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Rim Joists

A rim joist, also known as a band board, is an important component of a building’s framing system. This type of joist runs along the perimeter of a floor framing system, where the ends of the other joists meet. The rim joist forms the outer edge of the floor assembly and provides a nailing surface for the edge of the floor sheathing, which is usually made of plywood or oriented strand board (OSB).

Rim joists serve a critical function in distributing the load of the structure above to the foundation below. In a two-story home, the rim joists bear the weight of the wall, the upper floor, its wall, and the roof. They also work with two outer joists, forming a band for the floor’s framework.

Rim joists are made of a variety of materials, including solid wood, engineered wood products, and laminated wood. The conventional material for creating floor joists is solid wood free from knots. Engineered wood products, such as laminated veneer lumber (LVL) or oriented strand board (OSB), are other common materials used for rim joists. The material used for rim joists is often the same size as other joists to ensure that rims are flush with joists on top of them.

Rim joists can be vulnerable to moisture intrusion and air leakage, leading to decay, insect infestation, and energy loss. Sealing and insulating rim joists is important to prevent these issues properly. This can be done by installing a vapor barrier to prevent moisture from entering the space.


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What are Floor Joists?

Floor joists are another important component of a building’s framing system that work with rim joists to support the weight of the structure above. Unlike rim joists, which run along the perimeter of a floor framing system, floor joists run parallel to each other and are spaced at regular intervals.

Floor joists are typically made of solid wood, engineered wood products, or laminated wood and are designed to support the weight of the floor system, as well as any live loads such as furniture and people. They also help transfer the structure load above to the foundation below.
In addition to supporting the structure’s weight, floor joists must also resist deflection or bending, which can lead to sagging or bouncing floors. Proper spacing and sizing of floor joists are crucial for preventing deflection and ensuring the structural integrity of the building.

Like rim joists, floor joists can also be vulnerable to moisture intrusion and air leakage, leading to decay, insect infestation, and energy loss. Properly sealing and insulating floor joists is important for preventing these issues.

What Do Rim Joists Do?

A rim joist or band board provides lateral support for the other joists in your subfloor preventing them from collapsing under your structure’s weight. On top of this, it caps off the terminal areas of other joists. This in turn prevents cavities that might expose your home to outside air or moisture. Another thing that the rim joist does is provide a mass of wood for nailing during the installation of your sheathing, siding, or trim boards.

Telltale Signs of Failing Floor Joists

A sagging floor is the biggest telltale sign of damaged joists. Other signs to look out for include:

If you notice any signs of damage, it’s important to contact a professional to have your floor joists evaluated and repaired immediately. Timely repairs can prevent further damage to the structure and ensure the safety of occupants. A licensed and experienced professional can assess the extent of the damage and recommend the best course of action for repair or replacement of the floor joists.

Common Floor Joist Issues

Understanding typical floor joist issues is crucial to coming up with the perfect solution. Some common floor joist problems include:

Wood Rot

  • The support holding up your home comprises wooden joists. Without proper sealing and waterproofing, your crawl space becomes prone to moisture and humidity. These conditions create the perfect breeding ground for mold and wood rot. Over time, these fungi can destroy the integral strength of your joists, posts, and girders. Hence, your joists may sag and cause the floor above your crawl space to sink or slope.
wood rot mold in crawl space

Poor Construction

  • Sometimes, contractors build crawl spaces with joists that are too few, while other times, the spacing is the issue. When there aren’t enough joists to support the weight of your household, the beams may begin to weaken and sag down. This may cause uneven floors and create an unsafe environment for your home.
poor construction crawl space joint

Termite Damage

  • Termites eat just about anything that contains cellulose, including floor joists. When termites attack your floor joist, they compromise its integrity, leading to loose or saggy floors.
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Fixing and Protecting Your Joists

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Any damage to your floor joists presents a safety problem and needs immediate repair. To protect your frames from rotting, start by addressing the moisture issues in all areas of your home, especially the basement or crawl space. Ask your carpenter to check and fix broken or damaged floor joists. Where repairing is not sufficient, consider replacing the joists altogether.  If the floors above still feel wobbly after your joists are repaired, consider adding extra floor joists to provide additional support.

We also encourage you to have the crawl space, which houses the joists, sealed. What this will do is it will lock out moisture and make it difficult for pests to get in that may threaten the integrity of your joists. This is something Groundworks can help you with. To get started, schedule a free inspection.