Mould, algae and leaf stain after a wet Australian summer

A wet Australian summer writes on a deck whether you asked it to or not. Drip lines from a verandah beam. The oval of a pot that was never lifted. A green shadow where the fence keeps the sun off. The particular brown-black of eucalyptus leaves that sat in a gap through three storms and a humid week. By March the boards can look older than they are. The temptation is to treat the whole platform as a single disgrace and hire a machine. The more useful response is to sort what you are looking at.

Mould, algae and leaf stain like the same weather — moisture, organic food, not enough sun — and they are not the same substance. One is a living film on the surface. One is a living film that also loves gutters and the north side of a wet fence. One is chemistry, a dye from leaves and from the timber’s own tannins. Cleaning that ignores the difference wastes effort. Cleaning that ignores the cause — shade, drainage, a tree, a clogged gap — books the sequel.

Three stains that share a season

Mould and mildew on a deck are surface growths. They can be black, grey, or that tired green-black that people describe as “the south side”. On timber they sit in the grain and in any raised fibre. On composite they sit in the texture and in the dust the texture holds. They often wipe back, at least in part, when wet. They return if the bay stays damp.

Algae is the greener, sometimes slimier film, the one that makes a shaded step into a skating rink after a shower. It likes constant wet and a little light — the underside of a dripping tap, the bay under a leaking planter, the path that never sees afternoon sun. Algae on a deck is a slip problem before it is an aesthetic one.

Leaf stain and tannin stain are not growths. They are colour moved by water. Eucalyptus leaves are famous for it in Australian yards, but plenty of other species will do, and plenty of imported decking — merbau especially — will contribute its own dark tea. The stain can look like mould and refuse to behave like mould. Brighteners and time do more than biocides.

You can have all three on one platform. The barbecue corner can be grease plus mould. The tree bay can be tannin plus algae at the drip line. The south wall can be mould in a clean, even bloom. Walk after rain, when the maps are honest.

A stain is a record of water’s patience. If you only clean the record and leave the patience, the deck will write it again.

Why Australian summers are good at this

Humidity, heat and sudden water are a generous climate for film. A tropical wet season is the extreme. A temperate summer with a monsoon trough sitting off the coast, or a week of easterly drizzle in Sydney, will do a quieter version. The south-west, dry in summer, does this work in winter instead — which is why a Perth deck can look its worst in July. The biology does not read your calendar. It reads wet timber.

Shade is the multiplier. A beautiful screening and privacy decision can also be a drying decision. A dense screen on the north or west, a low eave, a neighbour’s new two-storey wall, a tree that earned its place in a heatwave — all of these keep boards from the one disinfectant Australian weather still offers for free: sun and moving air.

Drainage is the other multiplier. A deck that ponds, even in shallow dishes of cupping, is a deck with a microclimate. Gaps packed with leaf become wicks. Joist tops that never see air become quiet farms. The face of the board is what you notice. The top of the joist is what you will pay for.

Eucalyptus, tannin, and the tree you are not going to fell

Most Australian gardens that feel like Australian gardens drop something tannic. Leaves sit, wet down, and leave a print. The print is darkest where the leaf sat longest and where the board was most thirsty. An unoiled, weathered hardwood takes stain like paper. A recently cleaned, well-oiled board still takes it, only more slowly, and gives it up more willingly at the next wash.

You do not need to cut a good tree to have a good deck. You do need to decide who wins the weekly argument.

• Sweep or blow gaps on the same rhythm the tree drops, which in a heatwave can be often.
• Do not let a “natural mulch” form along the house junction or in the first metre under the canopy.
• Lift the furniture that sits in the drip line. The stain under a bench is a bench problem as much as a leaf problem.
• Accept that the bay under the tree will always be the first to need cleaning. Build that into the year rather than resenting it each autumn.

Oxalic brighteners can lift tannin from timber when the stain is still mostly in the surface. They will not bleach a deep, old dye out of a board that has been drinking leaf tea for years, and they are not a weekly product. Use them as a specific pass after a wash, rinse them properly, and let the timber dry before you oil. On composite, follow the board’s own rules; some caps dislike acids that timber will tolerate.

The timber’s health is the part owners skip while they argue about colour. A board that is repeatedly wet with leaf litter is not only stained. It is staying wet at the end grain and in the checks. That is the path toward softness, toward the restoration question, toward the joist-top decay you cannot see from the barbecue. Stain is the visible edge of a moisture problem.

