The framework of a floor consists mostly of regularly spaced wooden joists that run parallel to one another and cover the areas between supports and beams. The International Building Code is a model code that has been adopted by all 50 states as well as the District of Columbia. A related code, The Residential Building Code, has been adopted by all states except Wisconsin. These building codes set the minimum requirements for construction, including requirements for framing floor joists.
According to Section R502.4 of the 2009 edition of the International Residential Code, joists under parallel load-bearing walls must be big enough to support the weight of the load they bear. Doubled joists that are separated so piping or vents can be installed must be solid blocked with lumber that's at least 2 inches thick and spaced no more than 4 feet apart. Load bearing walls transfer loads from one point to another. The exterior walls of a house are usually the main load-bearing walls, but some interior walls may also bear loads.
Sections R502.3.1(1) and R502.3.1(2) of the International Residential Code include tables that list acceptable spans for floor joists. Additional tables are included that list the maximum allowable span of floor joists that support sleeping areas and attics. The American Wood Council maintains an online calculator that allows you to calculate the maximum span for joists by entering information such as the species and size of the wood you're using, spacing and exterior exposure.
Blocking provides the floor joists with lateral support, and it provides a way to transfer shear and vertical loads from the walls above to the foundation below. Section R502.7 of the 2009 International Residential Code specifies that joists must be laterally supported by solid blocking that's not less than 2 inches thick and the full-depth of the joist. Floor joists can also be attached to a full-depth header, rim joist or to an adjacent stud. Trusses, structural composite lumber, structural glue-laminated members and I-joists are exempted from this requirement, but must be supported according to the manufacturer's recommendations.
Additional support for the floor joists is achieved by nailing the ends of the joists into headers. Section R502.10 of the International Residential Code states that header joists can be the same size as the floor joists when the header joist span isn't greater than 4 feet but, if the header joist span is more than 4 feet, you'll need to double the header joist and ensure that it's capable of supporting the floor joist framing.
Notches and Holes
Problems can occur if framing is cut to make room for plumbing, electrical wires or ductwork. Cutting holes or notches into the frame weakens its structural capacity. According to Section R502.8.1 of the International Residential Code, notches in solid lumber joists can't be more than one-sixth of the depth of the joist or longer than one-third of the depth of the joist. In addition, cuts should never be made in the middle one-third of the span, and no holes should be made closer than 2 inches from the edges of the joists.