Everything You Need to Know about Floor Joists

Floor Joists

When moving about in your house, you are probably not thinking about what you are standing on. Of course, it’s a floor, but there are so many types of floors in the market today, all of them offering different attributes and upsides. You may be very surprised to find that the floor you are walking on is not sitting directly on a concrete slab, but rather that there is an underneath subfloor, underlayment, and floor joist support system. The raison d’etre of this article is to make sure that you understand everything that constitutes the floor joist system. First things first, what exactly is a floor joist?


There are floors that have been built directly on a concrete slab, and this is a very common occurrence. Some floors have crawling spaces, lower floors, and basements below them. In such instances, they almost always use floor joist systems to offer weight-bearing support for the furniture, flooring, people, and anything else that is in the home. Floor joists, essentially, are horizontal framing members that are the skeleton of the floor frame. They span across the room, according to Home Edit. On either end and even sometimes in the middle, the floor joists are supported by load-bearing beams or walls, so that the weight that is subjected to the floor joists is then transmitted to the loadbearing beams or walls. Thus, the joists may run from wall to wall, beam to beam, or beam to a wall, depending on the place itself and the area that they span, as well as the structural design of the space. Like the studs on a wall, the joists are spaced at regular intervals. After they have been installed, the floor joists are to be covered using subflooring panels, such as plywood or OSB, oriented strand board, to complete the flooring structure assembly and thus create a solid and continuous platform. The said platform is where you can install your carpet, tiles, and any other flooring finish. Floor joists can be made using a myriad of materials, but the most appropriate and most commonly used is engineered or solid wood.

Parts of a floor joist

  • The blocking; is made up of small wooden sections that are fastened in between the joists. They provide lateral support.
  • Header joists; header, also known as rim joist, is used as the frame of an opening in the floor. It, too, provides lateral support and stability.
  • Sill plate; is attached to the bottom of the foundation and is made from treated wood. The joists are then fastened to the sill plater.
  • Support beam; also known as the center beam, is installed if the joists are not long enough to reach the walls or beams, and thus offers structural support and stability.

The types of floor joists

When it comes to floor joists, there are three main types used, and they are I-joists, solid lumber, and open web floor trusses. Each of them offers its own upsides, but also has a couple of drawbacks, which is why the carpenters will have their own favorite or will recommend a specific one based on the space that you want to be worked on.

Solid lumber joists

This is the oldest type of timber joists, and once upon a time, it was the only option, according to Hunker. However, with the advent of modern building materials and the introduction of more effective and efficient construction building technologies, it is far less common in homes nowadays. Solid lumber is the most economical choice of all three types, though the pricing will vary a bit depending on the board size, the wood species, and the lumber grade you choose to use. These same factors will also affect the maximum joist span. Another upside aside from the fact that it is economical is the fact that solid lumber joists have the best survival rate when it comes to fires and floods.


I-joists are sometimes referred to as TJI joists. Then installed, they resemble the letter ‘I’. The Engineering Wood Association has stated that the bottoms and tops of I-joists are to be made using either laminated veneer lumber or solid wood, while the center is to be made using either OSB or plywood. One upside of the I-joists is that they are able to span a longer distance than solid lumber, and can reach up to 26 feet in distance. They also have minimal flex, which is a huge upside if you want to install certain types of flooring, such as ceramic tiles. They also have a higher load capacity than solid lumber joists. Some of the I-joists come with pre-scored holes, and even knockouts, that can be used to run the utility lines through. Since the joists are made using engineered materials such as OSB, and they are relatively smaller compared to solid lumber, they are the eco-friendliest joist option.

Open-web floor trusses

These trusses are constructed using 2 by 4 boards on the bottom and top and with a diagonal board on the middle, which is secured with metal plates. The reason why open web trusses are so popular amongst builders is that it is easier to run utility pipes and wires in the middle, where there are open spaces. Even though they flex more than I-joists, they offer less flex compared to lumber trusses. Additionally, they see less wood. All these are great upsides, but perhaps the biggest advantage when it comes to open-web trusses is that they can stretch, and offer the greatest span when it comes to joists.

