Dark patches on a shady driveway, green film on pool surrounds and black spots creeping up a garden wall are common in Gauteng, especially after the summer rains. Some of this is mould. Much of the green and black growth on outdoor paving is actually algae, moss or lichen. All of them need the same thing to grow: moisture that doesn’t dry out. This guide explains what you’re looking at, what the evidence says about health, and how to deal with it safely.
Mould, algae, moss or lichen?
- Mould is a fungus. It looks like fuzzy or slimy black, grey, green or white spots and often smells musty. Outdoors it shows up on damp walls, in grout and on surfaces that stay wet and shaded.
- Algae forms a thin green or black film, or a powdery deposit, on paving, concrete and walls. It grows fastest in wet, shady spots with poor air flow.
- Moss grows as green cushions, usually in paving joints and along shaded edges.
- Lichen appears as flat grey, yellow or orange crusts. It is a partnership between a fungus and an alga, and it is very hard to shift.
The UK’s Royal Horticultural Society notes that these growths don’t damage the surface they grow on, but they do make paths, patios, drives and steps slippery. On paving that people walk on, slips are the most practical everyday risk.
Is it harmful to health?
Mould is everywhere, indoors and outdoors, and contact with it doesn’t make everyone ill. According to the US Centers for Disease Control and Prevention (CDC), exposure to damp and mouldy environments may cause a variety of health effects, or none at all. Some people get a stuffy nose, sore throat, cough, wheeze, itchy eyes or a skin rash. People with asthma or mould allergies can react more strongly, and people with weakened immune systems or chronic lung disease are at higher risk.
The World Health Organization’s guidelines on dampness and mould focus on indoor dampness. They link damp, mouldy homes with respiratory symptoms and asthma, and recommend preventing persistent dampness and mould growth in buildings. Outdoor growth on a driveway is far less of a concern than mould inside your home. But damp walls, leaking gutters and poor drainage can let moisture into the house, so it’s worth dealing with the cause.
If you or someone in your home has symptoms you think may be linked to mould, speak to a doctor. This article is general information, not medical advice.
Why it keeps coming back
Growth on paving and walls almost always comes down to moisture that stays put. The usual causes are:
- shade from trees, walls or carports;
- poor drainage or low spots where water pools;
- sprinklers that wet the same area every day;
- leaking gutters, downpipes or taps;
- soil and debris packed into paving joints.
The US Environmental Protection Agency puts it simply: the key to mould control is moisture control. Clean it off without fixing the damp and it will return.
Practical prevention
- Fix leaking gutters, downpipes and taps, and point downpipes away from walls and paving.
- Keep drains and channels clear of leaves and soil.
- Prune overhanging plants to let in sun and air.
- Adjust sprinklers so they water the garden, not the driveway or the wall.
- Sweep paving regularly with a stiff broom, so spores and organic debris don’t settle in the joints.
- Keep block-paving joints properly filled with sand, which leaves less space for moss and weeds.
- Indoors, ventilate bathrooms and kitchens, and dry any leak or flooding within 24–48 hours, as the CDC and EPA recommend.
Cleaning small areas yourself
Small patches can usually be handled at home. The EPA suggests that if a mouldy area is smaller than about 10 square feet (roughly 1 m²), most people can clean it themselves.
- Scrub hard surfaces with water and detergent and a stiff brush, then rinse and let the area dry.
- If you use bleach, dilute it as the label directs, and never mix bleach with ammonia or other cleaners, because it releases toxic gas (CDC).
- Wear gloves and eye protection, and keep bleach runoff away from plants, lawns, pools and ponds.
- Take care with a home pressure washer. Too much pressure, or a nozzle held too close, can etch concrete, damage soft bricks and blast the sand out of paving joints.
When to call a professional
- The growth covers a large area of paving, a whole driveway or a long stretch of wall.
- It keeps coming back within weeks of cleaning.
- The paving is slippery and older family members or children use it every day.
- The bricks are old, soft or sealed, and you’re worried about damaging them.
- The joints have washed out and the bricks are starting to move.
- There is mould inside your home, or the damp is coming through a wall. In that case, also have a builder or plumber find and fix the moisture source.
For paving, we use high pressure cleaning to lift algae, moss, mould and grime from the surface and joints. We then re-sand block paving with dry kiln sand so the joints are full again, and can re-seal it in a matt or satin finish once it is clean. We can’t promise growth will never return, because that depends on shade and moisture. But a clean, properly sanded surface is far easier to keep that way.
Want a quote? WhatsApp a few photos of the area and your suburb to 082 956 4085, call 010 534 8472 or use our quote form.
Sources
- World Health Organization (2009). WHO guidelines for indoor air quality: dampness and mould.
- US Environmental Protection Agency. A Brief Guide to Mold, Moisture and Your Home.
- US Centers for Disease Control and Prevention. Mold: what to know.
- Royal Horticultural Society. Algae, lichens, liverworts and mosses on hard surfaces.