I have a Trex deck in Upstate NY. A former co worker helped install it in 2008-09. We had some extra, so we made planters out of it too. The deck is about 14 X 40. I use to buy Corte Clean locally, until one year, I used it and it didn't work so well. They sold me bags that were beyond the best if used by date. I've been ordering it online ever sense, for the last 3 or 4 years, to avoid this problem. I generally clean it every spring and fall to keep it clean. Last fall, I didn't have time. The black mold is apparent this year. I am very happy with the product.