What type of pipe cannot be threaded?

In plumbing, certain types of pipe cannot be threaded due to their material properties and wall structure. Cast iron soil pipe is made from a brittle material that is designed primarily for gravity drainage systems, which leads to its inability to be threaded effectively. The wall thickness and the inherent rigidity of cast iron make it unsuitable for threading, as attempts to do so may lead to cracks or other failures in the pipe.

In contrast, PVC pipe, galvanized steel pipe, and copper pipe can all be threaded. PVC can be manufactured with threaded ends that are compatible with threaded fittings. Galvanized steel, while often used in pressurized systems, has the ability to be threaded due to its robust construction. Copper pipe can also be threaded, though it is more commonly joined using soldering or compression fittings.

By understanding the characteristics of these materials, especially that cast iron is non-conducive for threading, one can effectively navigate plumbing installation and repair scenarios.