It is nice to have shade on the deck in the hottest summer days. But algae like it too. When the weather is humid, algae can grow rapidly over the areas that fall outside the drying effect of sun and wind. Failing to treat algae on a wooden deck can shorten the life of the wood. When pools or ponds are near the deck the conditions are even better for the algae. You know you have algae when the wood has black or green streaks. Then is the time to take quick action.
Trim all trees and bushes that are blocking the sun from the deck. Sweep the deck thoroughly. Remove all furniture, grills and toys from the deck. Add 4.5 litres (1 gallon) of warm water to a clean plastic bucket. Add 170 ml (6 ounces) of powdered oxygen bleach. Mix it well and wait until the crystals dissolve completely.
Fill the hand pump sprayer with the bleach solution. Start with the areas most affected by the algae and spray them liberally with the solution. Allow this to set for 15 minutes. Scrub the affected wood with the nylon brush as vigorously as possible for 10 minutes. Let the bleach stand and continue scrubbing for five more minutes.
Set the spray nozzle on the hose to a high-pressure setting. Carefully rinse away the bleach solution and the algae. The water that runs off the deck should be greenish. Let the deck dry overnight. Repeat the process the next dry day, and again, until the algae is gone.