How to clean composite decking

In shortWarm soapy water and a soft brush, twice a year, is all most composite decking needs, with a hose to rinse. A pressure washer is accepted by most makers if you use a fan tip, keep it moving and stay about 300 mm off the surface at a modest pressure. Never sand, oil or stain it, and clear grease and leaves quickly on uncapped boards.

The selling point of composite decking is that it needs no oiling, staining or sanding. That is true. It is not the same as needing no attention. A composite deck still collects pollen, leaf litter, bird mess, barbecue fat and the green film that grows on anything damp in a British garden. Left alone for a few years it looks tired and becomes slippery.

The work is light, though. Two washes a year, a quick response to spills and a careful hand with the pressure washer will keep most decks looking much as they did when fitted.

The twice-a-year wash

Pick a dry day in spring and another in autumn. Sweep the deck first and poke out anything wedged between the boards, because blocked gaps stop water draining and are where mould starts.

Then:

1. Wet the deck with a hose.
2. Mix warm water with a little washing-up liquid or a cleaner the board maker approves. Work one area at a time so the soap does not dry on.
3. Scrub with a soft-bristled brush, following the grain pattern of the board.
4. Rinse well. Soap left behind leaves a dull film.

That is the whole job for a capped board. Uncapped boards, the older style with wood fibre exposed at the surface, hold dirt more and may need a second pass.

Avoid wire brushes, scouring pads and anything abrasive. Avoid bleach or chlorine cleaners unless the maker’s guide lists them, because they can lighten the colour. Check the care sheet before trying a new product; a warranty claim can turn on it.

Can you jet wash composite decking?

Yes, with care, and with the maker’s limit in front of you. Most composite brands accept a pressure washer on these conditions: a wide fan tip rather than a pencil jet, the lance kept moving, a distance of about 300 mm from the surface, and a pressure at or below the figure in their care guide. Those figures differ a good deal from brand to brand. Some UK makers warn against pressure washing uncapped boards at all, because a close jet can etch the surface and leave permanent pale streaks.

So the rule is: fan tip, keep moving, stay back, and read the brand’s number before you start. Treat the pressure washer as a rinse for a deck that has already been scrubbed, not as a substitute for the brush.

If you have no pressure washer, you do not need one. A hose, a bucket and a deck brush do the same job with more effort.

Grease, mould and other stains

Barbecue fat and food spills should be dealt with promptly, and within a week on uncapped boards, before they soak in. Hot soapy water and a brush lift most of it. Capped boards shrug off far more, which is one reason they cost more.

Black spots and green film are mould and algae feeding on organic dirt. Wash them off as above. If they keep returning in the same place, the cause is standing water or shade: check that the deck drains and that the gaps are clear. Persistent algae also makes a deck slippery, which is covered in the note on whether composite decking is slippery.

Rust marks from furniture feet and chalky rings from plant pots are cosmetic; a composite deck cleaner usually shifts them. Stand pots on feet so water can get underneath.

What not to do

• Do not sand composite. It removes the finish and, on a capped board, the shell.
• Do not oil, stain or varnish it. Composite does not absorb them and they sit on the surface as a sticky film.
• Do not paint a capped board. It tends to void the warranty; the note on painting composite decking sets out the exceptions.
• Do not use a metal shovel to clear snow. A plastic one is fine; check the care sheet before using salt.
• Do not leave rubber-backed mats down for long periods. They trap moisture and can mark the surface.

Does composite decking fade?

A little, and mostly at the start. Uncapped boards lighten noticeably in their first two or three months outdoors as the surface wood fibre weathers; makers describe this as the board reaching its final colour, and samples are often shown in both states for that reason. After that the colour is fairly stable.

Capped boards fade far less because the pigment sits in a protective polymer shell, and the main brands back that with fade and stain warranties; Trex and Millboard both publish 25-year residential warranties. If colour matters to you, buy a capped board and read what the warranty says about fading before you sign.

Cleaning does not cause fading. A heavy-handed pressure washer can, by scouring the surface.

Timber for comparison

A softwood deck asks much more: a clean every spring, 48 hours to dry, then oil or stain, repeated every one to two years, plus preservative on any cut ends. Hardwood needs oiling once or twice a year to hold its colour or can be left to silver. That difference in effort, and in cost over twenty years, is weighed in the guide to composite or timber decking.

When cleaning is not enough

If boards have cupped, cracked or gone spongy, or the frame moves underfoot, the problem is below the surface and washing will not fix it. A sound frame can often take new boards once the joist centres have been checked against what the new board needs; see the note on decking joist spacing.

When you are ready for quotes, the town pages list installers whose websites we check: all towns.