What Is Floor Joist

In building constructions the term joist refers to a horizontal framing member that is the primary structural element of a floor or ceiling frame.

What is floor joist.

After joists are installed plywood subflooring and or drywall are fastened to the joists to complete the structural floor or ceiling. For example if you double the thickness or number of joists in a floor the distance the boards can span will increase by approximately 25. In buildings with basements the first level of floor joists serves the same purpose. The width of a board is an important factor in determining the distance a floor joist can span playing a much larger role than the thickness.

In the past 2x4s or 2x6s were sometimes used as floor joists. Floor joists are an important part of the supportive structure of a floor. Joists are spaced at regular intervals much like the studs in a wall frame. The floor sheeting is usually made from 3 4 or thicker sheets of plywood or osb.

Joists span across the tops of load bearing walls and may be supported at mid span by a beam or other member. The floor sheeting is also engineered to provide a solid floor. Floor joists installed on second or third or higher levels double as ceiling joists for the level below. In many countries the fabrication and installation of all framing members including joists must meet building code standards.

The floor joist is nailed to the plate material and spans the open area under the floor. Each type of wood has a specific span and spacing requirement. Joists must exhibit the strength to support the anticipated load over a long period of time. B the framing member that runs around the perimeter of your house and that the floor joists butt into often gets riddled with large holes this space is the logical exit point for dryer and furnace vents big electrical service wires and fireplace fresh air intakes.

This size is no longer acceptable when framing a structure. Floor joist spacing is the distance a floor joist board can be separated from an adjacent floor joist. Considering the cross section of a typical joist the overall depth of the joist is critical in establishing a safe and stable floor or ceiling system. This spacing is combined with a floor joist span to determine the maximum capacity load of a floor.

The rim joist fig. This sample table gives minimum floor joist sizes for joists spaced at 16 inches and 24 inches on center o c for 2 grade lumber with 10 pounds per square foot of dead load and 40 pounds of live load which is typical of normal residential construction.