What are Floor Joists?

Mary McMahon
Mary McMahon

Floor joists are an important part of the supportive structure of a floor. They hold up the weight of a building, absorb impacts on the floor, and create structural support so that the floor will be stable and secure. Joists are installed at a very early stage in the building process, and in older structures, they may eventually need to be replaced as a result of wear, or if they were installed improperly when the building was constructed. Many people never see them, unless they are involved in remodeling and construction projects.

View of second story floor joists from below.
View of second story floor joists from below.

These structural elements of a building make up part of the subfloor, the structure that goes underneath the flooring that people walk on. Subfloors are designed to provide support to the flooring, and to make the building itself more rigid and stable so that it will withstand years of heavy use. Joists are typically run perpendicular to beams, and they connect with the supportive framework of the structure to reinforce it.

Floor joists on new construction.
Floor joists on new construction.

It is very important to install floor joists properly. Their spacing is critical to engineering safety, and they must be perfectly flat and level, or the floor will be uneven. High quality wood must also be used, as bending and warping in the joists will throw the floor and the building out of plumb, causing structural problems and safety issues. In addition, contractors have to think about ease of access for repairs and other projects when they install the subfloor.

Plumbing, electrical lines, and ductwork may be installed in the subfloor at the time it is built, or on a later date. Thinking about these issues ahead of time is important when the building is erected, as badly-placed joists can make construction much more difficult than it needs to be. Contractors also have to think about issues such as the potential for water intrusion into the subfloor, which can cause structural problems with the building.

A number of different products can be used for floor joists. Wood is a classic material of choice, and it continues to be widely used around the world. It is also possible to use various fabricated metal products, along with specially engineered plastics. In all cases, reputable contractors will use materials that satisfy or exceed the building code, and they will take care when installing the joists and the other elements of the subfloor to reduce the need for maintenance in the future.

A standard car jack can be used to lift a floor joist during repairs.
A standard car jack can be used to lift a floor joist during repairs.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


You can also repair or "Reinforce" Joists using the Metwood Joist Reinforcers! They work great.


@EdRick - As @rugby girl said, word of mouth is a great place to get a head start on contractors to use. If you need other places to look for a reputable contractor who will meet or exceed the building code as stated in the article, try contacting your state's website listing for their residential construction commission once you get a company name to make sure that the contractor in question holds a *current* license with your state as well.

Your local city's building office listed in your city's website can tell you what the current building code they use as well. Additionally, ask them what type of permit is required to do the work and make sure the contractor you select pulls one (ask for a copy of it so you can show proof of the work done to your interested buyer). Good luck!


@EdRick - I feel your pain! That stinks. I hope you're able to salvage the contract.

Basically, you just need a licensed contractor to put in new timber floor joists.. But with the contract on your house, you'll naturally be concerned that the work be completed in a timely fashion. Ask around!

Ask literally everyone you know--people at work, the other parents at your kid's daycare, the people on your bowling team, etc. Ask who's had repair work done on their house, who did the work, and how satisfied the homeowner was with the timeliness and quality of the work. Good luck!


How do I go about finding someone to replace our floor joists? They are made out of wood and we just had a pest inspection as part of a contract to sell our house. Turns out we have extensive termite damage!

Thankfully, the buyers haven't run for the hills yet. (I probably would have!) But naturally, they want us to have all the problems fixed, and the joists need to be replaced. We haven't been homeowners very long. What kind of company do I call? How do I find someone good?

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