vulnerable.

Moisture retention. Render absorbs and holds moisture, especially on north-facing walls or areas sheltered from sun and wind. This creates the consistently damp conditions moss loves.

Sheltered locations. Walls under overhanging roofs (including flat roofs which can drip onto walls below), near trees, or in shadowy corners stay damp longer and provide protection from harsh weather.

Slow drying. Unlike smooth surfaces that shed water quickly, render’s texture means walls stay wet for hours after rain – perfect for moss establishment.

Why Pressure Washing Render Is a Bad Idea

Pressure washers are brilliant for concrete and block paving, but they can destroy render:

Surface erosion. High pressure strips away the outer layer of render, exposing the coarser material beneath. This creates an even rougher surface that moss colonises faster.

Water ingress. Forcing water into render at high pressure can drive moisture deep into the wall structure, causing damp problems inside your home.

Cracking and lifting. Pressure can exploit small weaknesses in render, turning hairline cracks into major damage and lifting sections away from the underlying masonry.

Uneven appearance. Even if you don’t cause obvious damage, pressure washing often leaves render looking patchy and uneven where different areas have been eroded differently.

The same concerns apply to aggressive scrubbing with wire brushes or abrasive tools. Render needs a gentler approach.

The Right Way to Kill Moss on Render

Successful render cleaning relies on chemistry rather than force. Let the moss killer do the work, then remove dead growth gently.

Step 1: Choose the Right Product

You need a moss killer that’s effective but render-safe. Avoid anything containing bleach – bleach can kill moss but often leaves permanent discolouration on white or coloured render.

Similarly, Jeyes Fluid and other tar-based products can stain light-coloured render. Professional moss killers designed for building exteriors are the safest choice.

Step 2: Protect Surroundings

Before treating, cover or move any plants directly beneath the wall. While render-safe products are gentler than bleach, runoff can still affect vegetation.

Close windows and doors nearby, and cover any outdoor furniture or fixtures that might get overspray.

Step 3: Apply Treatment Thoroughly

Using a pump sprayer or low-pressure garden sprayer, apply moss killer from the bottom of the wall upwards. This prevents streaking from product running down over untreated areas.

Work in sections, ensuring complete coverage. The textured surface of render means you need to be thorough – moss hiding in crevices will survive if not properly wetted.

For high walls, you may need a telescopic lance attachment or scaffolding. Don’t try to stretch beyond safe reach.

Step 4: Wait for Complete Kill

Moss killer takes time to work – typically 7-14 days for complete kill on walls. Don’t try to remove moss early. You’ll know it’s ready when growth has turned completely brown/black and feels dry.

Treating during the right season (spring or autumn) with good weather forecast helps ensure fast, complete results.

Step 5: Remove Dead Growth Gently

Once moss is completely dead, you can remove it using:

Soft brush. A long-handled soft brush is ideal for most render. Work gently – you’re removing dead moss, not scrubbing the surface.

Low-pressure rinse. A garden hose on normal pressure is fine for washing away loose debris. Don’t use a pressure washer.

Leave it to weather. Completely dead moss will gradually weather away on its own over a few months. If you’re not in a hurry, this is the gentlest approach.

Step 6: Consider a Second Application

For heavy infestations, particularly on older or more porous render, a second treatment 4-6 weeks after the first catches any surviving growth. This is especially important for preventing regrowth.

Different Types of Render

Treatment approach varies slightly depending on your render type:

Cement render (traditional). The most common type on older properties. Relatively robust but can be damaged by pressure washing. Treat as described above.

Silicone render. Modern render systems with water-repellent properties. Generally more resistant to moss but still vulnerable in shaded areas. Same treatment approach, but moss often comes away more easily after dying.

Monocouche (through-coloured) render. Single-coat coloured render. Be especially careful with aggressive cleaning as damage can’t easily be patched to match. Chemical treatment only.

Pebbledash/roughcast. Textured render with embedded aggregate. Very prone to moss due to extreme texture. May need heavier product application to reach all surfaces.

Tyrolean/spar dash. Decorative textured finishes. Treat like pebbledash – thorough application, gentle removal.

Preventing Moss Coming Back

Killing existing moss is only half the battle. To prevent rapid reinfestation:

Improve drainage around walls. If ground at the base of walls stays wet, moisture wicks up into render. Ensure soil slopes away from walls and consider improving ground drainage.

Trim vegetation. Cut back trees, shrubs, and climbers that shade walls or hold moisture against them. Even a small increase in sunlight and airflow makes a significant difference. While you’re at it, check nearby fencing for moss – it can spread to walls.

Fix leaks promptly. Leaking gutters, downpipes, or overflows create constantly damp patches that moss loves. Repair any plumbing issues feeding moisture to your walls.

Check roof overhangs. Short eaves or missing drip edges allow rain to run down walls rather than dripping clear. Extending overhangs or adding drip details helps keep walls drier.

Preventative treatment. A light application of moss killer once or twice a year (spring and autumn) prevents moss establishing before it becomes visible.

Consider specialist coatings. Some protective coatings make render less hospitable to moss. Consult a render specialist about options suitable for your specific render type.

When to Call a Professional

Consider professional help if:

• Walls are very high and require proper access equipment
• Render is already damaged or in poor condition
• You have specialist or heritage render
• Previous DIY attempts have caused damage
• Moss growth is extremely severe or widespread

Professional exterior cleaning companies have appropriate access equipment, specialist products, and experience with different render types. The cost is often worthwhile for difficult or high-risk situations.

Frequently Asked Questions

Can I use a pressure washer on low setting for render?

Even low pressure settings can damage render if the nozzle is too close or you dwell too long in one spot. It’s not worth the risk – chemical treatment followed by a garden hose rinse is much safer and just as effective.

How long will treatment results last on render?

On north-facing or shaded walls, you might see some regrowth within 12-18 months. On sunnier walls with good drainage, results can last 2-3 years or longer. Regular preventative treatment extends these timescales significantly.

Will moss killer damage my render?

Quality moss killers designed for building exteriors won’t damage render when used as directed. Avoid bleach-based products on coloured or white render as they can cause discolouration. Always test on an inconspicuous area first if concerned.

Can I paint over moss-stained render?

Never paint over active moss – it will continue growing under the paint and push it off. Kill and remove all moss first, let the render dry thoroughly (several weeks in good weather), then paint if desired. Masonry paint actually helps protect against future moss growth.

Why is only one wall of my house mossy?

Moss grows where conditions suit it. North-facing walls get less sun and stay damp longer. Walls near trees are shaded. Walls with leaky gutters have constant moisture supply. Address these specific conditions to reduce moss on problem walls.

Is the green growth always moss?

Not always – algae is also common on render and appears as a thin green film rather than the thicker cushions of moss. The same treatment works for both. You may also see black mould in persistently damp areas, which needs different treatment.

Ready to restore your render? View our professional moss killer – safe for render, effective on moss, and no pressure washer needed.