My 10' X 10' deck has 2X4's laid with the 4' up with about 1/2" spacing that is in fairly good condition. I want to lay "Weathershield" 5/4"x6" premium pressure-treated decking over the 2X4's. I will lay the new decking across (not parallel) the existing 2X4's. Should I lay the new decking directly on the old or should I cover the old with an underlayment like lauan plywood? Also, how should I attach the decking - adhesive with screws or nails or what? What should the spacing be between the boards? Thanks for your reply. Bob