Joist span considerations

The joist span is the distance or length that is between supporting structures. The supporting structures can be either beams or walls, as mentioned earlier. The structural engineers, and sometimes the builders, will calculate the span in order to ensure that accuracy is maintained. It is a rule of thumb that the larger the structure, the larger the joists will be used. That being said, there are a couple of other factors that have to be taken into account, like the type of floor joist that is being constructed, and the building codes of the area. There are no standard joist sizes.

Wood species

The most common types of wood used to create floor joists are hemlock, Redwood, southern yellow pine, and Douglas fir. It is important that you consider the strength differences between the wood types when selecting the joist material. The bending strength of wood will reveal the load that the lumber will withstand. It is measured by high, medium, or low bending strength. The stronger types of wood can span longer distances without needing additional support. An example of this can be seen with fire and redwood. Redwood has medium bending strength, while fire has high. This means that redwood cannot hold as much weight as fir.


Another factor that you have to take into consideration is the grade of the timber. The grade is essentially how many knots and other naturally occurring defects are in the wood. Timber with a higher grade has fewer defects and is thus stronger than the rest. However, the higher the grade of timber, the more it will cost you to purchase it.

Lumber width

The lumber width is a determinant of how far the joist will be able to go, that is the distance that the lumber is able to go before it develops a structural need for support, either by the foundation, or a supporting post. The top-to-bottom width does affect the strength of the joist board, and this is much more important than the thickness of the board.

The load capacity

The load capacity is how much weight the floor will handle. Load capacity is critical when it comes to floor joists. To determine the load capacity, you will have to have expert knowledge in the wood’s structural configuration and properties, as well as the building code requirements. There are two types of loads that have to be taken into account; live loads and dead load. Live load is the weight exerted on the floor by anything that is not connected to the structure, like furniture, people, and appliances. According to the International Residential Code, non-sleeping rooms need to be able to support a live load of at least 40 pounds per square foot, while none sleeping rooms need to support a load of 30 pounds per square foot, according to The International Residential Code. Dead load is the weight of the structure itself and the structures permanently attached to the floor. It is determined by the materials that make up the floor. Any normal wood-frame floor will have a dead load of about 8 pounds per square foot. Thus, any flooring material that weighs more will increase the dead load.

Deflection limits

Deflection is the sag or bend on the floor that is caused by loads. The maximum allowed deflection is a fraction of the span in inches.

Floor Joists installation basics

The floor joists are almost always installed in the home-building process. The complexity that is ordering the right products and installing them in the proper way is the reason why professionals should always handle the installation of floor joists. Floor joists that span between walls and or beams are installed parallel and at regular intervals. If the joist is installed above the foundation wall, then the sill plate for the solid lumber will be bolted to the concrete, and the joists are nailed at the top of the sill. If the joists are to span a long distance, then a beam or wall is installed at the center to offer support. The joists can rest at the top of the beam, or they can hang from either side using metal hangers.

Repairing common joist problems

Though it is sometimes inadvisable to do repairs on the joist installation yourself, homeowners can sometimes do it, depending on the nature of the damage. As a result of their technical nature, floor trusses and I-joists require the assistance of a structural engineer and should not be installed or repaired by anyone else. Even for lumber joists, when there is severe sagging, breaks, cracks, or twisting, you will have to get a structural engineer to come and inspect the area, make recommendations on what course of action is to be taken. That being said, for a majority of lumber joists minor problems, the best solution is to sister the joists. One way you can fix a joist is to sister it using a flitch plate. A flitch plate is a quarter-inch to a half-inch-thick piece of plywood or steel that is then bolted to the sagging joist. However, the most common way, because it is easiest to fix a damaged joist, is to sister it with an identical piece of lumber. At its most basic, the process of sistering begins by first removing any pipes, electrical wires, or any other utilities running through the joist that has been damaged. The new sister joist is then cut to be slightly shorter, or to the same length, as the old joist, and then inserted into the cavity. It is then snugged up against the old joist. If sagging is the issue, you will have to jack up both the damaged joist and a couple of other joists surrounding the damaged one. This is done so that the floor can be raised a bit, making it easier to put the sister joist in place without a lot of fuss. The sister joist will then be anchored to the old joist using construction nails and adhesive. Notches and holes, if and as applicable, are then cut through the new joist, and utility lines are threaded through and reconnected. If the floor frame has been blocked between the joists, a new blocking will be required, so it is cut and installed on both sides.

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