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 2x10 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