I recently worked on a similar building but with an exterior ring and a center post. I was told the living roof support structure needed a minimum slope of 1 on 12 even with the roof membrane. This project was not a residence but rather a classroom so it was required to resist seismic forces, hence knee braces were used. This one used cordwood instead of straw for the infill. There is no code accepted shear values for either straw or cordwood. (The third pig had no problem) Be careful, the living roof system is very heavy.