Slippery decking: why it happens and how grip is measured

A timber deck can be easy underfoot in summer and slippery after a wet week. The cause sits on the surface, and the grip of a deck can be measured.

Why a deck gets slippery

The Timber Decking and Cladding Association (TDCA) says slippery decks are the result of surface deposits such as mildew, algae and fine mosses. These get worse in wet conditions, as they do on any flat outdoor surface.

Cleaning it without damage

TDCA suggests sweeping a young deck with a stiff brush. An older or neglected deck may need more abrasive cleaning.

A pressure washer can do the work, and some have attachments made for decking. TDCA warns that too high a pressure can damage or bruise the timber. It advises a lower pressure, with the lance held back from the surface.

A scrubbing brush and a mild soap solution are another way. TDCA suggests a dry day, a methodical scrub and a good rinse with clean water. Let the deck dry before any further treatment.

How grip is measured

HSE uses the pendulum test as its standard method for assessing how slippery a floor is, both dry and when contaminated, for example by water. A swinging arm with a standard rubber slider, like an imitation heel, sweeps across the surface. The result is a pendulum test value (PTV).

HSE reads the PTV against these bands, taken from the UK Slip Resistance Group's 2011 guidelines:

HSE says the test needs a trained and competent person to run it and read the result. On a profiled surface, several tests in different directions may be needed, so only an experienced operator should assess it.

The kinds of fix

TDCA's supplier search covers anti-slip deck boards, specialised deck cleaning products and coatings. TDCA also runs DeckMark, a quality scheme that verifies the performance and longevity of accredited materials, or the standards of an approved installer.

A deck at a business

HSE's slips and trips pages are written for employers, workers and designers. Its slip resistance sheet is aimed at employers who need to measure floor slipperiness, and it helps them read a flooring maker's test data.

Building a new deck

In England, the Planning Portal says a garden deck is permitted development, with no planning application, as long as it stays within three limits. It must be no more than 30cm above the ground. Decks, extensions and outbuildings together must cover no more than half the garden. And no part of the deck can sit in front of the wall that forms the main front of the house.

Those allowances are for houses, not flats or maisonettes. In the grounds of a listed building, no deck is permitted development. In National Parks and other designated areas the limits are tighter, and in some places a planning condition or a local direction removes the rights altogether. TDCA gives the same height and garden limits for Wales, and says the rules vary across the UK. It recommends asking the council's planning department about anything beyond a simple low-level garden deck.

GOV.UK treats building regulations approval and planning permission as separate, and a deck can need both. The Planning Portal advises assuming that building regulations apply to any deck that needs planning permission.

Useful links