The section of the joists that overhangs the drop beam is generally known as a “cantilever.”
The 3:1 cantilever rule is a general rule where the length of the joists that is cantilevered over the drop beam is supposed to be equal to or less than one third the length of the non-cantilevered joist length.
For example, a joist with a total length of 16 feet would ideally have less than 4 feet cantilevered and more than 12 feet between the house and the drop beam (provided that the structure of the joist allows for a 4′ cantilever.)
Common green-treated lumber generally won’t allow for more than a 2 foot cantilever, but if using engineered joists or steel joists, a cantilever may allow for more than 2′ of a cantilever.
