View Residential Floor Joist Size Background . Is there a simple way to size floor joists? Then add two to the answer. Lawriter Oac 4101 8 5 01 Floors from codes.ohio.gov Residential wood framed floors and aquarium weights. About 0% of these are metal building materials, 0% are flooring accessories, and 0% are ceiling grid components. Floor joists, spaced on regular intervals, span the areas between supports such as walls, foundations, girders, and beams. There is specific engineering that determines the amount and size of nails to be used at the ends. The rule of thumb that i like to use when sizing uniformly loaded residential floor joists is half the span plus two.. Joist & beam spans & sizes. Floor joists are used to support a floor that spans over an open area, like over a they are used mostly for residential and light commercial construction like small offices. Find out...