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What Are Floor Joists?

Floor joist in a crawl space.

Floor joists keep your floors sturdy and level. Without them, your furniture, walls, and everything inside your home would be unstable.

Unfortunately, floor joists, especially those found in crawl spaces, are highly susceptible to moisture damage. In this article, we will explore what floor joists are and their crucial role in supporting your home’s structure.

Floor Joists Explained 

Floor joists are horizontal structural support beams found below the floorboards in a building. They transfer the weight of the floor to the foundation of the structure.

Floor joists need to be spaced evenly to provide maximum support. Since they play such an important role, floor joists should undergo regular inspection and maintenance, especially if you notice issues like a bouncy floor.

Types of Floor Joists 

Contractors choose from three types of joists for repair. Each type has its advantages and flaws, so it is important to assess the area to determine which floor joist will provide the best support. 


I-joists are primarily composed of OSB or plywood, with solid wood only at their ends. Named for their “I” shape, these joists sometimes span over 20 feet and have a high weight capacity, making them ideal for tile flooring. Some I-joists come with predrilled holes for utility lines, simplifying installation.

Solid Lumber 

Solid lumber floor joists are the traditional choice for construction. They’ve been used for a long time before newer materials came along.

During installation, builders need to follow safety rules and remember to create holes for pipes and wires. Overall, solid lumber joists are a simple, tried-and-true option for supporting floors.

Open-Web Floor Trusses 

Open-web floor trusses are a popular choice for builders. They’re made from 2×4 boards arranged in a web-like pattern, held together by metal plates.

This design leaves open spaces between the boards, making it easy to run pipes and wires. These trusses can cover larger areas than other types of joists. They’re also quick to install, which can save money on building costs.

Signs of Failing Floor Joists

Floor joists can fail over time, and it’s important to spot the signs early. Here are some things to watch for:

Rotting floor joist.
  • Sagging or uneven floors: If your floor feels bouncy or looks uneven, the joists are weak.
  • Cracks in walls: Wall cracks may appear when joists aren’t supporting the house properly.
  • Squeaky floors: Loud creaking when you walk might mean the joists are loose or damaged.
  • Mold growth: Dark spots on joists or walls can be a sign of mold, which eats away at wood.
  • Wood rot: Soft or crumbly joists may indicate wood rot and weakening, making them unsafe.

If you notice any of these signs, it’s best to have an expert check your floor joists to keep your home safe.

What Causes Damage to Floor Joists?

Several factors can impact your floor joists and cause them to malfunction. 

Puddle of water in a crawl space.
  • Poor support design: Incorrect spacing or worn-out floor joists can lead to sagging floors.
  • Increased home weight: Adding stories or expansions may overload existing joists, causing cracks.
  • Moisture and humidity: High humidity and moisture causes damage to floor joists.

Read more about the causes behind floor joist damage in this guide.

Fix Floor Joist Damage With AFS 

Reduce moisture levels under your home to keep your floor joists in optimal condition. Otherwise, the joists will absorb water, increasing the risk of wood rot and mold.

Crew installing crawl space encapsulation.

Consider working with AFS to install a crawl space encapsulation system that keeps water out with various tools, including:

Damaged floor joists should be repaired as quickly as possible. If you notice signs of floor joist damage, contact AFS today. We proudly serve homeowners in Tennessee, Alabama, Florida, Kentucky, and Georgia, so contact us today and schedule a free inspection with our team. 

Floor Joist FAQs

Yes, but only through pre-drilled holes or notches. Work with a professional to ensure you don’t compromise the joist’s structure.

Floor joists should be inspected annually. It’s also wise to check after major weather events or if you notice any changes in your floors.

Well-maintained floor joists can last 50-100 years or more. Their lifespan depends on factors like material, environment, and maintenance.

Related Resources

Ted Dryce

Ted Dryce

Content Writer

Ted is an SEO Content Writer who has been with Groundworks since 2021. He’s covered home repair topics ranging from crawl space encapsulation to regional soil conditions. When he’s not working, Ted is performing improv comedy and working on his own creative projects.

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