Hey, I am building a storage shed in the backyard and would like to build a flat roof which houses plants instead of a shingle roof. The slope of the roof will be around 1 foot rise per 6 feet run. The dimensions of the building are, 12 feet by 16 feet. floor is built with 2x10's and the floor joists are 24 inch o.c. The walls are 2x4, studs 16 inch o.c. I am a bit confused with the roof because i am not sure how much wet soil weighs. The roof will be covered equally with 5 inches of soil. I was thinking of using 2x6's to make the rafters but how close should they be? 12 inch o.c. or is that an overkill? The location is Florida, so there is a potential for a lot of rain and wind during hurricane season.