We had the deck installed on the back of the house 2 years ago, a portion of it was built over the concrete pad that was outside our bedroom door. The only thing on the pad was the grill and the only time I used it was when grilling steaks once every month or so. The concrete was mostly covered in moss and every couple of years I go out with some bleach and a broom to try and eradicate the green stuff, never 100% successful. They were supposed to get rid of the concrete pad, but it was poured like 8″ thick so after wasting a day with a jack hammer and only nibbling around the edges I told them to go ahead and just build over it. Big mistake, the moss just jumped up the 6″ onto the wood of the deck. I spent a couple of hours this morning trying to get all the moss of the deck. Just like the concrete pad I was not 100% successful. I got a big portion of it off, it is no longer slick in spots and the green color is almost gone. I used some deck cleaner stuff in a bottle and scrubbed with a big stiff brush. I think to do a thorough job I am going to need a pressure washer. Must be some one at work with one I could borrow, instead of spending several hundred bucks on one.