Looking at this table you will see there is a choice in the size of floor joist (2 X 6, 2 X 8, 2 X 10 or 2 X 12) and there is a choice in the joist spacing (12", 16" or 14"). The floor joist spacing is the distance between the centers of any two installed joists. See the image below for an example of joists spaced 16" on center (16" o.c.). Since the example house we are designing for is 12 feet wide, we need to find, in the floor …