Cleaning algae on composite deck
Compared to painting or staining your wooden decking floor every year or two the composite decking is a great low-maintenance alternative and holds up longer. But nothing is entirely maintenance-free. All exterior building materials gets dirty and can develop biological growth stains, Green Plank deck is no different, it's important to clean it properly.
The greenish discoloration you're seeing on the decking is probably not mildew, which is usually dark grey or black in color, and doesn't survive well in the open air. More likely it's lichen, algae or moss growth. Exact identification isn't important, however, because getting rid of it involves only one remediation methodology.
You'll need to remove what you can of the organism, disrupt its growth, and get rid of its food source at the same time. If clean, our composite decking won't support life forms. But “food” in the form of dirt, pollen and other pollutants fall out of the sky and onto surfaces like decks. And that's what supports algae or lichen growth.
We do not usually recommended pressure washing our composite decking, cladding or fencing products. Pressure washing also risks to spray particles of soap on other surfaces. If done incorrectly, it can be a big mistake. Pressure washing too close or using too much pressure can be ruinous for your composite product. Once the material has been "opened up", this can lead to a host of other issues including chipping, sun damage, and mold staining of the plastics inside the opening. Once mold has stained these plastics, it can be extremely hard to remove. In most cases, a garden hose should be sufficient for regular duty cleaning. If you must pressure wash, just remember to use a household pressure washer with "spray nozzle", keep it at the lowest pressure possible and no closer than 25 cm from the surface. So a gentler approach would be to use manual scrubbing and a hose for rinsing.
Choosing the right cleaner is important. When purchasing a cleaner, always check the label and NEVER use a Chlorine Bleach based "cleaner". If the active ingredients read – Sodium Hypochlorite, it contains Bleach. Chlorine Bleach is extremely corrosive and will "eat" any material it's applied to. Eventually, it will discolor your deck, damage the surface (not to mention your plants). Avoid this costly mistake and choose a non-toxic, sodium percarbonate based cleaner.
Sodium percarbonate based cleaners are highly effective because when mixed with water they create a bubbling action much like "alka-seltzer" and when applied to composite decking these bubbles will actually dissolve and lift any organic material to the surface which can be easily cleaned off.
Cleaning is simple, apply the mixture to the affected area of the deck, allow it 15 minutes to penetrate and to start breaking up the stains, while the area is still wet take a deck scrub brush and in circular motions scrub the spots out, when cleaning is complete rinse the area well with water.
The key, when cleaning is to clean small sections at a time and always keep that area wet with the mixture while scrubbing, if the area dries before the cleaning is complete the organic matter could settle back below the surface and the process would have to begin again. Be sure to scrub well between the boards to get at anything hiding in there. Do several boards and then rinse with a hose. Repeat until you get to the end of the deck. Give it a few hours to dry and then move your furniture back into place.
At least some green discoloration will likely return in a year or two. While you should try to hose and broom off your decking floor several times a summer to keep it clean, nothing can stop the eventual return of these types of biological growths. So it's best to plan on doing this job periodically. It is “low-maintenance, not no-maintenance”. But compared to chipping and scraping paint every year, it's really not much hard work.