I had wondered about the solid roof material usage as well so I hollowed a roof in Prusa, exported the STL then sliced in Cura to discover it uses around the same amount of material as it being completely solid with my typical printing settings. I like using Cura’s “Lines” infill setting as it prints a line in one direction then skips on the next layer to print in the opposite direction. This gives pretty dense bridging support at low infill percentages. I’m printing at 6% infill currently and could potentially go lower depending on the part. Obviously other infill setting will be stronger but I feel the infill is just required to complete the print, the walls are strong enough by themselves.