Black algae on render: why it appears and how to clean it safely
Render looks crisp for the first few years, then dark patches start to appear. Usually they begin under window sills, behind downpipes and on whichever wall gets least sun, and they spread outwards until a once pale house looks grubby from the road.
Those marks are almost always algae. The good news is that render can be cleaned back without being damaged, as long as the method is right. The bad news is that the most obvious method, a pressure washer, is the wrong one.
Why render stains
Render is slightly porous and textured, so it holds moisture after rain and gives airborne spores somewhere to settle. On shaded walls, near trees or where water runs off sills and ledges, the surface stays damp long enough for algae to establish. Black and red algae are the most common culprits, alongside a general green film on cooler elevations.
Newer finishes described as self-cleaning resist staining for longer, but they are not immune. Once the surface is colonised they stain in the same way.
Where it usually starts
- Below window sills and ledges where rain drips and runs.
- Either side of downpipes and around leaking gutters.
- North-facing and tree-shaded walls that rarely dry out.
- Near ground level where splashback keeps the base damp.
Why pressure washing is the wrong answer
A jet wash removes the visible staining by force, but it also opens up the render surface, can mark the finish and drives water into the wall. The algae is rarely killed at the root, so it returns quickly into a surface that is now rougher and more absorbent. We do not use pressure on render for that reason.
How we clean render
Render is cleaned by soft wash. A solution is applied at low pressure so it can soak into the staining and kill the algae and its spores. After it has had time to work, the wall is rinsed gently. The finish is left intact, and because the growth is killed rather than just knocked back, the wall stays clean for longer.
Heavier or older staining sometimes fades further over the following days as the treated residue weathers. That is normal. Windows, doors and planting are protected while we work.
Dark patches spreading on your render?
Send us a photo of the worst wall and we will explain what is causing it and how we would clean it. Quotes are free.
Sky Scrapers Ltd · info@sky-scrapers.co.uk
Keeping it clean afterwards
Most of the causes are about water. Fixing leaking gutters and downpipes, clearing blocked gutters, and trimming back shrubs that sit against the wall all help the render dry faster. Many customers book render cleaning alongside gutter and fascia cleaning for exactly that reason.
Can you do it yourself?
Small, low patches can be tempting to tackle with a brush and a bucket, but scrubbing hard marks the finish and household cleaners rarely kill the growth at the root. Anything above ground floor also means ladders, which is where most accidents happen. If the staining covers a whole elevation or sits high on the wall, a professional soft wash is the safer and longer-lasting option.
Our own photographs
Images from Sky Scrapers Ltd work showing the methods described on this page.
Where this applies locally
Towns where this comes up most often on our patch:
- Softwashing & Biocide Treatment in BedfordBedfordshire · MK40
- Softwashing & Biocide Treatment in AmpthillBedfordshire · MK45
- Softwashing & Biocide Treatment in LutonBedfordshire · LU1
- Render cleaningMethod guide
- What sets the price of render cleaningCost guide
- Cleaning driveways, patios and paths properlyMethod guide
Questions we get asked
Is black mould on render dangerous?
The black marks on external render are nearly always algae. They are unsightly rather than a health hazard, but they do spread if left.
Can I use a pressure washer on my render?
We would advise against it. Pressure can damage the finish and makes restaining faster. We do not use pressure on render.
Will soft washing remove all the staining in one go?
Most staining lifts on the day, and remaining marks usually fade further over the following days as the treated growth weathers.
Does this work on painted render?
Painted and through-coloured render both suit low-pressure treatment. We will look at the finish before we start.
Can you clean render and gutters on the same visit?
Yes. Booking them together also helps fix one of the main causes of render staining.
In short
Black and green marks on render are algae thriving on damp, shaded walls. Pressure washing damages render and brings the staining back faster; a low-pressure soft wash kills the growth and leaves the finish intact.