How To Repair a Cracked Floor Joist

Cracked floor joists are broken beams that can lead to sagging floors and structural issues in your home. Fortunately, repairing a cracked floor joist is a manageable task for homeowners with basic carpentry skills.

In the video above, This Old House general contractor Tom Silva demonstrates how to repair a cracked floor joist in an older home and restore the floor’s structural integrity. We’ve also included step-by-step instructions and other guidance below.

Understanding Floor Joists and Common Issues

Floor joists are horizontal structural boards that support a home or building’s floor. They span from wall to wall or beam to beam, creating a foundation for the subfloor and finished flooring above. Over time, these important components can develop issues for a variety of reasons.

Causes of Cracked Floor Joists

Several factors can contribute to cracked floor joists:

• Age and natural settling of the house
• Excessive weight or load on the floor
• Improper notching or drilling during construction or renovations
• Moisture damage or wood rot
• Poor installation practices or inferior quality materials
• Termite or other pest infestations

Signs of a Cracked Floor Joist

Identifying a cracked floor joist early can prevent more severe structural problems down the line. Look out for the following indicators:

• Doors that stick or don’t close properly and windows that are difficult to open
• Furniture that wobbles or feels unstable on the floor
• Gaps between the floor and baseboards
• Sagging or uneven floors
• Squeaky or bouncy floors
• Visible cracks in the joist when inspecting from below

Assessing the Floor Joist Damage

Before attempting any repairs, assess your floor joist damage. This will determine the best repair method and materials.

Inspecting the Joist

1. Access the area below the floor, typically in the basement or crawl space.
2. Use a flashlight to examine the joists for visible cracks, splits, or other damage.
3. Check for any signs of moisture, rot, or pest infestation.
4. Measure the depth and length of any cracks you find.
5. Use a moisture meter to detect hidden moisture within the joist, if necessary.

Determining the Repair Method

The appropriate repair method depends on the severity of the damage:

• Minor cracks may be fixed with wood glue and reinforcement.
• More significant damage might require sistering the joist, a process where a new joist is attached to the damaged one.
• Extensive damage or rot may call for complete joist replacement.
• For structural integrity, sometimes additional support, like adding steel plates, might be best.

Repairing a cracked floor joist is a feasible do-it-yourself (DIY) project. However, if you’re unsure about the damage’s extent or your ability to make the repair, you should hire a professional contractor.

Tools and Materials Needed to Repair a Floor Joist

Gather the following tools and materials before starting the repair:

• 2×4 post
• Carpenter’s wood glue
• Cloth for wiping excess glue
• Hammer
• Hanger nails
• Hydraulic jack
• Metal joist hanger
• Moisture meter (optional)
• Safety glasses and work gloves
• Supplemental steel plates (optional)

Steps To Repair a Cracked Floor Joist

1. Apply a generous amount of carpenter’s wood glue into the crack of the floor joist.
2. Slide a metal joist hanger onto the end of the joist.
3. Position a 2×4 post and hydraulic jack directly beneath the cracked end of the joist.
4. Carefully pump the jack to push the crack closed.
5. Secure the joist hanger to the beam using hanger nails.
6. Wipe away any excess glue from the joist with a clean cloth.
7. Allow the glue to dry completely before removing the jack and post.
8. If you choose to add steel plates for extra support, screw them onto both sides of the joist using bolts.

Silva demonstrates this process in the video, emphasizing the importance of using the correct joist hanger type and firmly closing the crack before securing the hanger.

Reinforcing the Floor Joist Repair

To further strengthen the repaired joist and prevent future issues, consider these extra steps.

Install Cross-Bridging

Cross-bridging helps distribute the load across multiple joists, reducing stress on any single one.

1. Cut diagonal braces to fit between joists
2. Install the braces in an X pattern between joists
3. Secure the braces with nails at each end

Cross-bridging not only adds stability to the floor but also helps lessen vibrations and squeaks that might occur from normal household activities.

Add Solid Blocking

Solid blocking provides additional support and stiffens the floor.

1. Cut pieces of lumber to fit snugly between joists
2. Install the blocking perpendicular to the joists, making sure it fits snugly
3. Stagger the joints of the blocking for maximum strength
4. Secure the blocking with nails

Solid blocking is particularly useful in preventing joist twisting and increasing overall floor rigidity. Distributing force evenly across floor surfaces can prevent future sagging and potential cracks.

Sister the Joist

For more significant damage, sistering the joist can provide added stability and support:

1. Cut a new joist to match the length of the damaged one.
2. Position the new joist alongside the damaged joist.
3. Secure them together with nails or bolts at regular intervals.
4. Check that the new joist is level and aligns well with the surrounding joists.

Sistering is a good way to increase a damaged joist’s strength, especially in older homes where replacing entire joists isn’t feasible without extensive renovations.

Preventing Future Joist Damage

To avoid future issues with your floor joists, follow these guidelines.

• Address any moisture issues in your basement or crawl space quickly.
• Avoid excessive notching or drilling of joists during renovations.
• Conduct regular inspections of your home’s structure.
• Distribute heavy loads evenly across the floor.
• Ensure there is good insulation around joists to prevent condensation-related damage.
• Install proper ventilation in your crawl space or basement to reduce humidity.
• Use pressure-treated lumber or rot-resistant materials, especially in moisture-prone areas.

When To Call a Professional

The following joist repair situations should be handled by professionals:

• Compromised beams or foundational elements
• Lack of confidence in your ability to safely complete repairs
• Multiple cracked joists
• Signs of extensive rot or pest damage
• Structural issues beyond a single joist

A professional contractor can assess the situation and recommend the best course of action for more complex repairs. They may also have access to specialized tools and materials that can make the project go faster.