Although our decking is profiled, not normally the case in Japan, it can become slippery with algae and even mosses. This needs to be frequently removed e.g. with a wire-brush when dry, or a power hose when wet. These tools are advised and used by many. So far we used noble handwork for this.
The photo shows my basic equipment. In all these years I managed to keep the electric and pneumatic tools out of the garden. First of all because it still does not feel right in a Zen garden. Secondly because cleaning-up the mess left after using these tools may be more laborious than manually scrubbing the deck.
The cleaning product we use is a regular detergent, like the well known Blue Wonder, solved in hot water.
With regard to timing I can say that I preferably do this after a couple of rainy day’s. Then the alga gets easily removed with only little cleaning fluid. After the scrubbing, the deck gets sprayed clean with water using a garden hose. Depending on the area size of the dech that needs cleaning I may also use a rough broom.
Maintenance-hint: An easy way to remove algae and mosses from the veranda wood, or to at least keep control on a regular basis, is to spray the deck with household vinegar.
Only do this when you do not expect rain for a couple of days. Then let it do its job for a couple of days and after the alga has died, broom it with a rough broom. That’s it.
Note: This will also work to get rid of lichen from the roof-tiles and other places where it is unwanted.