How deep does the dirt need to be under the chicken run?

[SamuelPanama]

I'm building my chicken run on top of a wooden platform and am planning dirt on top. How deep does the dirt need to be? Is 6 inches enough?

Related questions...

1. Does the type of dirt matter?
2. Clay, sand, or does it need to be good quality topsoil?
3. Should I plant anything on it or is dirt alone good?
4. If the dirt gets very salty (like from ocean water), will that cause any problems?

 

[LaurelC]

Why are you putting dirt on top of wood? This seems like a recipe for rotten wood.

Why would ocean water be getting on the dirt? Does this area flood?