What Is a Floor Joist in Construction?

February 4, 2024

A floor joist in construction is a crucial component that transfers the weight of your house to its foundation or beams. It is designed to support both the dead load (the weight of your walls, roof and fixtures) and the live load (weight of furniture and occupants). There are three main floor joist types and each one has its own unique purpose.

Solid-Lumber Joists

Solid-lumber floor joists are the most common and can be found under most homes built before 1990. They typically require a smaller distance between each joist and can have a higher load capacity than other floor joist systems. The drawback of solid-lumber joists is that they are more susceptible to wood rot and must be regularly painted or treated.

Open-Web Floor Joists

Open-web floor joists use metal connectors instead of nails or screws to connect the joists together. These connectors help reduce the amount of friction between joists and also increase the overall strength of the floor. Additionally, they are less likely to shrink or bow over time. They are a good option when there are large gaps to span between walls or load-bearing walls.

Regardless of which floor joist system you choose, they must be designed by an engineer to ensure they can support the loads placed on them. These engineers will take into account things like the spacing, the type of lumber and how much the joists can be deflected. It’s important to follow recommended building codes when sizing and spacing your floor joists. Joists that are too small or too far apart can cause sagging and bouncing in the floors of your home. This can lead to unstable flooring, skewed windows and doors and even sinking crawl space supports.


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