A decking system is made up of a variety of decking boards and other components are disclosed. In some aspects, the decking boards are connectable to each other so that adjacent boards will provide a water barrier and a drainage channel. Some versions of the boards may have a hollow region to accept the provision of heating elements or other accessory structures. A connector piece is disclosed in various embodiments span the gap between the butt ends of the boards to provide a water barrier at the butt ends of the boards. A gutter and downspout system is disclosed, as well as structures for protecting the ends or sides of the deck structure.