It's one of the most common things people notice about an older or poorly built deck: fine in summer, dangerous in a wet autumn. The cause is rarely one single thing, which is why a quick clean often only buys a few weeks.
What causes decking to turn slippery in shade?
Algae and moss grow on timber that stays damp for long stretches without direct sun to dry it, and Bournemouth's wetter months make that worse.
The Met Office's nearest official station, Hurn, averages around 877mm of rain a year across 1991–2020, with November the wettest month at roughly 108mm. A deck under mature tree canopy, or on a north-facing wall, holds that moisture on the surface for longer between dry spells than a deck in open sun, and algae takes hold on a damp surface far faster than a dry one. Once it's established, it's untidy and a genuine slip hazard underfoot.
Does material choice change the risk?
Yes, to a degree. A capped composite surface generally drains and dries faster than untreated softwood, so it holds less of the moisture that algae needs.
Composite decking is engineered to drain better and absorb less water than timber, which means it's less likely to hold the standing moisture algae relies on. It doesn't make a shaded deck immune, but it changes how often it needs attention compared with untreated softwood in the same spot. See the composite vs timber guide for the fuller comparison.
Does the frame or fall matter, not only the boards?
Yes … a deck built with the correct drainage fall sheds water faster than a flat one, whatever the board material.
A deck built flat, or with boards laid straight on top of a frame without proper joints, holds water in low spots rather than shedding it. A properly jointed frame, built with a slight fall away from the house, drains faster and gives algae less to work with. This is a build-quality question as much as a material one.
Can an existing slippery deck be treated rather than replaced?
Often, yes. Cleaning and re-treating can improve a tired timber deck, but if the frame underneath is already rotting, cleaning only buys time.
Pressure-washing and re-treating (or a proper decking cleaner and stiff brush, rather than anything that damages the wood fibres) removes the surface algae and can meaningfully improve grip. It won't fix a frame that's already rotting or sinking underneath, though, and on an older deck it's worth checking the subframe condition before spending on a cosmetic clean.
Which Bournemouth gardens see this most?
Shaded gardens under mature tree canopy, and north-facing spots that get little direct sun, are the most exposed, which in practice means the wooded inland suburbs.
Talbot Woods and Branksome Woods are heavily wooded, so more gardens there sit under mature canopy and stay damp for longer between dry spells. Mature trees in those suburbs may also carry a Tree Preservation Order. Work affecting one needs written permission from BCP Council first, and doing it without permission is a criminal offence. See the guide to building near protected trees for that detail if a TPO tree is part of the picture.