What Are Floor Joists and How Do They Work?

Floor joists are horizontal structural members that span an open space, often between beams, which subsequently transfer the load to vertical structural members. These joists, part of the floor system, carry the weight of everything inside a room, including walls, furniture, appliances, and even people.

Most often, floor joist spacing is 16 inches apart on center, but this can vary depending on building codes and the requirements of the structure in the blueprint.

Floor joists help distribute the load of a structure. When weight is applied to the floor and the joist, wood fibers on the bottom of the joist go into what is known as tension. The top fibers go into compression, and this helps distribute the load evenly.

The Anatomy of a Floor Frame

the anatomy of a floor frame

Prior to building your frame make sure that the ground is compacted and the foundation is sturdy. These are the major components of a floor frame (note: this is not a comprehensive list of everything that goes into a floor frame):

  • Blocking – Bridging is made up of smaller wood sections fastened between joists. This provides what is known as lateral stability.
  • Header joists – A header, or rim joist, is used to frame an opening in the floor. It provides lateral stability.
  • Floor joists – Floor joists are large wood framing members that are positioned on their narrow ends. They are evenly spaced and are connected to the sill plate. A subfloor is attached to these joists, which carry the weight of the loads to the walls.
  • Sill plate – The sill plate attaches to the top of the foundation, usually made from treated wood. Joists fasten to the sill plate.
  • Support beam – The support beam, also known as the center beam, bears the first-floor joists if the joists aren’t long enough to reach between foundation walls.

Types of Floor Joists

types of floor joists

There are several different sizes and types of floor joists. The most common sizes are 2×8, 2×10, and 2×12.

Solid Lumber Joists

Solid lumber joists are contiguous boards, usually made from old-growth trees. Their span distances are affected by things such as species, board size, spacing, and deflection. Solid lumber joists on a job site are still common, but the supply of trees is being exhausted, and using younger trees for the joist can result in warped wood.

Pros

  • Last longer during a fire
  • Less expensive than engineered solutions

Cons

  • Not environmentally friendly
  • Limited span distance

I-Joists (TJI Joists)

I-joists, also known as TJI joists, get their name because they look like the capital letter I. Different sections of the I are made with different materials. The tops and bottoms are often made of wood or a laminated veneer. The center support is usually plywood or oriented strand board (OSB).

Pros

  • Longer spans than solid wood
  • Lighter for easier maneuvering

Cons

  • Fail more quickly in a fire
  • Cost more

Open-Web Floor Trusses

Open-web floor trusses make use of wood pieces cantilevered to help support floor loads. Many builders like these trusses because they don’t have to measure and make holes for things such as pipes and electrical wires.

Pros

  • More rugged construction than I-joists
  • Even longer spans
  • Can accommodate HVAC, plumbing, and electrical without cutting

Cons

  • Have specific lengths — can’t be trimmed
  • Higher cost

Joist Span Considerations

A joist span is the distance covered between supporting structures. Usually, structural engineers calculate the spans to ensure accuracy.

As a rule, the larger the structure, the larger the joists that are used. However, considerations such as the type of floor joist and the dimensional lumber used to construct them, as well as building codes, factor into floor joist spans.

It’s important to realize that there are no standard joist sizes. That means many factors need to be considered when determining the correct joist size and span.

Wood Species

The common types of wood used to make floor joists are redwood, hemlock, Douglas fir, and southern yellow pine. However, not every type of wood product is available across the country.

It’s important to understand the strength differences between wood types when choosing joist material. The bending strength of a wood reveals the load the lumber can withstand perpendicular to the grain. This is usually measured by low, medium, or high bending strength.

Stronger wood can span longer distances without needing additional support. For example, redwood has a medium bending strength, so it can’t take as much weight as fir. So while one type of wood may look better to you, you have to consider its strength. Staining is always an option if you wish to change the appearance of an exposed joist.

Grade

Another strength factor to consider when selecting wood floor joists is the lumber grade. The grade is determined by how many knots and other defects are found in the wood. Higher grades of lumber have fewer defects and are stronger than the lower grades. However, the higher the grade, the higher the price.

The most common lumber for floor joists is #2 grade. This lumber has more knots and defects than higher grades, but it doesn’t lose much in the way of strength.

Lumber Width

The width of lumber plays a large role in determining how far a joist can span — the distance a piece of lumber can cover before it needs to be supported by a foundation or support post. The strength of a joist board is affected by its top-to-bottom width. This is much more important than the thickness of the board.

Load Capacity

Load capacity is critical when using floor joists. Determining the load capacity of a floor joist involves expert knowledge of wood’s structural properties as well as an understanding of building code requirements.

There are two types of weight a floor must bear: live and dead loads.

Live Loads

The live load is basically the weight of anything that isn’t connected to the structure, such as furniture, appliances, and people. Commonly, non-sleeping rooms must support a minimum live load of 40 pounds per square foot and sleeping rooms 30 pounds per square foot, according to the International Residential Code.

Dead Loads

The weight of the floor structure, and structures permanently attached to the floor, is known as dead load. This load is determined by what materials make up the floor. A normal wood-frame floor has a dead load of about 8 pounds per square foot. Flooring materials that weigh more increase the dead load.

Deflection Limits

Deflection is defined as the bend or “sag” in a floor caused by loads. The maximum allowable deflection is usually given as a fraction of the span length (L) in inches. For floor joists, dead and live loads shouldn’t exceed L/240, as specified by the 2012 International Building Code (IBC), Section 1604.3.

Joist Sizing Calculations

Sizing the depth needed for a floor joist can be done with a simple calculation: half the span plus two. For example, if you had a floor in a room that spanned 16 feet, you would divide that number in half (eight), and then add two, to get 10. Therefore, the depth of the joist will need to be 10 inches.

Conclusion

Floor joists do the heavy lifting in a room, holding up the floor and whatever is in the room. It’s important to choose the right floor joists (material, size, etc.) for your specific needs. However, while there are span tables and calculators to help determine joist needs, a structural engineer should calculate the required joist specifications in accordance with the building’s engineering as well as local building codes.

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