Fuel gas piping installed across a roof surface must be elevated at least _____ inches above the roof.

The correct answer is based on plumbing and gas piping installation standards which require that fuel gas piping is elevated above the roof's surface to prevent potential hazards. When fuel gas piping crosses a roof, it must be elevated to avoid damage from snow accumulation, standing water, or debris, which could obstruct flow or cause corrosion over time.

Specifically, a height of at least 3 1/3 inches is mandated to ensure that there is sufficient clearance. This measurement helps maintain the integrity of the gas piping system while also adhering to safety regulations that minimize risks associated with fuel gas leaks or other incidents. Ensuring that the piping is installed at the correct elevation significantly contributes to the long-term performance and safety of gas systems in a residential or commercial setting.

Other heights specified in the other choices, such as 2 inches, 3 inches, or 4 inches, do not meet the industry standards set forth in the relevant codes that emphasize a minimum elevation aimed at addressing both safety and functionality. Therefore, the requirement of 3 1/3 inches is crucial for compliance with these safety measures.