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What Are Floor Joists And How Do They Work

What Are Floor Joists

What is a floor joist?

floor joists laid out

Floor joists are used to support a floor that spans over an open area, like over a basement, crawl space or lower floor level or between shed floor beams. It is like a mini bridge that is only supported at each end. When many joists are installed next to each other and a sub-floor sheeting like plywood or Oriented Strand Board, (OSB), is nailed on top they create a large floor area called a “floor system”.

They are used mostly for residential and light commercial construction like small offices. They are fairly inexpensive, between $1.00 to $2.00 per lineal foot, for the benefit they provide. This article answers the questions of What Are Floor Joists and What Is A Floor Joist by by describing their uses, how they are made and the types of applications they are used for.

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In this article, you will find information about:

  1. Types Of Floor Joists
  2. How Far They Can Span
  3. How They Bear At The Ends
  4. Parts Of A Framed Floor System
  5. Mechanical Equipment In Joists
  6. Floor Joist Tricks

step 1 Types Of Floor Joists

The two most common types of floor joists are natural lumber like a 2×10 and manufactured like a “I” joist. These are the characteristics of each type:

  • Natural Lumber: These are simply boards that are cut from trees. They are solid wood. They cannot span as far as engineered lumber.
  • Engineered Lumber: These are made by gluing together thin strips of wood in specific directions. engineered lumber is stronger and lighter than natural lumber. engineered floor joists create a flatter and more predictable floor system.
i joist framing

step 2 How Far They Can Span

The distance between the supports is called the “joist span”. They can span from a few feet to over 20 feet. The span is determined by several factors:

  • Joist engineering: Each manufacturer has engineering for each of their joist products. The tables take into account the joist depth, length, spacing and desired floor strength.
  • The depth of the joist: this is the height from top to the bottom of the joist when it is standing on its long side.
  • The length of the joist span: The joist span is the distance between the two supports. The longer the span the weaker the joist will be
  • Joist spacing: This is just a fancy way to say how close the joists are installed to each other. The closer they are installed to each other the stronger the floor system will be. Typical spacing is 16 inches on center but they can be spaced as much as 19.2 inches on center and as close as 12 inches on center.
  • Floor strength: There are several ratings for typical floors. The ratings for floor systems are rated by how far they “deflect” or sag when typical weight is put on them. The strength ratings are also on the engineering charts.
floor joist span

step 3 How They Bear Weight At The Ends

Since floor joists run perpendicular to the bearing walls they must be supported at each end. The typical requirement for bearing is at least the width of the joist. There are several ways for a joist to be supported at its ends.

Bearing on a Sole Plate: A sole plate is a piece of wood attached to a concrete foundation. The joist simply sits on the wood plate. It is attached to the plate with nails. There is specific engineering that determines the amount and size of nails to be used at the ends.

framing floor sole plate joist

Bearing on a center wall: When joists span great distances between two foundation walls they are supported in the center on a “bearing wall”.

joist bearing on bearing wall

Bearing on a joist hanger: These hangers are simple metal straps that hold the ends. They are commonly used when attaching a joist to a beam, like over a doorway in a basement or to keep a room open. The engineering calculations that come with the hanger will determine the size and quantity of the nails used to attach a hanger to a beam.

joist hanger

step 4 Parts Of A Framed Floor System

A framed floor system consists of:

  • Wood Plate: A plate is a piece of wood bolted to the top of a concrete foundation wall or nailed to the top of a wood wall. The plate is made of wood so the floor joist can nail to it.
  • Floor Joist: The floor joist is nailed to the plate material and spans the open area under the floor.
  • Floor Sheeting: The floor sheeting is usually made from 3/4″ or thicker sheets of plywood or OSB. The floor sheeting is also engineered to provide a solid floor. It is important to nail the floor sheeting with the required size of nails spaced according to the engineers specifications.
  • Rim Joist: On the ends of the joists a board is run perpendicular to the floor joists. This board is called a rim joist. It holds the joists upright and bears the weight of the wall and roof along the edge of the house.
  • Optional Parts: Other parts of the floor system may include beams and hangers. These are only used when the design of the house and floor require it.

step 5 Mechanical Equipment In Joists

Mechanical equipment is often installed in the cavities between the joists. This keeps the ceiling below the floor joists higher. There is engineering that tells how much of the webbing between the top and bottom may be cut away and still retain its strength. Most often it is not allowed to cut away parts of the joist near the ends of the joist but it is ok to cut away some material in the center of the span.

mechanical in floor joists

step 6 Floor Joist Tricks

Cantiliever: The cantiliever is created by running the floor joist past the sole plate. It creates a situation where the joist is not bearing at its end.

cantiliever floor joist

The Anatomy of a Floor Frame

What Are Floor Joists

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.


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


  • 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).


  • Longer spans than solid wood
  • Lighter for easier maneuvering


  • 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.


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


  • 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.


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.


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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