Piping Systems, Materials, and Joints
Know the material before the joint
Piping questions test whether you can match a material to its service and to the right joining method.
Copper tube comes in K, L, and M: K is the heaviest wall (underground and heavy-duty), L is standard plumbing, and M is thin-wall for light duty. Soldered joints use lead-free solder (0.2% lead maximum in solder; 0.25% weighted average in wetted surfaces), brazed joints suit higher temperatures and medical gas, and press or push fittings must be installed per their listing. Where copper meets steel, use a dielectric union or insulating connection to stop galvanic corrosion.
Plastics: PVC serves drainage, cold water, and irrigation - not hot distribution. CPVC handles hot water, and solvent-cement cure times stretch with larger diameter and colder weather. ABS joints with ABS cement only, never PVC cement. PEX needs listed insert or crimp fittings and must be protected from sunlight, freezing, and physical damage; check local amendments for exposed or plenum locations.
Cast iron uses no-hub couplings torqued to the manufacturer's spec, or hub-and-spigot lead-oakum where still permitted. Galvanized steel appears on exterior and some fire lines, with threaded joints and dope applied to male threads only.
Traps: assuming any plastic pipe takes hot water, cementing CPVC with PVC cement, and forgetting that lead limits apply to the whole wetted system, not just the valve.
Rule of thumb: on 2 in. PVC solvent weld, hold the joint roughly a minute before release, and double the published cure time in cold weather. Check the cement can.