Typical Floor Joist & Beam Specifications

Floor joists or beams provide the below-surface support for the wood subfloor of a room. Though joists may be any size that you desire, using typical floor joist and beam specifications ensures that you can find hardware to fit the joists and that the floor provides adequate support for furniture and other items in a room.

Joist Sizes

The wood boards used as floor joists generally range in size from no smaller than 2-by-4 inches to no larger than 2-by-12 inches. Two-by-6 inches is a joist size frequently used in both indoor floors and as deck joists. In joist sizes, the larger size, such as 6 inches, represents the height of the joists, while the 2-inch size represents the joist width or the area on which the floorboards sit.

Joist Spacing

Joist spacing may vary even more widely than joist sizes. In many home constructions, joists sit 16 inches apart beneath the floor. This spacing provides enough support for common usage, such as the placement of furniture and foot traffic. If a floor needs to support more than common items, you may choose to reinforce the floor by spacing joists closer together. Joists may be spaced as closely as you like, but spacing joists 10 to 12 inches apart should handle almost any home use.


The term "beam" generally refers to pieces of wood that run thicker than standard joists. Instead of having a 2-inch surface area on which to place flooring, beams may be 4 inches, 6 inches or even wider. This extra surface area provides more support beneath the floor and also serves decorative purposes. When beams are used to support the second floor of a home, they may be left exposed at the bottom so they remain visible on the ceiling of the bottom floor.

Joist Hardware

The installation of floor joists or beams generally occurs during the building of a house's original structure. When installed at this time, beams or joists generally attach to the perimeter of the flooring structure with bolts, though, in some instances, joists may also be bolted to the floor. Joists may also be installed using joist hangers. These hangers attach to both the perimeter of the flooring structure and the joists or beams with screws and may be installed at any time, even after completion of the original structure