Building Code for Sistering Joists

To sister a joist means to fasten a new joist alongside an existing one. It’s a tried-and-true method for repairing damaged joists, strengthening weak ones, reinforcing sound joists to bear more load, and/or extending joists to expand your deck.

Before you start, I recommend you learn what the building code says about sistering. That’s important because substandard structural work puts your safety and the longevity of your deck at risk. There could also be legal consequences for not following code.

What Is Sistering Joists?

Sistered joists are paired joists, one existing and one new. The two are fastened side by side to increase the structural strength of the deck framing. By doing this you correct for sags and excessive deflection (bounce) due to joists that are damaged by rot, insects, or age. Since sistered joists can carry more load than a single joist, they enable your deck to support more weight, such as a hot tub or outdoor kitchen. You can also use sistering to extend the reach of the existing joists, allowing you to enlarge the deck.

Building Code Requirements for Sistering Joists

The building code doesn’t mention sistering joists specifically, but the ICC Digital Codes on spacing cite the guidance on joist spans and deck spacing as provided by the International Residential Code (IRC). With this information—mainly IRC sections R502 to R507 for wood framing and decks—you can calculate how much weight (load) the sistered joists can support.

The key considerations and requirements include

• What type of lumber to use. The sister joist should match the wood type (typically pressure-treated fir or pine) and size (typically 2×8 or 2×10) of the existing joist. IRC Table R507.6 provides allowable spans for deck joists depending on lumber size, species, and spacing. The maximum cantilever is equal to one-fourth the permissible span. If your deck’s joists are already cantilevered, extending them will likely require additional support to be code-compliant.

• How much to overlap the old joist. Sistered joists used for structural reinforcement should extend over at least 1/3 of the original joist length (ideally the full span) to transfer the load adequately. The minimum overlap is 3 feet, with the overlapped sections bearing on the same or similar support as the original joist.

• How to fasten the joists. You can use nails, screws, or bolts per IRC R502.9. Building officials typically expect to see through-bolts, lag screws, or structural screws/nails applied in a staggered pattern every 16 to 24 inches. Use at least two fasteners at each end of the sister joist. Be sure to fasten the sistered joists to supporting members.

• How much bearing support to provide. This will depend on the kind of wood you use, joist size, and the span. At a minimum, the new joist requires support equal to or greater than that of the existing joist.

As always, check with your local building authorities for additional insight. Local codes are often more detailed.

Best Practices for Sistering Joists

For long-term deck stability and safety, install the sister joists using accepted best practices. Above and beyond the code, you should:

• Plan how you’ll deal with structural wood blocking, wiring, plumbing, or ductwork that is in the way. Ask a pro for help if you’re unsure.

• Use lumber that is free of significant defects, especially at the fastening locations.
• Run the sister joist over the full span, from ledger to beam, if possible.
• Use a jack to lift sagging joists back into position before you attach the sister joist. Make sure the new and existing joists align along their full lengths without any gaps.
• To protect the deck joists, cover the top of them with joist tape. This waterproof membrane will prevent any source of moisture—such as sodden tree litter—from reaching the joist and rotting it.

• Install the sister joist crown side up. The slight arch will help the new joist stay in plane with the other joists as natural settling occurs.
• Apply exterior-rated construction adhesive to the joists and clamp them together before you fasten them with nails, screws, or bolts.
• Seek professional advice if you plan to sister new deck joists to your house joists, since many of today’s houses use prefab I-beam wood joists, not solid wood.

Inspection and Permits

In most areas, a significant structural alteration to your deck requires one or more building permits.

By pulling a permit, you will trigger scheduled inspections from the local building authority. In this way, your community ensures that its structures meet local building codes and engineering standards. This protects against unsafe shortcuts that could cause the deck to fail.

In the case of sistering deck joists, inspectors might verify that the damaged joist isn’t part of a larger structural failure; that the sister joist is appropriately sized and fastened; and that the load-bearing capacity and deflection are code-compliant.

Inspectors typically check that:

• Only structural fasteners were used (bolts/lags/nails);
• Fasteners are applied in a staggered pattern, not in a straight line, with two fasteners at each end, and regular fastener spacing along the length;
• The joists are correctly aligned and without gaps;
• The sister joist bears fully on supports and/or is hung properly.

Keep in mind that building rules and regulations vary by location, so seek the advice of your local authorities. Many provide information, worksheets, and forms on their websites.

Cost and Time Considerations

When considering material costs and your time commitment, first decide whether sistering is work you can do yourself or is better suited to a professional. Next, count how many joists you’re reinforcing. It’s best to cover each joist’s full span, and getting access probably means removing some portion of the deck’s fascia and maybe a section of the rim joist.

To estimate the cost of the materials required, I’ll assume the joists are 2×8, 2×10, or 2×12 and are 8 to 16 feet long. Cost for one joist: $15 to $45.

Fasteners—nails, through-bolts, or structural screws—will cost $10 to $20 per joist. If you need joist hangers, add $2 to $5 per joist. Add another $1 per joist for the cost of adhesive.

Butyl joist tape retails for about $0.50 per foot, meaning you’ll spend between $4 and $8 per joist.

The result: A per-joist cost somewhere between $30 and $75 for DIYers.

As for the time required, it depends on how accessible the deck joists are and how skilled you are. Expect to spend 1.5 to 2 hours on each joist.

Pros will do the job faster, but it will cost more. It’s difficult to determine how much because professional labor rates vary by location, and complex or large jobs take more time than simple or small ones. My rough estimate for a pro to handle the job is $100 to $200 per joist.

For low, small decks with easy access and without utilities to work around, sistering joists is a job suitable for many DIYers. If, however, the deck is high, difficult to access, or involves dealing with plumbing and electrical work, hire a pro instead.

Conclusion

Difficult or easy, the job of sistering joists must meet code, or you risk the safety of the people using the deck and the durability of the work. Plus, you could be subject to a fine or forced to redo the work if it was not done correctly and without a building permit.

Sources

Sistering Joists: What It Is and How to Do It Right. Retrieved October 14, 2025. https://trexrainescape.com/blog/sistering-joists/

What Is the Building Code for Sistering Joists and Why You Should Know It. Retrieved October 14, 2025. https://todayshomeowner.com/general/guides/building-code-for-sistering-joists/

2024 International Residential Code (IRC) – R507.6 Deck joists. Retrieved October 14, 2025. https://codes.iccsafe.org/content/IRC2024V1.1/chapter-5-floors#IRC2024V1.1_Pt03_Ch05_SecR507.6