How to clean composite decking (and what will damage it)

Composite decking does not need staining, oiling or sanding, which is the reason most people buy it. It does need cleaning, because anything left outdoors in Britain grows algae in the shade and collects grit in the sun. This guide is the routine we give our own customers: what to do twice a year, what to do the same day something is spilt, and the two tools that will damage a composite board.

The short answer

Sweep it when leaves fall, wash it with a stiff brush and warm soapy water in spring and autumn, and wipe grease and wine the day they land. Hose it, do not jet wash it. That is the whole regime, and a deck treated this way looks the same in year ten as in year one.

Twice a year: the proper wash

1. Clear it. Furniture, pots, the barbecue. Sweep off leaves, seed heads and grit with a soft broom; grit is what scratches, so get it off before you start scrubbing.
2. Wet the deck with a hose so the cleaner does not dry on it.
3. Wash with a stiff-bristled brush (not wire) and a bucket of warm water with washing-up liquid, or a composite deck cleaner from any DIY shed. Brush along the grain of the boards, or along the grooves if the grooved face is up. Work in sections you can rinse before they dry.
4. Rinse thoroughly with the hose. Soap left to dry leaves a film that attracts dirt.
5. Let it dry before the furniture goes back, and put felt pads under the feet while you are at it.

An hour for a 3.6m by 3m deck. Do it in April when the pollen and winter algae are off, and in October when the leaves are down.

The same day: spills

• Grease and barbecue fat: warm water and washing-up liquid on a cloth, straight away. On an uncapped board, fat left for a week soaks in and leaves a shadow that takes a summer to weather out. A mat under the barbecue avoids the problem.
• Red wine, ketchup, sun cream: same. Soapy water, cloth, rinse.
• Bird droppings: soak with water first so you are not grinding it in, then wipe.
• Rust from metal furniture feet: a deck cleaner containing oxalic acid shifts it. Then felt pads.
• Chalk marks, mud, footprints: the hose, or wait for rain.

Green algae and black mould

A north-facing deck, a deck under trees or a deck against a wet fence will grow a green film over winter. It is on the surface, not in the board, and it comes off with the spring wash. If it is heavy, use a composite deck cleaner or a diluted outdoor patio cleaner and give it ten minutes before brushing. Black spots are mould feeding on leaf debris; the same treatment clears them.

To slow it down: keep the 40mm airflow gap under the deck clear, trim back anything overhanging, and sweep leaves before they rot. Algae needs standing damp and food; deny it both. The problems guide covers why some decks grow more than others.

What not to use

• Pressure washers. The most common way a composite deck is ruined. At close range the jet lifts the surface texture, leaves permanent lighter streaks and can force water into a hollow core. If you must, keep the fan nozzle at least 30cm off the surface and under 1,500 psi, and move constantly. A hose and a brush does the same job without the risk.
• Wire brushes and abrasive pads. They scratch through the embossed grain.
• Bleach, undiluted. Lightens the board unevenly. Diluted 1:10 it is acceptable for a stubborn mould spot, rinsed well.
• Solvents, acetone, paint stripper. They attack the plastic in the composite.
• Sanding. Removes the surface finish and exposes a rougher core that then stains. If a scratch bothers you, wait: it weathers to match in a few months, and the grooved face hides it anyway. The face guide explains.

Snow, ice and winter

Shovel with a plastic shovel, not metal. Rock salt is fine on composite (it is not on timber). Do not chip ice off; let it melt or salt it. Grit sand for grip gets swept off in spring. More on winter grip in the winter guide.

Does it ever need sealing?

No. Composite is not porous the way timber is, and sealers sit on the surface, peel and make cleaning harder. Some people apply a composite deck brightener after a few years to freshen the colour; it is cosmetic and optional. The colour change that happens in the first six months is the board settling, not fading, and no product reverses or prevents it; see does composite decking fade.

The routine on one line

Sweep when there are leaves, wash in April and October, wipe spills today, hose not jet. That is it. Everything else is what timber owners have to do and you do not.