If you have been walking around your house in the past few weeks and have noticed some dark spots on your render, you’re not alone. It’s one of the most common calls we get, and as a matter of fact, most householders cannot tell, just by looking, what the infestation is: mould, mildew or something less alarming like lichen. That confusion isn’t due to your lack of knowledge; these three growths are truly quite similar if looked upon from a distance, and you may waste money with the wrong approach, or you may withhold treatment for one that has subtly been working its way into the crevice in your wall.
The tips in this guide explain how to detect mould on walls, the difference between mould and mildew, and why lichen is completely another creature. We’ll also go through a true scenario that we had a few years ago which helps to explain this better than a checklist.
The Patch That Wasn’t Just “Dirt”
A client of ours in a townhouse had been unable to spot a grey-green area on the north-facing render area of her property for a month or so. She believed that it was garden dust misted up by the sprinklers, and continued to brush and water the spot with soapy water. Regrowth was always thorough within a few weeks, making it bigger each time.
When she contacted us, the patch had become patchy black in the regions facing the downpipe on the shaded side of the wall, and the original grey-green spot remained intact on the other part of the same wall facing the sun. It was a textbook example of two problems happening at once: lichen on the sun-affected and less damp render, and black mould on the corners of the exterior walls where water was building up from a blocked gutter, yet less sunny. After we rectified the drainage problem and treated the two differently, the problem of the wall being affected was resolved. She didn’t want to run into a moisture problem with a surface stain solution, so scrubbing was never going to work.
Mould vs Mildew: What’s the Difference
Although these words are used interchangeably, they’re not one and the same, and there’s a difference of perception when using these words and how you should use them.
Mildew is normally an aerial fungal growth which colonises the surface. It generally appears dull, dusty, grey or white and is not very tacky or does not adhere to the surface well, often removing with relative ease. It is common to find around window frames and under eave areas where frequent condensation occurs with good circulation, or in the cladding area.
Mould, however, is the other side of it, and is more aggressive. It will normally be raised, sticky or hair-like in appearance and, for the most part, green, grey and black in colour. Mould can invade porous renders, timber-faced and screw-clad surfaces, and grout lines, eating away the organic substance in the rendering itself. That is why the early identifying mould on walls is important; the longer it stays, the more deeply it gets into the wall, and the more problems it will cause if the mould is not treated professionally.
Here are a couple of tips to help distinguish them:
- Colour: mildew is grey or white; mould is more brown, green, grey or black and becomes a little slimy when wet.
- Smell: mould typically has a musty and damp smell, but not as strongly as mildew.
- Cleaning response: mildew is removed quickly by a wet cloth; mould smears, stains, or will come back quickly.
- Location: humid areas with poor air-convection, e.g., low airflow areas, are those most favoured for mildew; mould is favoured anywhere it comes into prolonged contact with moisture, such as inside porous render.
What Black Mould on Exterior Walls Actually Looks Like
There is only one sort of black mould that causes concerns on exterior walls, and that one is a very sensible one to watch for. It rarely forms a uniform layer, but is often found as patches or lines and is typically found around damp spots like window sills, near downpipes, around overgrown shrubs or where there is little sun on the walls. If the mould has been around for a period of time, there may be some softness to the render or cladding around it, as well as any sign of yield.
To see if it’s mould vs. dirt or staining, you will notice the surface will feel a little fuzzy or uneven, and may smell musty when you examine it closely: and if the patch comes back soon after cleaning, it’s mould, not dirt or staining. Mould that is a persistent problem on render is a good indication that there is not only an issue on the surface but an underlying moisture source.
Where Lichen Fits In
Lichen, however, is quite a distinct organism, a fungus as well as algae, and isn’t in fact a sign of a moisture issue, unlike mould or mildew. Lichen tends to form flat, crusty grey, green, yellow or even orange patches and develops over a long period by being deposited along a wall surface, such as walls facing the sun. It adheres well to render and masonry, making it hard to wipe off with a cloth as mildew does, and is normally only removed by a proper soft wash or specialised treatment.
The difference is that lichen grows on walls that are not wet, or located in a sunny, well-drained area, whereas mould and mildew grow in damp, shaded, or poorly drained areas.
Why Is It So Important to Get the Name Right?
Mould, mildew and lichen are often treated in a similar manner, but this approach seldom proves successful. A mildew-lifting bleach-based cleaner may only skim the surface of existing mould, and a chemical solution used to remove mould not sufficiently strong enough for lichen may result in staining to the render. Another sign of mould on exterior walls that hasn’t been resolved may indicate a larger problem, such as leakage, drainage, or wind washing that has left the exterior wall without its waterproof coating; all of which should be corrected instead of just dealing with the visible mould.
As a general guideline, as a homeowner:
- If it comes off easily and has a slight musty odour, then mildew is probably the source.
- If it’s any fuzzy, dark material which reappears and smells strongly damp, that’s mould.
- Is it flat, crusty, slow-growing, and growing on a dry wall? If the answer is yes, it is likely lichen.
Just always remember, if in any doubt, or if the area is quite large, it always pays to get expert advice rather than a guess, and maybe choosing the wrong treatment where there may be a bit of broken render.
Getting Professional Help
We see mould on render, mildew and lichen on exterior walls or cladding regularly, and we can’t treat them all the same, as they all require specific types of treatment. If there’s mould present, we also inspect for the source of moisture triggering the mould growth, as mould will often return within a season unless there’s been a fix to the water penetration or ventilation problem causing it. Blocked gutters are a common culprit — our guide on storm season gutter cleaning covers what happens when that moisture source is left unchecked.
When you have dark patches, black mould on exterior walls, or you simply don’t know what you’re dealing with, it’s important that you get it identified correctly to provide a lasting solution, not a temporary patch-up. We service properties across Adelaide, and if you’d like to see the results of a similar job first, have a look through our recent work. Call +61 415 277 399 or send us a message to get your render or cladding properly identified and treated.