Cleaning green algae and grime off decking boards

Why green algae forms on decking and where it spreads fastest

Algae needs damp and shade, and Estonian weather supplies both for most of the year. Long autumns, wet snow that sits on the boards through winter, and short summer days mean decking rarely gets the sustained dry heat that would stop growth before it starts.

The worst spots are north-facing terraces, areas under trees or overhangs, and the gaps between boards where leaves and needles collect and hold moisture. A terrace that gets full sun most of the day tends to stay far cleaner than one tucked against a fence or a wall.

Composite decking is not immune. It does not rot, but the surface texture holds moisture in the same way, and algae will grow on composite just as readily as on wood if the spot stays shaded and damp.

What tools and cleaning agents you actually need

For a light film, a stiff outdoor broom and warm water with a mild detergent will often do the job. For anything established, you want a proper deck cleaning agent, sold at most Estonian hardware shops, formulated to lift algae and green growth from wood or composite without bleaching the surface.

Avoid plain household bleach or strong chlorine solutions. They can lighten wood unevenly, corrode any metal fixings, and are harsh on garden plants and soil if they run off the edge of the terrace.

A stiff synthetic-bristle brush is the main tool for working the cleaner into the grain. Wire brushes are too aggressive on softwood decking and will scratch composite boards, so leave them for stonework.

• Stiff synthetic-bristle deck brush
• Deck or terrace cleaning agent (wood or composite formula, as needed)
• Bucket, garden hose, and a watering can for rinsing
• Rubber gloves and eye protection
• A pressure washer with a fan nozzle, if you have one
• Plastic sheeting to protect nearby plants from run-off

Setting a pressure washer without gouging the boards

A pressure washer speeds things up but is also the single easiest way to ruin a terrace. Use a wide fan nozzle, never the narrow turbo or pinpoint nozzle, and keep the wand moving constantly rather than holding it on one spot.

Work at a moderate pressure setting and hold the nozzle at least 20 to 30 centimetres from the surface, testing on an inconspicuous board first. If you see the wood fibres lifting or fraying, you are too close or too strong.

Always spray along the grain, not across it or straight down into the joints, and pass over each board once rather than dwelling on stubborn patches. Repeated passes at close range are what leave the fuzzy, striped marks you sometimes see on older decking.

The cleaning sequence from start to finish

Start by sweeping off loose debris, leaves, and grit, since this stops you grinding dirt into the wet surface later. Move furniture and plant pots clear so you can reach every board edge to edge.

Wet the decking first, then apply the cleaning agent according to the instructions on the bottle, working in manageable sections rather than the whole terrace at once. Leave it for the stated time, usually ten to fifteen minutes, so it can break down the algae before you scrub.

Scrub with the deck brush, working along the grain and paying attention to the gaps between boards where growth is often thickest. Rinse thoroughly with the hose or pressure washer on a low setting, and let the terrace dry fully before walking on it or putting furniture back.

The most common mistake, and when cleaning alone is not enough

The mistake most people make is reaching straight for the pressure washer at full pressure and skipping the cleaning agent altogether. It looks effective at first because the green disappears, but repeated high-pressure blasting wears down the surface fibres, opens the grain, and leaves wood more likely to soak up water and grow algae back faster than before.

If cleaning brings up soft, spongy patches, boards that flex underfoot, or screws that have pulled loose from rotten wood underneath, the problem has moved past algae and into structural decay. That is a repair job, not a cleaning one, and it is worth having someone assess it before the damage spreads to neighbouring boards.

Once a terrace is clean and properly dried out, it is also the right moment to think about oiling or varnishing, which is a separate job from cleaning and covered in its own guide. If you would rather not spend a weekend scrubbing and rinsing, our decking and terrace service starts from 90 € per job, and for apartment associations or offices with outdoor areas to keep on top of, our building maintenance callouts start from 49 € per hour.