The span of a beam is dependent on a few variables: The grade and species of lumber, size of lumber and the load it carries. Fewer posts on upper-level decks are typically more desirable to the occupants and this drives the use of larger framing materials for longer spans. Beam span maximums are based on a maximum anticipated live load as well as other factors. Building codes for residential decks only require 40 psf in some areas, but check your local requirements to make sure you are aware of any additional local guidelines. In addition, many residents prefer the fell of a deck that is designed for higher loads. The longer the joist, the more area of deck the joist supports, and thus the beam supports more area as well.