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Sistering Joists 101 – A Beginners Guide

sistering joists
sistering joists
ByMariel McClung
Table of Contents

Looking for information about sistering joists? If so, you’ve landed on the right page because that’s what we’re going to review in this article. We’ll review what floor joists are, why they might need sistering, what it means to “sister” a joist, and more.

What Are Floor Joists?

If you have a crawl space foundation, floor joists are essential structural elements used to support your home’s ground floor. They’re long, horizontal, wooden members placed parallel on top of beams. Building codes and regulations dictate the minimum floor joist spacing and size requirements. See the illustration below for the difference between joists and beams (also known as girders).

floor joists are essential structural elements used to support your home’s ground floor.

What Does It Mean to “Sister” a Joist?

When builders need to increase the load-bearing capacity of floor joists, they often use a process known as “sistering.” This process involves attaching a new joist alongside an existing joist to increase its strength. This restores the structural integrity of the flooring system, providing a long-term solution with minimal disruption and cost compared to replacing the entire joist.

Sistering a joist is a common solution for older homes with weakened or damaged floor joists and for new construction where design changes require additional support. It’s an effective way to repair and reinforce weak spots in the structure.

Why Might a Floor Joist Need Sistering?

There are several reasons why a floor joist may require sistering. These include the following:

  • The joist has deteriorated – Aging and exposure to moisture can cause the joist’s wood fibers to weaken. When this happens, the joist can’t support as much weight as before.
  • Termites have damaged the joist – Termite infestations can destroy significant portions of the joist, compromising its load-bearing capacity.
  • The joists aren’t big enough, or they’re spaced too far apart – Improper installation, such as under-sizing or inadequate spacing, can put excessive stress on a joist and cause it to sag or deflect under loads.

While joist sistering can provide a quick and effective way to strengthen compromised joists and maintain the overall strength and stability of the floor, it’s not the only possible solution for supporting weakened floor joists. Other options include adding additional beams, blocking or bridging the joists to keep them straight (when they’re straight, they’re stronger), and adding support columns or posts under the beams. See the illustration below:

6 Types of Crawl Space Floor Support

How Long Does It Take To Sister a Joist?

The time it takes to sister a joist depends on several factors, including the size and condition of the joist and the project’s complexity. In general, sistering a joist can take a few hours to a full day. The process may only take a few hours for a simple project, such as sistering a small, lightweight joist. However, the task may take closer to a full day for larger or more complex projects, such as those involving larger, heavier joists or intricate designs.

Sistering a joist isn’t a task that should be rushed. It requires high precision and attention to detail to ensure the support joist is attached correctly and aligned with the existing joist.

For more information, see What Are Crawl Space Jacks And When Do You Need Them?

How Much Does Joist Sistering Cost?

The cost of joist sistering varies depending on factors such as the type of lumber, the length of the joist, and the severity of the damage to the existing joist. On average, you can expect to pay anywhere from $100 to $300 for each joist, not including the labor cost. In cases where there’s significant damage to multiple joists or where additional supports such as posts or beams are needed, the costs may increase significantly. Other cost factors include the cost of lumber, nails, and other materials required for the project and your geographical location.

While it may seem costly, the benefits of joist sistering far outweigh the costs. By investing in this technique, you can extend the life of your foundation and prevent further damage, saving you money in the long run.

What About DIY Joist Sistering? 

Joist sistering is not a DIY project. It should only be done by experienced foundation repair professionals. Hiring a licensed contractor to sister a joist will help ensure the quality of the work done.

Signs There’s a Problem in Your Crawl Space

Your crawl space may be out of sight, but it shouldn’t be out of mind because it plays a crucial role in maintaining the structural integrity of your entire property. Here are some things to look out for:

  • The floor over the crawl space is sagging – One of the most common signs there’s a problem in your crawl space is a sagging floor. This occurs when weakened floor joists start to bend. Your floors may look uneven or even feel spongy when you walk on them.
  • Furniture shakes when you walk by – This is a telltale sign there’s a problem with the wooden structures in the crawl space.
  • Your home smells musty – Mold in a crawl space can make your home’s living area smell musty.
  • Doors and windows don’t open and close correctly – This could mean a problem with the support structure in the crawl space.
  • Cracked walls – If the wooden structures in your crawl space are no longer stable, it can cause various problems, including cracked walls.
  • There’s mold on your baseboards or floor above the crawl space – This is a sign the crawl space is full of mold, which means the crawl space is full of moisture.

If you think there might be a problem with the joists in your home’s crawl space, contact AMC911 today to schedule a crawl space evaluation. If we find a problem, we’ll give you a repair estimate.


Mariel McClung

Mariel McClung is AMC911’s Vice President and full-time mother. Mariel studied Marketing at The College of William & Mary and is the operational brain behind AMC911.

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