Final answer:
Ceiling joists overlapping above a loadbearing partition commonly require a 12-inch overlap, though the specific measurement may vary by local building codes or practices.
Step-by-step explanation:
When ceiling joists must be overlapped above a loadbearing partition, the overlap must be at least 4 inches. However, commonly a 12-inch overlap is recommended or required by most building codes for added stability and to adequately transfer the load across the overlap. The specific building codes or engineering practices in your region may require a different amount, so it's crucial to consult local building codes or a structural engineer when planning construction.