This appears to be a great site for calculating header and beam requirements: http://learnframing.com/wood-beam-calculator/ But it distinguishes between interior and exterior beams -- and the results are very different. I am adding an addition to my one-story house. I am knocking a 10' wide door between the existing building and the addition. This door is going into the existing "exterior" wall. But when things are finished it will be in an "interior" wall. I have calculated the weight, and added the weight of the new roof to the existing. But I can't understand the tool's differences for interior vs exterior and hence not sure which to use.