Slippery Decking: Making It Safe Again

Why decking becomes dangerously slippery

Decking is porous, organic timber, which is exactly the kind of surface algae and cyanobacteria love. A thin biofilm builds up gradually, often unnoticed while the boards are dry, because it looks like nothing more than a slight darkening or a faint green tinge. Add water, whether from rain, dew, or a garden hose, and that film turns into something closer to wet soap on wood.

North or east-facing decking, areas under trees, and decking near ponds or water features are worst affected because they stay damp for longer and get less direct sun to dry the film out. This is why the same deck can feel fine in July and become a genuine hazard by October.

Cleaning decking safely without damaging it

The instinct is to blast it with a pressure washer on full power, but that is usually the wrong move. High pressure at close range can strip the fibres of softwood decking, raise the grain, and leave a rougher, splintery surface that traps dirt even faster next time. Composite decking can scar or discolour under an aggressive lance in the same way.

The better approach is a proper decking cleaner or diluted algae treatment applied first, left to dwell for the time stated on the product, then agitated with a stiff (not wire) brush before a gentle rinse at low pressure or with a hose. This lifts the biofilm rather than blasting a groove into the wood. For larger or badly affected decks, a soft wash approach with a biocide gets into the grain and knocks back regrowth for longer than a rinse alone.

Anti-slip options once it is clean

Cleaning solves the immediate problem but not the long-term one, so it is worth pairing it with an anti-slip measure. Self-adhesive anti-slip decking strips or tape work well on steps and high-traffic routes. Decking oils and stains with a fine grit additive give grip across the whole surface while still looking like a normal finish, and they double up as a protective coat that slows algae regrowth.

For older, softer decking that is showing its age, a light sanding before oiling opens the grain back up and improves both grip and appearance in one job.

Keeping it that way

Trimming back overhanging shrubs and branches lets more light and air onto the boards, which slows algae far more effectively than any product. A yearly clean before autumn, when falling leaves and damp weather combine, keeps the problem from ever building up to a dangerous level again.