Shade, airflow and the south-side problem

In much of the country the south side of a house is a mould factory: less sun, colder boards, dew that lingers. A courtyard that is a well of shade can behave like south no matter which way it faces. So can the strip under a dense pergola that was glazed or covered until it stopped being a pergola and became a room with a moisture habit.

You cannot always add sun. You can often add air and subtract water.

Keep gaps open. Keep the undercroft, if there is one, from becoming a shed of forgotten offcuts and plastic. Do not lay an outdoor rug on a damp bay and leave it for a season; you are building a darkroom for film. If a screen is solid to the deck, consider a gap at the bottom that lets air move across the boards. If a planter sits against the house, it is a wet object against two surfaces that wanted to dry.

Privacy and shade are real needs in Australian suburbs. The design essay on screens is about living with neighbours and with west sun. This essay is the maintenance half of that choice: a shaded deck is a wetter deck, and a wetter deck is a cleaning schedule, not a failure of the species you chose.

Cleaning the film without making a worse one

The method is in the cleaning essay and is worth repeating only where mould changes it. You want to lift the growth, not grind spores into the air as a dry dust, and not drive dirty water into the frame.

Wet the boards first. Use a cleaner that is actually meant for outdoor mould or for decks, not a random bleach from under the sink. Agitate with a brush. Rinse thoroughly. Do the work on a day that will dry the timber, or you will have washed the film into a new, even dampness it will enjoy. Take the runoff seriously if it will hit a garden you care about or a waterway that did not ask for your chemistry.

Algae on steps and on the first metre off the lawn may need more frequent, milder passes rather than an annual punishment. If a step is chronically green, look at the irrigation, the overflow, the shade, and the fall. A non-slip issue that you treat only with cleaner is a non-slip issue you have accepted.

After a wet summer, people skip the dry. They wash in a break between storms and oil the next morning because the surface looks matte. The section is still wet. Oil on that is a way of sealing the season in. Wash, wait, then finish if the finish is due. Put the waiting on the annual calendar as a real step, not as an inconvenience.

Composite deserves a special sentence after a wet summer: it will look as if it has failed when it has only been used as a plate for pollen and spore. Clean the plate. If the colour has blotched in a way that cleaning does not lift, you may be looking at moisture in the board, at a worn cap, or at a stain from metal or leaf that needs a specific product. You are not looking at rot in the sense a joist rots. You may still be looking at a frame underneath that has had a much worse summer than the plastic.

Health of timber, health of people

A mouldy deck is slippery. That is the first practical harm. The second is the temptation to attack it dry with a grinder or a ferocious washer and to stand in the plume. Wet it down. Work upwind if there is a wind. Do not treat outdoor mould as harmless dust.

It is also not helpful to talk about a backyard the way one talks about a sealed bathroom with black mould in the plaster. They are different environments. The move that matters outdoors is the same move that matters indoors: find the water, stop it sitting, let the material dry. A deck that cannot dry — closed undercroft, ponding, permanent shade, irrigation that arcs onto the boards every dawn — will keep a film no matter how expensive the bottle.

For the timber, repeated film is a moisture report. If the same boards bloom after every wet week, they are not getting enough sun or enough air, or they are holding water in cupping and in packed gaps. Probe the ends. Look at the joists. A restoration or a repair decision can start as a mould complaint. That is not drama. That is the climate being literal.

After the wash: change the conditions a little

You will not redesign the house because August was green. You can change a few conditions.

Move the pots through the year. Shorten the irrigation cycle that wets the deck. Put a simple drip line on a downpipe that currently drums onto one board. Clear the screen’s foot. Thin a tree rather than removing it, if thinning is what a decent arborist would do. Choose a mat that can be hung up, or choose no mat. If you are planning shade, plan for the boards as well as for the people: slats that move air, a roof that does not drip in a line, a fall that takes water off the platform.

A wet summer is not a verdict on the deck. It is a season. The boards that come through it looking tired but hard, well fastened, and willing to dry, are boards you can clean and keep. The boards that come through it soft, or sitting over joists that smell like a closed shed, are telling you a longer story. Listen to that story before you buy another tin of something that promises to make March look like November. November, in this country, has its own weather. The deck will meet that too.