Final answer:
The header joist may be a single member the same size as the floor joist if its span does not exceed four feet, in accordance with the International Residential Code (IRC) for residential construction.
Step-by-step explanation:
When discussing the construction of a floor system, specific rules apply to the dimensions and allowable spans of joists, header joists, and other structural elements. The span limit for a header joist is one such rule. For residential construction, as per the International Residential Code (IRC), a header joist may be a single member the same size as the floor joist when the header joist span does not exceed four feet. It's important for engineers, architects, and builders to adhere to code recommendations to ensure the safety and integrity of the structure.