How heavy does a roof need built to support a 185# / per square foot load?

*The depth of the plant growth media is between two to six inches on extensive green roof systems. The weight specifications for these roofs are 16 to 35 pounds per square foot, so it is important to use a light-weight soil media. “You almost need to be an engineer when designing these types of soil mixes because you are working with so many materials of different sizes, weights and measures*

Text above taken from Source website

If you're going to be just using this soil for growing turf, two inches would be the most soil you need, so your 2" x 8" tongue and groove sheathing over log rafters spaced 4' apart to me seems over engineered depending on the size of the logs and over engineered is probably the way to go.

Carpenter & Joiner