What Are Floor Joists?

What Are Floor Joists?

Floor joists span an open space, usually between beams. These components are horizontal support elements that transfer loads to vertical members. Joists carry weight from any person, animal or object inside the room. These elements require specific spacing, often 16 inches apart, but this can vary depending on the structure's blueprint requirements and building codes. 

Joists help distribute a structure's load. When you apply weight to the floor and joists, wood fibers at the bottom of the board go into tension. The top fibers experience compression, helping distribute the load evenly. Several factors can impact the function of a floor joist, including the joist type, size, material, space between joists, load on the floor and more. 

What Size Are Floor Joists?

There are many types of floor joists, but the most common measure 2 feet wide by 8, 10 or 12 feet in length. These elements are essential components of a structure, so choosing the right size is critical. Your floor plan, floor surface, span lengths, air quality, floor finishing and fire protection will all impact the type of joist you need. The three most common floor joists are:

  • Solid lumber: These floor joists typically come from old-growth trees and are contiguous boards. The span distances for these joists rely on factors such as spacing, board size, species and deflection. Although using these joists is common, few old-growth trees are available, and using younger trees can result in warping. Solid wood joists are less expensive than some engineered solutions, but they aren't suitable for all applications because they have a limited span distance and are not environmentally friendly. 
  • I-joist: I-joists, sometimes known as TJI joists, use different materials, with the bottom and top often made from laminated veneer or solid wood and the center usually consisting of oriented strand board (OSB) or plywood. These joists are light, allowing for easy maneuvering, and can provide longer spans than solid wood joists. However, due to their abilities and additional processing requirements, these joists are often more expensive. Their multilayer construction also means they will fail more quickly in a fire. 
  • Open-web trusses: These joists support floor loads with wood pieces. Many construction workers and builders prefer open-web trusses because they don't need to measure or craft holes for elements like electrical wires or pipes. These floor joists have a more rugged construction than others and provide longer spans. Additionally, these components make it easy to accommodate wiring, plumbing and HVAC systems without cutting. The enhanced performance of these joists makes them the most costly on this list. Additionally, you cannot trim these joists because they come in specific lengths. 

Elements to Consider for Floor Joists

Joist span refers to the covered distance between supporting elements. Structural engineers often calculate these spans to ensure accuracy. Typically, large structures will need larger joists. However, there are several other factors to consider when determining which joist you need. There are no standard joist sizes, but you should consider these elements:

  • Wood species: You can often find southern yellow pine, redwood, Douglas fir and hemlock wood joists, each of which offers a different level of strength. You'll want to refer to the bending strength noted by the supplier to determine the loads each type of lumber can withstand. Stronger wood will let you create longer span distances without requiring additional support. 
  • Grade: Lumber grade refers to the amount of defects and knots in the wood you're using. You will find fewer defects in high-grade lumber, which indicates that the material is stronger than that you'll find in lower grades — and that the cost will be higher. Since floor joists will not be visible in the finished construction and their aesthetics are unimportant, a #2 grade is common for this application. This grade still offers ample strength but has more defects than higher grades, lowering the cost. 
  • Load capacity: When choosing joist load capacity, you must account for dead loads and live loads. Live loads refer to people, appliances, furniture and other objects lacking a connection to the structure. Dead loads refer to permanent structures and the floor structure's weight. Flooring materials can increase this load.
  • Lumber width: Lumber requires support, but the width determines how far you can span the joist before needing a support post or foundation. Although the thickness of the board is less relevant, you can rely on the top-to-bottom width to determine joist board strength. 

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