Playhouse flooring options
If it is open air then i think you construct just like a deck with 2x6 deck boards so water can get through.
Either pressure treated, trex or cedar/redwood.
Your 2x4 joist appear to NOT be treated. I would use treated 2x6, unless it will not be in the elements for long.
Why 17&quot; on center, that is an odd number?