Composite decking is sold as the material you never have to look after, which sets up two disappointments: it does get dirty, and it grows mould like everything else in Sydney.
It also comes with warranty conditions that are considerably more specific than timber, and ignoring them is an easy way to lose a 25-year warranty on a deck that cost a great deal more than hardwood.
What composite actually is
Wood-plastic composite: wood flour or fibre bound in a polymer, usually polyethylene, with UV stabilisers and pigment.
The important distinction is uncapped versus capped. Uncapped boards — mostly older generation — have the composite material exposed on all faces. Capped boards have a polymer shell bonded over the core, which resists staining and fading far better.
Uncapped composite still contains wood, and wood feeds mould. This is why people with uncapped decks are surprised to find exactly the growth they were told they had escaped.
What the warranties say
Specifics vary by brand, so read yours. The common conditions:
- Pressure limits. Most permit a pressure washer but cap it, commonly around 1500 psi, with a fan tip held at a stated distance and kept moving. Exceeding it typically voids the warranty and leaves permanent marking.
- No abrasives. No wire brushes, no scouring pads, no sanding. Abrading a capped board breaches the cap, and once the core is exposed the board stains and weathers from there.
- Approved cleaners only. Usually a mild soap or a composite-specific product. Many explicitly prohibit chlorine bleach, acid, and solvent-based cleaners.
- No deck oil, stain or paint. Composite is not meant to be coated and most warranties exclude boards that have been.
- Documented maintenance. Some require periodic cleaning as a warranty condition.
This is the same principle as our piece on what manufacturers actually say about cleaning their own products — the document that governs the claim is the one nobody reads until there is a problem.
The routine clean
Twice a year suits most Sydney decks, more in heavy shade.
- Clear the deck and sweep thoroughly, including between boards
- Rinse with a hose
- Apply warm water with a mild soap, or a composite deck cleaner
- Scrub with a soft bristle brush, always along the board direction, never across the grain pattern
- Rinse completely, because residue left behind shows as streaking
If you use a pressure washer, stay well under the stated limit, use a wide fan tip, hold it back, and keep it moving. Honestly, a brush does a better job with none of the risk.
The mould problem
This is the complaint we hear most about composite in Sydney.
Mould on composite is usually growing on the surface film — pollen, dust and organic debris sitting on the board — rather than in the material. On uncapped boards it can also feed on the wood content.
It is worst in shade, where airflow is poor, and in the grooves of a textured board where debris collects.
Treat with a composite-safe mould cleaner, or an oxygen bleach solution, which most manufacturers accept. Not chlorine bleach unless your warranty specifically permits it — it can lighten the pigment unevenly and is a common exclusion.
Prevention matters more than treatment: keep the deck swept, keep debris out of the gaps between boards, and keep the underside ventilated. A composite deck with blocked airflow underneath stays damp and grows mould from below.
Specific stains
Tannin from leaves. Common and usually comes off with a deck cleaner if treated reasonably promptly.
Rust from furniture feet or fixings. Needs a composite-safe rust remover. Oxalic acid works on timber but check it against your warranty before putting it on composite.
Oil and food. Capped boards release these readily. Uncapped boards can stain permanently, so clean spills quickly.
Ice melt and harsh chemicals. Not a Sydney concern, but pool chemicals are. Spilled chlorine granules on a composite board will bleach it.
Scratches. Not fixable. Composite cannot be sanded back the way timber can — sanding removes the cap and makes it worse. This is the real maintenance trade-off against hardwood, where a sand and re-oil resets the surface entirely.
Fading
All composite fades in Sydney UV, most of it in the first year or two, then it stabilises. This is normal and generally covered as a limit rather than a defect — warranties usually specify a maximum fade over a period rather than none.
It cannot be restored. Painting or staining to recover the colour voids the warranty and creates a coating that will peel.
The underside
Worth a look once a year, because it is where problems start: debris packed between boards, poor ventilation, moisture trapped against the joists, and on an older deck, rusting fixings.
For a professional clean within manufacturer limits, see deck cleaning. Bring the warranty document — on composite, that is the specification we work to.
