What is the minimum distance a single-wall metal vent pipe must be installed above and below a roof made of combustible material?

The correct answer indicates that a single-wall metal vent pipe must be installed a minimum of 18 inches above and 6 inches below a roof made of combustible material. The reason for the specified clearance is primarily to ensure that there is adequate separation between the vent pipe and any combustible materials, thereby reducing the risk of fire hazards.

The 18-inch clearance above the roof is critical as it allows for proper venting of gases while ensuring that the heat generated by the vent pipe does not ignite any combustible roofing materials. This distance helps maintain safety standards and complies with building codes designed to protect structures from fire risks associated with heating appliances.

The 6-inch clearance below the roof is also important. It provides enough space to prevent heat from the vent pipe from transferring to the combustible materials directly beneath it, further mitigating any potential ignition risk.

In contrast, other options suggest either greater or lesser clearances which do not align with the requirements for maintaining safety around combustible materials. Maintaining the appropriate distances ensures that the installation adheres to industry standards and reduces hazardous conditions associated with venting systems.