Arsenalfan:
Write your own spread sheet to do the required calcs. as a function of field measure sizes and spans and assumed species and grade. As suggested you can study the grading rules over the years, there are classes for becoming a lumber grader, which you could take. Hundred year old houses might have been made of any good local wood, ask some old contractors, restoration people, old lumber guys, not the kid at the bog-box. To do what you are talking about the right way, you have to do an inspection of the/each roof system and measure sizes and spans. Probably, more importantly, look at the entire roof and all the important details for condition or defects and deficiencies. Is the roof sway-backed from years of permanent set and deflection, indicating too flexible a roof for the existing loads? Are the joints intact, not pulling apart, crushing or splitting, enough bearing area, properly cut and fitted, no rafter thrust problems at their bearings, etc. Now, you can start to make a current condition judgement, and start thinking about any excess rafter cap’y., or how to add some reinforcements, or how to distribute your new point loads to several rafters to meet your needs. With time you may gain some experience and efficiencies at doing the analysis and making the judgements about possible options, but I think you pretty much have to do the inspection, etc. to arrive at an educated and engineered sale. One size does not fit all, and one simple table will not do it for you either.
