Composite decking is sold as maintenance-free, grows mould anyway, and can absolutely be damaged by a pressure washer — a combination that surprises nearly everyone who owns it. Safe deck cleaning on Trex and its cousins is a matter of following the manufacturer’s numbers.
Why ‘maintenance-free’ grows mould
Composite boards are wood fibre in a polymer matrix, usually with a harder cap layer. The embossed grain texture that makes them look like timber also gives organic material somewhere to sit: pollen, leaf tannin and airborne spores settle into the pattern, and in shade the surface grows mould exactly like wood does.
It cleans off readily — the material is genuinely easier to clean than timber — but it does need cleaning, which is the part the marketing skips.
The warranty numbers
Most composite manufacturers publish care guidance with a maximum pressure and a minimum nozzle distance, and several warranties are conditional on following it. Exceed the numbers and the cap layer can be permanently marked — often as a lighter, roughened patch that no product will restore.
The safe approach is the one we use anyway: an appropriate cleaner, soft brush agitation in the grain direction, and a low-pressure, wide-fan rinse from a respectful distance. The chemistry does the work; the water rinses.
The two composite-specific problems
Early-generation uncapped boards stain deeper and may retain some tannin shadowing after cleaning — honest expectations apply. And spot-cleaning one bad area often leaves a visibly cleaner patch, which is why the right unit of work is the whole deck, not the stain.
Know your board brand if you can. It changes the chemistry we choose and what we promise about the result.
Tell us what the deck is made of when you send photos — composite and timber are quoted as different jobs because they are. Read more about deck cleaning →