Plans call for a 3-foot cantilever off the second floor for a bump-out that carries the exterior wall and roof above.

For a 3-foot cantilever using 2x10 joists at 16" o.c., the key rule is the maximum allowable cantilever length based on the backspan (the portion of the joist inside the bearing wall).

IRC Rule (R502.3.1): The cantilevered portion must not exceed 1/4 of the backspan. For a 3-foot (36-inch) cantilever, the required backspan is:

Backspan = 36" × 4 = 144" (12 feet)

So, the joist must extend at least 12 feet back into the building from the support point.

Additional Considerations:

• Deflection Limits: The IRC limits deflection to L/240 for cantilevers under live load (R502.3.8). A 3-foot cantilever is generally acceptable for 2x10s at 16" o.c. if the backspan is met.
• Blocking: Install tension blocking (full-depth 2x10 blocking) between joists at the support point to resist uplift forces.
• Bearing: Ensure the joist bears adequately on the exterior wall—minimum 1.5" bearing on wood or masonry (R502.7).
• Load Path: The roof and wall loads above must transfer through the floor system into the cantilever—verify adequate joist size and connection details.

Bottom Line: With 2x10s at 16" o.c., a 3-foot cantilever requires a 12-foot backspan, proper blocking at the support, and compliance with deflection limits. If the existing structure doesn’t meet the backspan requirement, the cantilever is structurally unsound.

IRC R502.3.3 (2021/2024) governs this. A 3-ft cantilever on 2×10s @ 16″ o.c. carrying a bearing wall + roof is at the absolute limit and likely requires an engineered design.

Prescriptive limits (Table R502.3.3(1)):

• Non-bearing wall/roof: Max cantilever = L/4 (joist span ÷ 4).
• Bearing wall + roof (your case): Max cantilever = L/6 (joist span ÷ 6) AND max 2 ft for 2×10 @ 16″ (per footnote a for heavy loads).

The Math: For a 3-ft cantilever under bearing-wall loads, you need a minimum 18-ft backspan (3 ft × 6). Even if your span hits 18 ft, the table hard-caps 2×10 @ 16″ at 2 ft for this load condition.

Required Fixes (pick one):

1. Engineered Design (IRC R301.1.3): Hire an engineer. They will likely specify deeper joists (2×12 or I-joists), tighter spacing (12″ o.c.), or flitch plates.
2. Reduce Load: If the roof load is tiny (e.g., shed dormer, no snow), you might fit "supporting roof only" column (L/4, max 3’-4” for 2×10 @ 16″), but a full exterior wall + roof almost always triggers "supporting wall + roof."
3. Add Support: Drop a post/column to the foundation or a beam below to kill the cantilever.

Detailing if Engineered:

• Full-depth blocking at bearing wall (R502.7).
• Rim board nailed per Table R602.3(1).
• Uplift straps at cantilever tips if wind uplift > 200 plf (R802.11).
• Protect untreated lumber per R317.

Bottom line: Do not frame this prescriptively. Get a stamped detail.