Floor joists are the framing components which support a house’s finish floor and subflooring. Joists are usually 2 x 10 or 2 x 12 inch lumber. A newer product, engineered wood i-joists, can create stronger and more reliable floors, since they eliminate the effects of shrinkage seen in traditional joists. Joists are typically installed at a spacing of 16 apart from each other, center to center. The joists are fastened to the foundation at the sill plate.
For larger foundations, a thick solid wood beam, or in some cases a heavy gauge steel beam, is installed in a beam pocket or on top of the walls of the foundation. The purpose of the floor beam is to provide intermediate support for the floor joists in order to limit the amount of floor deflection where the joists span over the foundation. The joists can either span over the top of the beam or be interrupted by the beam, and be nailed to each side of it. The amount of stability provided by the two methods is about the same.
The use of engineered I-joists can allow the elimination of the intermediate support beam. The i-joist is shaped such that it can resist deflection more and may be strong enough to span the foundation without support, depending on its dimensions and the joist design. Another way to eliminate floor beams is to build in concrete grade beams or piers to the foundation which support the joists from below. Also, if the dimension from one side of the foundation to the other is short enough that a standard 2 x 10 or 2 x 12 joist will span it without support, a beam may not be required.
Floor joists are rated for span and load carrying capability in a given situation; variables include the cross section dimension, length, the wood type and grade, or with steel, type and gauge. The joist rating also specifies how much weight can be placed on the joists when the home is built and occupied. The ratings come in the form of load and span charts or tables and are available from the manufacturer or building trade associations.
Architects, builders and engineers consult the rating tables when designing the house frame. The local building department responsible for permitting also checks the tables and charts to verify the house meets minimum structural integrity and safety requirements.
Although building codes specify a minimum joist size, a builder will often use the exact minimum allowable, which saves money, but can result in a floor that has a small deflection, and seems bouncy and cheap. If you are having a home custom built, make sure to tell your architect to specify a minimum floor deflection which will not feel bouncy.
Wood joists are sometimes supplied with a crown from one end to the other; a crown is a natural curvature along the length of the narrow edge of a joist. Framers install crowned joists with the crowns facing up, so that it gives the whole floor structure a rounded, slight raised area in the middle. After the subfloor sheathing panels are fastened over the joists, and more weight is added from other framing members used in walls and partitions, the crowns flatten and level out.
It is important when installing crowned joists that they all be installed crown facing up; if some joists are installed crown up and some crown down, then the floor will be uneven, and if they are all installed crown facing down, the floor will sag in the middle. Engineered wood i-joists do not have a crown, they are designed and manufactured perfectly flat on the top and bottom edges, and create a flat, even floor frame structure.