Is composite decking slippery?

In shortNot in itself. A clean, well-drained composite deck gives a reasonable grip, and most slips on any deck come from algae, wet leaves and standing water rather than from the board. Before buying, ask for the board's wet pendulum test value and look for 36 or more, the UK threshold for low slip potential.

Composite decking picked up a name for being slippery in its early years, when most boards were a plain mix of wood flour and plastic with no protective shell. The question still gets asked, and it deserves a straight answer rather than a sales line.

Any outdoor floor can be slippery. Wet stone is, wet timber is, and so is wet plastic. What matters is how a surface behaves once it is wet, how quickly it grows algae, and how easily it comes clean. On those three points composite does reasonably well, with the caveats set out below.

How slip resistance is measured in the UK

The British test is the pendulum test, described in BS 7976-2 and in the European standard BS EN 16165. A weighted rubber slider swings across the wetted surface and the friction it meets is read off as a Pendulum Test Value, or PTV. The higher the number, the better the grip.

The Health and Safety Executive and the UK Slip Resistance Group read the result in three bands.

Composite makers test their boards and publish the results, often by board and finish. The figures vary enough between products, and between colours of the same product, that this note does not repeat any of them. Ask for the wet PTV of the exact board you are buying and want to see 36 or more. A number quoted for a whole range is not the same thing.

Why decks get slippery

The board is rarely the main problem. Algae is. A thin green or black film forms wherever a surface stays damp: the north side of a house, under trees, next to a lawn that drains towards the deck, or any low spot where water sits. Wet leaves left over winter do the same job faster.

Grooved softwood is a special case. The grooves on a standard UK softwood board are a traditional pattern, not an anti-slip feature. The Timber Decking and Cladding Association’s guidance is that grooves hold water and algae and that the smooth face sheds water better. A grooved softwood deck that has gone green can fall well below the low-slip threshold.

Uncapped composite shares some of that weakness. Its wood fibre sits at the surface, takes on moisture and gives mould a hold. Capped boards, with a bonded polymer shell on three or four sides, give algae far less to feed on and come clean with less effort. That is the real slip difference in composite, and it is a bigger one than the gap between composite and timber.

Millboard sits apart from both. It is a resin and mineral board with a rubbery Lastane surface rather than a wood-plastic composite, so it weathers and moves differently; judge it on its own test figures.

Composite against timber, honestly

Clean and dry, neither is slippery. Wet and neglected, both are. Composite has one real advantage: a capped board resists the algae that makes an old timber deck treacherous, and it needs no oiling to stay that way. Timber has one of its own: a tired softwood deck can be sanded, re-oiled or fitted with anti-slip inserts, whereas a composite surface cannot be sanded or recoated. If it goes wrong, you clean it or you replace it.

The wider trade-offs, from cost over twenty years to heat and warranties, are in the guide to composite or timber decking.

What to ask before you buy

• The wet PTV for the board and colour, and which standard it was tested to.
• Which face is meant to be walked on, where a board is reversible.
• What the maker says about shaded or north-facing sites.
• How the installer will set a slight fall so water runs off, and how the gaps between boards will stay clear.

A good installer will have the test sheet to hand or know where to find it. The guide to choosing a decking installer lists the other questions worth asking at the quote stage.

Keeping a deck grippy

Most slips are prevented with a brush. Wash the deck twice a year with warm soapy water and a soft brush, clear leaves before they rot, and keep the gaps between boards open so water drains instead of standing. Cut back whatever is casting the shade if you can. The full method, including when a pressure washer is and is not a good idea, is in the note on how to clean composite decking.

If a deck is already slippery and cleaning has not cured it, the options are retrofit anti-slip strips or tape on the steps and main walking lines, or, for timber, replacement boards with resin and aggregate inserts set into the grooves. The TDCA runs a certification for anti-slip decking products, DeckMark Plus, which is a useful filter when comparing them.

A word on heat

The sister complaint about composite is that dark boards get hot in direct summer sun, hot enough to be uncomfortable barefoot. Dark timber warms up too, but composite of the same colour gets noticeably hotter. Lighter colours and a little shade are the answer. No published surface temperature is reliable enough to quote here.

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