Floor joist spacing should be at regular intervals, spanning the area between supports such as walls, foundations, girders, and beams. Typical spacing is 16 inches on center (from center to center). Though some floors may have joists on 12 inch or 24 inch centers. Joist sizing and spacing is regulated by building codes, which are based on engineering requirements. Check your local area for your
. building codes. Check this website for more information http://www.hometips.com/how-it-works/floor-joists-framing.html