Rafter is a commonly misused term, similar to the way “Kleenex” is used for facial tissue or “Wirsbo” for crosslinked polyethylene tubing used for in-floor heat and other plumbing applications.  A rafter describes a constructed system used to support roof sheathing, felt, shingles, and loads placed upon it, like snow.  Since a rafter is built at the site during the construction of a building and is more of a general construction technique built by many different people of varying skill levels, there is a serious lack of consistency and proven acceptable loads.  A truss is an engineered system used for the same purpose but is typically constructed at a facility under strict standards using specifically rated lumber to support calculated loads.