Sistering Joists: What It Is and How to Do It Right

Joists are the lumber just beneath your deck boards and are an essential structural element. In cases where a joist is undersized or becomes weak, you can reinforce it by fastening another joist alongside it. Called a “sister joist,” this new lumber improves the structural integrity of your deck and extends decking lifespan.

It may sound simple, but the job can be pretty complicated. Keep reading to find out what you should know about using this practical solution, including whether to handle it as a DIY project or to hire a professional.

What Is Sistering Joists?

Sistering joists involves attaching new, additional joists alongside the existing ones to reinforce or repair damaged or undersized joists. This method improves structural integrity by distributing the deck’s load more evenly across the joist system. It’s a practical solution when your deck sags or has too much bounce. It can also restore solidity to joists damaged by rot, insects, or age. By sistering joists, you can also extend the reach of the existing joists to support a deck addition or renovation.

Causes and Signs of Joist Damage

There are several ways in which joists can deteriorate, and it is essential to know what to look for if you suspect a problem.

Common Causes of Joist Failure

• Water. Prolonged exposure to moisture leads to rot and weakens the wood fibers of the joist, compromising its structural integrity.
• Insects. Termites and carpenter ants can hollow out joists, reducing load capacity.
• Overloading. Placing more weight on the joists than they were approved for causes sagging or cracking.
• Excessive notching. Cutting into joists for plumbing or wiring can weaken structural integrity.
• Poor installation. Incorrect spacing, excessive spans, and insufficient support can lead to failure.
• Wood defects. Knots, splits, and warping reduce the strength and durability of joists.
• Compromised fasteners. Rusted nails or screws can loosen, causing joists to shift or detach. Undersized fasteners are also a danger.

Warning Signs

Common warning signs of compromised joists include:

• Sagging deck. A bowed or bouncy deck could indicate weakened, undersized, or overloaded joists.
• Unevenness. Shifting or settling joists can cause deck boards to twist, rise, or sink.
• Rot or mold. Joists that look dark, feel soft, or harbor fungal growth often signal that moisture is causing decay.
• Insects. Signs of an insect problem include tunnels, fine wood dust, and hollow-sounding wood.
• Splits and cracks. When the wood isn’t solid, especially at the joist ends, it has less load-bearing capacity.
• Loose or squeaky boards. A gap between the deck boards and joist indicates movement, meaning the joist could be loosening or separating.
• Rusty or missing fasteners. Corroded fasteners or missing nails or screws can undermine the deck structure.

How to Sister a Joist, Step-by-Step

1. Identify the Problem Area

Is the problem limited to one joist, or are others also compromised?

2. Select Proper Materials

The sister joist should match the wood type (typically pressure-treated fir or pine) and size (typically 2 x 8 or 2 x 10) of the existing joist.

3. Remove Obstructions

If structural wood blocking, wiring, plumbing, or ductwork is in the way, remove or loosen them to make room for the sister joist.

4. Measure and Cut Sister Joist

Sister joists can cover part or all of the existing joist. The more coverage, the better. Add joist tape to the top edge of the new joist.

5. Apply Construction Adhesive

After verifying the fit, glue the joists together with an exterior-rated construction adhesive. Hold them together with clamps while you prepare to fasten them with screws or bolts.

6. Fasten the Sister Joist Securely

Using lag screws, structural screws, or carriage bolts, attach the sister joist to the existing joist. If space allows, use joist hangers to support each end of the sister joist.

7. Add Blocking or Bridging (if needed)

Replace any blocking or bridging that you removed to install the sister joist. Add more blocking between joists to prevent them from twisting.

8. Inspect and Test

Double-check your work to ensure that you used the correct size and number of fasteners and that they are spaced correctly. Ensure both joist ends are securely fastened to the bearing points. There should be no movement or gaps.

DIY vs. Hiring a Professional

Sistering a deck joist can be a manageable DIY project if you have the necessary construction knowledge and own the right tools. In other cases, it’s best to hire a professional.

Do It Yourself

Here are some situations where a DIY approach may be effective.

Minimal damage. If a joist is only slightly compromised and/or doesn’t need to rest on bearing points, DIY makes sense.

Accessibility. The easier it is to reach the joist without dismantling the deck, the more likely you are to do the work yourself.

Hire a Pro

There are times when it’s best to hire a professional.

Extensive damage. If the damage to the joist is severe or multiple joists are affected, the deck might have structural issues that require professional assessment.

Complex repairs. If the joist repair involves intricate work, such as dealing with wiring or plumbing lines, or if it requires reinforcing connections with other structural elements, seek a pro.

Safety concerns. If the deck’s structural integrity is compromised or if the repair involves working at heights that pose safety risks, let a pro handle the job.

Frequently Asked Questions About Sistering Joists

How Much Does It Cost to Sister Joists?

For a DIY job, plan on:

• Lumber (2 x 8, 2 x 10): $20 to $40
• Fasteners: $10 to $30
• Construction adhesive: $5 to $10
• Joist tape: $5

The total, not counting your time or the cost of tools, will likely range from $40 to $85 per joist.

If you hire a professional builder:

• Labor (per hour): $60 to $120
• Materials: 15 to 25 percent higher than retail prices

The scope of the work is a big factor. Assuming that four to eight sister joists are needed, expect to pay between $650 and $2,500, depending on complexity and access. You can also expect to see the job done faster and the work should include a warranty.

How Long Does Sistering Joists Take?

The bigger the deck, the more joists it has, and that means more chances for joist issues to develop. Nonetheless, assuming good access to the joists and a professional crew, sistering joists takes 1 to 2 days. If the damaged joists must be replaced instead of being sistered, the job will take longer.

For DIYers, plan extra time for layout, lifting, and fastening, especially if you’re working alone.

Does Sistering Joists Increase Load Capacity?

Yes, by installing a new joist alongside the original, you effectively double the lumber carrying the load. Full-length sistering, from bearing point to bearing point, provides the most strength. However, if you plan to increase the deck load by adding a hot tub or outdoor kitchen, it is recommended that you hire a structural engineer for their expert advice.

What is the Bolt Spacing for Sistering Joists?

Bolt spacing for sistering joists is a building code question best answered by local authorities, but typically, you should aim for:

• Horizontal spacing of 16 to 24 inches along the joist’s length
• Placement at least 2 inches from the top and bottom edges of the joist
• Staggering of the bolts vertically in two rows.

Conclusion

By adding new joists adjacent to the existing ones, sistering strengthens your deck. It’s an effective and economical way to avoid replacing the entire joist assembly. Many sistering jobs are within reach of experienced DIYers. Just be sure you comply with all building codes and do the job right.