Strong floor joists are the key to a structurally safe home. The cost of floor joist repair will often come down to what is causing the problem.
What Is a Floor Joist? Overview, Spacing Requirements, and More
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Floor joists are horizontal wooden structures that span between beams to support your floors, ceilings, subflooring, walls, furniture, and all foot traffic.
Joist spacing depends on wood species, lumber grade, board width, and the weight your floors need to carry from furniture, appliances, and daily use.
Sagging or uneven floors signal joist damage from moisture, mold, improper drilling, or pest issues that you should address before the problem worsens.
Hiring a local flooring company provides reliable help with joist inspection, repair, and replacement to restore your home's structural integrity and level 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.
Flooring services reviews from real Angi homeowners in Columbus, OH
4.4★ avg. • 41 total reviews
- Sarah E.
Joseph did a great job with the staircase and trim. Timely and efficient, clear communication about the work, very fair pricing. For the stairs in particular, done very skillfully in an old house (1893) in a tight spiral, and very creative with how to do the handrail. Did a nice job with...
Vouis Trim Carpentry L.L.C. - Jena P.
This company does it right. Firstly, I want to say that Landan was a joy to work with. He was so kind, transparent, and had everything under control. His entire team showed up ready to work and they were all very respectful of the fact that we were in an office space. A little backstory – we...
Krumlauf Floors - Gina M.
Finished Basements Plus was wonderful from beginning to end! First, they came out quickly for an estimate. I had 4 estimates and theirs was by far the best. (2 of the estimates were more than $20K higher than for the exact same work). Beyond their fair pricing, the work was excellent. They...
Finished Basements Plus Inc. - Jeanann S.
Very professional, knowledgeable and helpful! Completed the install on time and as expected.
Ward's Services - Grace G.
Excellent, Prompt, Courteous Service!!! They were super friendly and quite personable. I felt very comfortable with the gentlemen that did the installation. They installed it exactly where I asked them to and they did an excellent job!! I loved their service so much.
Tether House - David E.
Window Depot did an amazing job installing the windows in my basement! The process from start to finish was by far more than what I expected. I would highly recommend Window Depot if you want professionalism from the the consultation to the installation. Dont settle for mediocre when you can...
Window Depot USA of Central Ohio - Tom B.
AFS was excellent from start to finish. Jess, the sales associate, was fantastic...very knowledgeable and very professional. The installation crew was excellent. I couldn't be more pleased with the process. Best of all, my new floor looks fantastic!
America's Floor Source - Bob T.
We were having 2”x 2” tile installed . There was a small installation error and the new additional replacement tile was from a different lot and the colors didn’t match. Joe assured us that he would rectify everything , which he did without hesitation. We decided to go with the 18” x 36” tile...
CarifaTile - Jake H.
Did an amazing job transforming my yard. Also installed sod in places, would recommend to everyone.
Cardinal Landscaping & Construction LLC - Bob T.
We hired Jim to replace our hardwood floors with new hardwood. He and his helper Jerry did a fantastic job. Their work ethic is impeccable!! We would highly recommend McColl Floors!
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