System Design, Plan Reading, and Estimating
Read the drawings before you count
The plan view shows layout; the riser diagram or isometric shows vertical relationships and sizes; the schedules list fixtures and equipment; the general notes and specifications govern materials and methods. Learn the symbols: soil versus waste versus vent versus cold and hot water, floor drains, cleanouts, valves, and gas cocks. Note the scale and the north arrow, and remember that scaling off a drawing is never a substitute for a dimension string - questions often hinge on a dimension explicitly shown.
Design calculations
The design path is: count fixtures by type, assign fixture units (WSFU for supply, DFU for drainage), sum by branch and by building, convert supply units to probable demand in gpm, then size for velocity and pressure. Drainage sizing uses DFU and slope. Developed length includes fittings and elevation - estimate it as measured pipe plus a percentage for fittings rather than treating it as pure length.
Estimating
A takeoff lists pipe by size and material, fittings, valves, hangers, fixtures, and equipment, each with a waste allowance (often a few percent on pipe, more on specialty fittings). Add labor units by item, then overhead and profit. Specifications and submittals tell you what the installed product must be - a cheaper substitute that does not match the spec is a change order, not a savings.
Traps
Using total connected fixture count instead of fixture units; forgetting a floor drain or a cleanout shown only on the plan; mixing up a section detail with a plan note; and assuming the isometric shows every horizontal dimension (it usually does not).
Worked example
A building with 40 water-supply fixture units of flush-tank water closets converts through the demand curve to roughly 20–22 gpm of probable peak demand - far less than the 100+ gpm those closets would flow if all opened at once. Size the service to the probable demand, and verify pressure at the top.