Water Supply and Distribution Piping, Materials, and Sizing
What sizing actually requires
Sizing a water distribution system starts with fixture units (WSFU). You total the supply fixture units for the building, decide whether the occupancy is private or public (flushometer valves change the math), then convert to probable demand in gallons per minute. That demand must be delivered at the most remote, highest fixture with adequate residual pressure after subtracting every loss between the meter and that fixture: meter loss, friction loss through the developed length, and elevation loss.
Pressure arithmetic
The single most useful number: 1 foot of elevation ≈ 0.433 psi, and 1 psi ≈ 2.31 feet of head. Add the developed length measured along the pipe (not straight-line distance), include fitting allowances, and check velocity so the system stays quiet.
Components and protections
Know where and why these go in: a pressure-reducing valve when street pressure is excessive; a thermal expansion tank on a closed system with a backflow device or check valve; vacuum breakers on hose bibbs and siphon-prone connections; backflow preventers sized for the hazard; water hammer arrestors at quick-closing valves; and freeze protection, thrust restraint, and sleeving where pipe passes through masonry or slabs.
Materials
Copper, PEX, CPVC, and galvanized are the recurring names. Requirements you should be able to state: lead-free solder and flux and lead-free wetted surfaces; tubing installed per its listing (PEX fittings, bend supports, and manifold labeling); proper support spacing; and no dissimilar-metal contact that promotes corrosion.
Worked example
Street main delivers 60 psi static; the farthest fixture is 24 ft above the meter. Elevation loss = 24 × 0.433 ≈ 10.4 psi. Meter and friction losses for a 12 WSFU branch run another 18 psi, leaving about 31.6 psi residual - well above a typical minimum requirement, so the size chosen is acceptable.