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What Is a Floor Joist? Overview, Spacing Requirements, and More
Flooring’s backbone
Photo: MoMo Productions / DigitalVision / Getty Images
Photo: MoMo Productions / DigitalVision / Getty Images
Highlights
Floor joists are the horizontal structures between beams.
Single and double joists are the most common in modern homes.
The size of your joist spacing is determined by the size, species, and grade of your lumber, as well as final structural loads.
Damaged joists can lead to sagging or uneven floors.
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Healthy floor joists are crucial to the structural integrity of your floors and ceilings. You may hear the term “joist” come up during a home inspection or when you hire a contractor to repair sagging floors. This article will teach you all you need to know about what a floor joist is, its specifications, signs of a damaged joist, and how to approach the repair.
What Is a Floor Joist?
Floor joists are the horizontal structural parts of your floor framing. They span the open space between beams so the load can be transferred to the vertical structural pieces. Ultimately, floor joists carry all the weight in a room, including subflooring, flooring, walls, appliances, furniture, and foot traffic.
Types of Floor Joists
In traditional timber-frame buildings, you may find single-set joists and double-set joists. Single sets carry the floor and the ceiling for the room below using the same floor frame. On the other hand, double sets have one set of joists for the floor and a separate set for the ceiling underneath.
Floor joists can also be categorized by structure. The three most common types of floor joists include the following:
Solid lumber joists, which are continuous boards made out of old trees
I-joists, which consist of a center plywood or OSB board with two wooden bars or laminated veneer pieces on the top and bottom, forming an “I” shape
Open-web trusses, which are lumber frames with an opening in the center featuring metal connectors that hold diagonal and vertical pieces to the two horizontal wooden frames on the top and bottom
| Joist Type | Pro | Con |
|---|---|---|
| Solid lumber joists | More fire-resistant, more affordable than engineered joists | Not environmentally friendly, limited in span distance |
| I-joists | Allows longer spans than solid lumber joists, lighter than solid wood | Less fire-resistant, more expensive |
| Open-web trusses | Longer spans and higher construction flexibility that can accommodate HVAC and utilities without cutting, more reinforced than I-joists | Can’t be trimmed since the length is fixed, costs the most |
Joist Spacing
Photo: anatoliy_gleb / Adobe Stock
The spacing, or span, is the distance covered between supporting structures. The larger the structure, the larger the joists. While most joists are placed at 16 inches on center, some can also be placed between 12 and 24 inches apart.
Since there are no standard joist sizes, a structural engineer needs to calculate joist spacing accurately based on the following factors:
Wood species: Stronger wood species can span a longer distance before needing support because they don’t bend as easily.
Wood grade: Higher-grade lumber has fewer defects, making it sturdier.
Lumber width: The width from the top to the bottom of your joist affects the joist board’s strength.
Load capacity (dead load): How much weight from furniture, appliances, floors, and walls must be supported by the joists?
Live loads: A pro will calculate foot traffic, including residents, maximum guests, and pets.
Deflection limits: This is the maximum load before the flooring begins to sag.
Joists vs. Beams
Joists and beams are two of the key pieces in your floor frame. Joists are the horizontal pieces in your support structure, and beams are the vertical pieces in the framework. Joists are thinner than beams and are often less than 10 feet long.
Beams can span across the entire home, so you don’t need as many of them. Joists are almost always made with lumber, except for I-joists, which use plywood or OSB board as the center panel. However, beams can be made of lumber, LVL, or even steel.
Signs of Joist Failure
Various factors, including moisture and mold damage, improper drilling, pest damage, or having too much weight on top of the joists can damage your floor’s entire structure.
Failing joists can cause uneven floors or creaking as the subflooring bends or falls into the hollow space. Bad joists can even be the reason why your floor is sagging, although that can also be caused by having too much load in one area (if the weight exceeds your joist’s maximum allowance).
Repairing Floor Joists
Leaving joist damage unattended means that the floor problems you’re facing will get worse. Since floor joists are a critical piece in your home’s structural integrity, you should hire a professional flooring company to inspect, repair, or replace damaged joists. A professional will be able to calculate the proper span, replace existing joists with the best alternative, and ensure the flooring is restored properly after they’ve finished the repair.
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