What type of pipe must be connected vertically at least 1 foot above the grade plane to connect an outside sheet metal conductor or downspout to a house drain?

The correct answer highlights the specific requirements for connecting an outside sheet metal conductor or downspout to a house drain. In this context, cast iron pipe is advantageous primarily due to its robust structural integrity and resistance to corrosion, which is particularly important in external applications where pipes may be exposed to various environmental conditions.

Connecting the pipe vertically at least 1 foot above the grade plane is essential to prevent backflow and ensure proper drainage. Cast iron pipe meets the necessary standards outlined in plumbing codes that pertain to drainage systems, ensuring not only compliance with regulations but also durability and longevity in the connection.

Other types of pipes, such as PVC, ABS, and iron, may have uses in plumbing but may not meet the same level of performance or code requirements for this specific application. PVC and ABS, for instance, although lightweight and easy to install, are typically used in different contexts and may not provide the same level of durability against external elements as cast iron when installed in an outdoor setting. Iron pipe, while durable, may not be as commonly utilized in modern plumbing systems compared to cast iron, which is often favored for its performance characteristics.