For building a treehouse, I'm putting 5/4 thick decking boards across the joists to make the floor (Home Depot #1001754835).
My default would be to use deck screws that have a smooth shank that's long enough that only the smooth part ends up in contact with the deck board. However, a couple of considerations which make me wonder if that's actually not ideal are:
- The structure will be subject to forces from wind will move it and this in turn might cause some slack to develop between the deck boards and the joists such that squeaking could happen when walking on them (i.e. rubbing between the screw shank and the board)
- If that does happen, a good portion of the deck will actually be below subfloor (i.e. for the indoor portion of the treehouse), so it would be difficult to go back later and tighten them to remove the slack
Would it be best to still use smooth shank and are these concerns unfounded, or would it be better to use threaded and just make sure I'm putting enough pressure on the boards when screwing them in?