Water accumulating after rain along with general dirt and debris build-up can make your deck slippery. This is a hazard to your family and a potential lawsuit, if a visitor falls. The best way to avoid a slippery deck is to keep water off and clean it. Decks require regular maintenance--sometimes daily depending on the weather and the location and condition of your deck. What you expend in time, you reap in peace of mind.
Sweep the deck surface as often as you notice debris--especially after a storm--using a stiff broom to remove dirt and excess water.
Spray the deck at least once a year, usually in the spring or fall, with a garden hose outfitted with a sprayer to remove mildew, moss, algae or other deposits.
Apply a deck cleaning product recommended by your decking manufacturer to remove tough stuck-on build-up. This Old House recommends an oxygenated-bleach cleaner in lieu of a deck cleaner. Follow the specific application directions provided with the product.
Scrub the deck surface with a soft scrub brush after applying the cleaner.
Rinse the surface with the garden hose sprayer after cleaning.
Avoid washing the deck in the summer, as the heat and the water will stress the wood.