I simply must ask: WHY? Why use bales as a roof insulation? The amount of time, effort, cost, and embodied energy in such a task seems to me to heavily outweigh the benefits. If two string bales yield an R-value of R-40 (debated by the way) across the 18″ direction, then one would only achieve an R-40 roof at best (perhaps a bit more with the inclusion of the other construction materials. A standard roof assembly provides R-38 so what on Earth is the point?
If you want a super insulated roof, add a layer of rigid insulation on top of the roof assembly or add an extra framing member to deepen the joist cavity and add more insulation. There are environmentally sound insulations out there and ultimately, the roof will have less embodied energy and will be more environmental if done in a standard frame. Consider engineered lumber uses smaller trees, cotton insulation uses a renewable resource. Now compare that with ferro cement, poles big enough to handle the weight of the bales and the cement, 1×4 nailers, a heavy moisture barrier, chicken wire, and all his is experimental and may fail. To me this is another example of trying to place bales in a section of a building where they do not belong. My two cents.