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What Size Are Floor Joists In Residential Construction?

Common sense tells you that huge floor planks can carry more weight. Also, spacing joists that are joint closely also enhances the load-bearing ability of a floor. But bigger is not often better when builders are building a home or doing a room addition.

When a builder frames a floor with 2 x 10 joists instead of 2 x 12, the additional two inches of vertical distance can be absolutely important, for instance. Therefore, the hard thing for builders is to select suitable joists for the load they hold while maximizing the area.

How Size Floor Joists In Residential Construction Is Affected?

A question always arises that what size are floor joists in residential construction. In this article, we will tell you all the details. Figuring out the load capabilities and choosing the spacing and right sizes of the floor joist is a complicated task. It is even more important than you might guess since there are several variables at play:

  • Wood species.
  • Grade of the lumber.
  • Thickness and width of the boards.
  • Spacing between joists.
  • The burden was placed on the floor.
  • The length that the joists span.

Wood Species

Various kinds of wood differ in resistance and stiffness to bending, with few species being way stronger than others. Even within the same kind of species, there is a great difference in their strength. It depends on the growing circumstances of the tree.

The wood from areas that grow slower and have more growth of end grain rings per inch. Both of these are much denser and stronger than the trees that grow faster of the same species.

Grade Of The Lumber

One more aspect of lumber joists is the wood quality that is being used. All wood manufactured for commercial sale is classified. The highest class – preferred – is generally free of any knots, blemishes, and other deformities that can make the lumber weak. Going for lumber can bear a good amount of loads.

After choosing a class, lumber carries a class of either 1,2, or 3. Lumber classified as one is of higher quality as compared to 2 or 3, and so on. Joist duration tables comprise maximum span lengths for all four classes for each variety of lumber.

Suppose you are not clear as to the class of lumber you are buying. There must be both a black tag attached to the lumber indicating the class and a white stamp on the wood itself with the class and species.

Size of Lumber (Thickness and Width of the Board)

The board’s width is a vital component in deducing the length a floor joist can span, playing a way bigger role than the thickness.

For instance, if you double the number of joists or thicknesses on a floor, the extent the boards can stretch will enhance by nearly 25%. But if you double the board’s width, the extent to which the joists can stretch increases around 80% to 100%. Though, you’re utilizing similar board feet of lumber.

Proper Spacing Between Joists

Like the joist size, the building code for your structure will specify the adequate gap between floor joists. The code itself is specified by the engineering laws of a specific building. Generally, floor planks have a spacing of 16 inches apart in the center. It means from the center of one also, with some buildings, floor shafts must be 12 or 24 inches distant in the center.

Are Floor Joists Load-Bearing?

Yeah, the floor joists must be weight-bearing. They act as the backing for the upper grades, constantly shifting weight to the vertical supports of the foundation. Though, when it gets to the upper levels, the floor shaft supports the walls.

Accordingly, the walls give support to the next floor. Yet, if a wall is positioned parallel to the planks, it isn’t weight-bearing. Only those walls which are placed upright to the floor joists can be weight-bearing.

But what you should know is that floor joists carry the burden of the whole house. While walls normally only carry part of the load, with a few not carrying any load at all. Floor joists are significant for this specific reason alone.

Joist Span

Formulating all these aspects jointly provides the minimum size joists required for several kinds of

  • Lumber.
  • Grades of lumber.
  • Sizes of boards.
  • Spacing of joists.
  • And the load that will be placed on the floor.

You also need to evaluate the code of your local building before you start construction. Also, discuss it with a structural engineer in rare or severe circumstances.

Can I Repair Floor Joist Myself?

Generally, a professional should do floor joist repairs. It is the structural quality of your whole residence. You can not put it at risk. Therefore, it requires it to be performed by somebody with knowledge and understanding of floor joists.

For easy repairing of floor joists, you should consider utilizing a flitch plate. A flitch plate is a delicate piece of plywood or steel attached to a weak joist to give support. Though, another alternative is to use a similar board and ensure it to the old one.

You can still leave the old board because the new board is only there to carry it in place. It will become the new weight-bearing board in the place. Nothing is risked, and you don’t need to do any compromise.


The size of your floor joists relies on numerous factors. Besides, just staring at your joists and determining they are too little is not a reasonable way to know the usefulness of your joists. You need to know all the factors that define the span length of the joist to evaluate your joints.

If you are constructing a new house, your house’s proposal will be stamped by an engineer. Then, your joists will be measured appropriately. But if you are constructing an addition yourself or any other structure, you must follow the joist span tables in the IRC.

Keep in mind that failure to measure floor joists accurately can result in vibration, wobble, and even full failure. Talk with a professional before you construct. Also, you need to understand that overbuilding is anyway better than underbuilding. We hope that your search of “what size are floor joists in residential construction” will end up here!

1 reply
  1. Eli Richardson
    Eli Richardson says:

    It really helped when you talked about floor joists and how you need to use the right size for your project. Recently, one of my uncles said he’s interested in building a barn in his land. My uncle wants to start a farming business, so I’ll be sure to share your tips on wooden floor joists. Thanks for the advice on wood quality and how it’s classified.


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