University Park, Md.
A: Trex composite decking comes in two basic types: the vintage kind you almost certainly have, which was marketed beginning in 1998 and consists of a uniform mixture of wood fibers and plastic, and newer kinds made beginning in 2010 that have a shell that caps each board on the top and sides. The cap protects the underlying composite material and allows the manufacturer to add grain pattern and more color than is in the core.
As you have already discovered, some cleaning methods that are okay for the newer products aren’t recommended for the older ones because they are too harsh and will damage the boards. “There are also cleaning methods that we used to recommend on the early boards that we do not now for the same reason,” Rochelle Thurston, a customer care representative for Trex, said during an email exchange.
Homeowners who don’t know the age of their decking have a couple of ways to identify which type they have. “They can tell these two generations apart by looking at an end cut on the boards to see if they are the same color from top to bottom or if a thin top layer is one color and the rest is a brown/gray color,” Thurston said. Or they can call the company’s customer-service line at 800-289-8739. If a phone discussion isn’t enough to determine the product type, “we are also able to help customers determine this by photos, as we are trained on the different lines that we have made through the years,” Thurston said.
For the early Trex products, the company does recommend cleaning with soap and water to get off most dirt and debris, but it also suggests cleaning twice a year with a composite deck cleaner, and it makes additional recommendations for especially problematic stains. For rust stains, ground-in dirt and grime, and pigment staining, Trex recommends applying a deck brightener, a type of product that contains oxalic acid, and waiting 10 to 15 minutes before rinsing. Thurston said the green and brown stains you describe are probably mold and mildew, which come in a variety of colors besides basic black. To remove mold from older Trex products, the company recommends using a deck wash that includes detergent and sodium hypochlorite, commonly known as bleach. It specifically recommends Olympic Premium Deck Cleaner ($9.98 per gallon at Home Depot) and Expert Chemical’s Composite Deck Cleaner & Enhancer ($19.98 a gallon at expertchemicalinc.com).
Bleach-containing cleaners may lighten your decking, so test first in an inconspicuous area.
Although many deck cleaners promise no-scrub cleaning, when the decking is composite material, the two recommended products call for scrubbing with a long-handled bristle brush before rinsing with a garden hose. Application details differ slightly, though, so consult the label for specifics. Labels say the Olympic product cleans 300 square feet per gallon, and Expert Chemical’s treats 100 square feet per gallon. Trex warns that whatever cleaner you use, you might need to treat the deck several times to completely remove mold.
If you want to avoid products that contain bleach, Trex recommends UltraMean, but it warns that you will need to scrub vigorously for it to remove mold. UltraMean Exterior Stain Remover costs $33 a gallon on the manufacturer’s website, rhinohide.com.
Because you most likely have the original-style composite decking, be wary about substituting other deck cleaners. TimberTech Composite Deck Cleaner, for example, has a warning on the label that it’s not for wood or non-capped composite wood.
Capping composite decking provides a tougher surface. So when you clean that with soap and water or use a deck brightener and still see stains, Trex says you can turn to a pressure washer — provided it is set no higher than 3,100 pounds per square inch, you use a fan tip, and you keep at least eight inches above the decking.
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