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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!


What Is Residential Construction?

What is Residential Construction? Constructing a building isn’t about building a skyscraper full of office spaces. Rather, skyscrapers are also built for residential purposes. Thus, residential construction means to specifically construct a building that is going to be used for living. It is quite a lucrative business, as many people have earned their fortune through it. Therefore, many people who aspire to become a builder must know the difference between commercial and residential buildings, and the different categories of residential buildings. Also, the cost that is incurred during the construction phase of residential buildings should be known. And how successful builders become so successful.

Different Types Of Residential Buildings

Different types of residential buildings

Following are the different types of residential buildings. What are their pros and cons? How one is beneficial to a certain group, while the other is beneficial for someone else.

Single-Family Homes

Single-family homes

These types of homes are made for families that want to live on their own. They are mostly in the country. Therefore, such houses have a small backyard, and some of them might even have a small farm for keeping animals. People can easily make a single-family home, and plan it in any way they want because they won’t have neighbors to restrict them to a particular style of house building. Hence, people are free to experiment and make the house of their dreams. These homes sell like hot-cakes because people see them as a great source of investment, and therefore, their market value is high. But their downside is that the owner is solely responsible for maintaining them, hence, he/she has to bear the entire cost.



Townhouses, like single-family homes, are made for single families but they don’t have to be maintained like single-family homes. Also, they don’t have the extra space like a single-family house, and thus people have to manage on their own. The only con of a townhouse is that they don’t offer a lot of amenities like a condo or a single-family house.



Called condos for short, these are mostly found in big cities. Some of them are also found in small towns as well. Condos are preferred mostly by single families or single people. Also, they are one of the most sought-after properties. Condos usually have a big market value because of the amenities like pools, gyms, and lounge areas. Thus, these amenities add to the value of the condos considerably. Also, condos have HomeOwners Association (HOA) fees that cover most of the maintenance of the condos. But some of the costs aren’t covered under this fee. Therefore, they have to be covered by the owner.

Multi-Family Homes

Multi-family homes

These homes aren’t as popular, but contractors still built them on request. Contractors also built them for investment purposes as well. Thus, contractors can earn a little money from them as well. They can rent the place out to multiple families, and then collect monthly rent from them. This way, they can easily earn back their investment in a short amount of time. These houses are rented out as units, and the owner of each unit is responsible for maintaining his/her own unit.

These multi-family houses also consist of garages, swimming pools, and sheds as well. Thus, a contractor can employ subcontractors for installing basic amenities such as electricity, plumbing, and water lines. These amenities are installed by professionals who do these types of work. Most of the residential buildings are graded on how safe they are in case of a fire or any other emergency. Hence, successful contractors try to build the safest building that they can possibly build.

Costs Of Building A Residential Building

A residential building, although costs less than a commercial building, can cost a huge amount to construct. Thus, people tend to rent condos and multi-family houses then buy them. The cost of maintaining a building is also considerably high. That is why most residential buildings charge HOA apart from the monthly rent. Thus, most successful contractors tend to build these houses for short-term investment. Therefore, they quickly build these residential buildings and move on to the next one.

Hence, successful builders don’t tend to rely on one project, but rather they continue to work on multiple projects. They also have men ready to work at a moment’s notice, and they also save a lot of money by getting materials at competitive prices.

Hire J Ray Construction, If You Are Looking For Residential Construction Contractors