We have two composite decks, at two houses. One is along the California coast and one is not. The 9 year old TimberTech deck, with hidden fastiners, is on the coast, which gets lots of morning dew, due to the fog resulting in green and black molds. The grey Trex is inland and stays dry most of the year, but still gets dirty and moldy over the winter. Corte Clean works great for cleaning both. The mold use to grow up from the deck around the built in BBQ, onto its cover, damaging it. I sprayed the Corte Clean on the decking and bought a new BBQ cover, problem solved. Great product which I just ordered more for this season's cleaning. Be sure to use in direct sunlight for best results!