If you want a better understanding of how roof and floor loads accumulate and are carried by beams and headers this article provides a good two part study guide.
http://www.umass.edu/bmatwt/publications/articles/calculating_loads_on_headers_and_beams.html
It also has an overview of the different types of engineered wood products that are now replacing the milled timbers of the old days when trees were BIG.