I've been working on a truss system to replace the ceiling rafters. The advantages to using a truss instead of a solid beam are that less wood is needed (in theory) and there is less thermal bridging. In order to get an R-60 dense-pack cellulose roof a thickness of around 18" is needed. For R-80 24" is needed. For those kinds of roof thicknesses a truss especially makes sense. Here are a few images of my latest truss designs.