What is cast iron pipein construction?

Two reasons dominate: sound and fire. Cast iron is dense enough that you barely hear water falling through a stack, while PVC drainage above occupied spaces is a common noise complaint. Cast iron is also noncombustible, so it avoids the restrictions plastic pipe faces in return air plenums and in noncombustible building types. Many commercial specs allow plastic underground but require cast iron above grade for exactly these reasons.

No-hub pipe has plain ends with no bell, and joints are made with a shielded coupling: an elastomeric gasket inside a stainless steel shield tightened with band clamps. It replaced the older hub-and-spigot system on most commercial above-grade work because it installs faster and fits in tight joist and chase spaces. Hubless pipe and fittings are made to CISPI 301 and the couplings to CISPI 310, which is the language you will see in the spec.

Count one coupling for every joint: each stick-to-stick connection plus every fitting opening. A sanitary tee consumes three couplings, a quarter bend two. A quick check is to count fittings from the riser diagram, multiply by openings, and add roughly one coupling per 10 ft stick of straight run. Confirm whether the spec requires standard or heavy-duty couplings, because the price difference across hundreds of joints is real money.