Warm, wet, shady. That's all mould and algae need, and the north coast supplies plenty of all three.
Where it shows up first
South and shaded sides of houses, under trees, along fences that never get sun, and in the cracks between pavers. Timber stays damp longer than concrete, so decks and fences go green first. The slippery film isn't just ugly, it's a fall risk on wet steps.
Fixes you can do yourself
- Sweep leaves off regularly. They hold moisture against the surface.
- Trim back plants and hedges so air and sun can reach the area.
- Scrub small patches with a stiff brush and a mild cleaning solution made for outdoor surfaces. Follow the label.
When a pressure clean makes sense
When it's spread over a large area, or the surface is too rough to scrub. It gets the grime out quickly, but the pressure has to match the surface. Too much on timber leaves furry, scarred boards. See pressure cleaning for how we handle different surfaces.
Doing it yourself, safely
If you've got a pressure washer and want to try it, go gently. Use a wide fan tip, not a narrow jet, and keep the nozzle moving along the grain of the timber. Start with a test patch somewhere out of sight and keep the nozzle a decent distance back. Too much pressure or too close a distance raises the grain, leaves marks and can splinter softer timbers. Check any loose boards and nails first, since they lift easily.
For composite decking, follow the manufacturer's advice. Some brands tolerate more pressure than others, and the wrong approach can void a warranty.
Timber, composite or concrete?
Timber grows mould easily and is the most easily damaged by cleaning. Composite resists rot but still grows surface mould and algae in the shade. Concrete and pavers take more pressure, but pavers can lose their joint sand if blasted. The surface decides the method.
Let it dry before you seal
If you plan to oil, stain or seal a deck after cleaning, the timber has to dry out first, which can take a day or more depending on weather and shade. Sealing damp timber traps moisture and the finish can fail.
Mind the drain
In most places the stormwater drain goes straight to a creek or the ocean, untreated. Don't wash detergents or strong chemicals into it. Rinse onto lawn or garden where the soil can filter it, and keep it away from sensitive plants.
Keep it from coming back
You won't stop it entirely. But sealing a clean deck, keeping gutters flowing so water isn't dumped on it (gutter cleaning), and cutting back overhanging growth all help. Mould returns faster in shade than in sun, which is worth knowing when you decide how often to clean.
Part of the wet season checklist.
Is it dangerous?
Mostly it's a slip hazard and a cosmetic problem. If mould is inside the house, on ceilings or walls, that is a different matter and worth getting advice on from a building professional.
What to ask for in a quote
Say what the surface is (timber, composite, concrete, pavers), roughly how many square metres, and whether you plan to seal or paint afterwards. Send a photo.