I am installing 150mm wide 38mm thick tannalised decking onto 150mm x 47mm joists. What should I use to attach the decking to the joists and what gap should I leave between the decking?
If I have to use screws/nails do they need to be piloted and counter sunk? How far from the edge of the decking should the screws/nails be placed?

What treatment, if any, should be used to prevent algae formation?