Why a Roadmap, Not Another Technical Guide

This studio already covers the individual technical pieces of plumbing design in depth: code selection, water supply sizing, drainage and venting, water heating, backflow prevention, grease interceptors, medical gas, storm drainage, and testing/commissioning. Each of those articles answers "how do I design this subsystem correctly?" This guide answers a different question: what order do I tackle these in, and how does the output of one step become a required input to the next?

That sequencing question matters because plumbing design is not a checklist of independent tasks — it is a chain of dependencies. Sizing a water heater before you know your fixture count and simultaneous-demand assumptions produces a system that is either wildly oversized or unable to meet peak load. Laying out DWV venting before finalizing fixture locations means re-routing stacks after the architectural plan has already moved on. Treat the sequence below as the backbone of a plumbing design project, and use the linked deep-dive articles when you reach the step that needs them.

Step 1 — Fixture Unit Counting and Code Selection

Every downstream calculation in a plumbing system traces back to two decisions made at the very start of a project: which model code governs the design, and how many fixtures of each type the building will contain. The IPC and UPC assign different fixture unit values, different pipe sizing tables, and even different vent configurations to the same fixture — so code selection has to happen before any sizing work, not as an afterthought during plan review. See IPC vs UPC: Navigating Plumbing Codes for how to determine which code applies to your jurisdiction and where the two diverge technically.

Once the code is fixed, tally fixtures by type and occupancy: water closets, lavatories, kitchen sinks, showers, floor drains, service sinks, and any process or special-use fixtures the building program requires. This fixture schedule is the single input that both the water supply calculation (Step 2) and the drainage/vent calculation (Step 3) consume — get it wrong here and every downstream pipe size is wrong too. Minimum fixture counts by occupancy type (how many water closets per 100 occupants, for example) also come from this same code chapter and should be checked against the architectural program before fixture locations are finalized.

Step 2 — Water Supply Sizing and Pressure Analysis

With a fixture count and code in hand, convert fixture counts to Water Supply Fixture Units (WSFU), then to a probable peak demand flow rate using the Hunter curve or code table. From there, work through available pressure, pipe sizing by velocity or pressure-loss method, and — critically — determine whether the building needs pressure-reducing valves, pressure zones, or booster pumps. Full methodology is in Domestic Water Supply System Design.

This step produces two numbers that later steps depend on directly: the simultaneous demand flow rate (GPM) for cold and hot water separately, and the available pressure at the top and most remote fixtures. You cannot size a water heater or a recirculation system without the hot-water demand number this step produces, and you cannot finalize riser layout in a multi-story building without knowing whether pressure zones or booster pumps are required.

Step 3 — Drainage, Waste, and Vent (DWV) Layout

In parallel with (or immediately following) supply sizing, lay out the sanitary drainage system: assign Drainage Fixture Units (DFU) to each fixture, size branches and stacks from the code table, and select a venting strategy for each fixture group — individual vent, common vent, wet vent, circuit vent, or air admittance valves where permitted. Trap arm length limits and vent pipe sizing both come out of this step. The full method, including the four trap-seal failure modes venting is designed to prevent, is in Sanitary Drainage, Traps, and Venting Systems.

DWV layout is where fixture locations, not just counts, start to matter — trap arm distance limits and wet-vent geometry are physical routing constraints, not just a tally. This is also the step most sensitive to late architectural changes: moving a restroom core after stack sizing is complete usually forces a rework of this step, so confirm fixture locations are reasonably fixed before investing heavily in DWV design.

Step 4 — Water Heating System Sizing

Water heater sizing cannot happen first, and it cannot happen in isolation — it depends directly on outputs from Steps 1 and 2. The hot-water fixture count and the hot-water WSFU demand established during water supply sizing determine peak hot-water draw; the building's usage pattern (continuous low draw vs. short high-volume peaks, as in a hotel or gym) determines whether a storage-type, tankless, or semi-instantaneous system is the better fit; and recirculation loop sizing (if used) depends on the hot-water distribution piping already laid out. Skipping straight to water heater selection without first completing fixture counting and supply sizing is the single most common sequencing mistake in plumbing design — it produces a system sized to a guess rather than a calculated demand. See Water Heater Selection and Sizing for full sizing methodology.

Step 5 — Special Systems (as Applicable to the Building Type)

Not every building needs every special system below — apply the ones relevant to the occupancy and program:

  • Backflow prevention is required at every hazardous cross-connection: irrigation systems, boiler feed, chemical feed, fire sprinkler connections, and any point where a non-potable source could reverse-flow into the potable system. This is not optional or building-type-specific — nearly every building has at least one cross-connection requiring an assembly. See Backflow Prevention and Cross-Connection Control.
  • Grease interceptors are required wherever a commercial kitchen, food service tenant, or similar fixture discharges fats, oils, and grease to the sanitary system. Size and locate these once kitchen fixture counts and flow are known. See Grease Interceptor Design and Sizing.
  • Medical gas systems apply to healthcare occupancies — hospitals, surgical centers, dental and veterinary facilities with anesthesia or oxygen delivery. These run as a parallel, independently-coded system (NFPA 99) alongside the potable and drainage systems described above. See Medical Gas Systems Design per NFPA 99.
  • Storm drainage coordinates roof drainage and site drainage with the civil/site design — this is frequently handled as its own coordinated deliverable rather than folded into the sanitary DWV design, since roof drain leader routing interacts with structural framing and site storm infrastructure. See Storm Drainage Systems: Roof, Site, and Siphonic Design.

Other special systems this studio covers — private wells and septic design for sites without municipal service, and greywater/rainwater reuse for water-conscious or code-incentivized projects — apply in specific site or sustainability contexts rather than to every project; evaluate whether the project scope calls for them at this stage as well.

Step 6 — Testing, Disinfection, and Commissioning

The final step, performed after installation and before occupancy, verifies that the system as built actually performs to the design intent: pressure testing the water supply and DWV piping, disinfecting the potable water system before it's placed in service, and commissioning any special systems (backflow assembly testing, medical gas verification) against their governing standards. This step cannot be shortened or skipped regardless of how much schedule pressure exists at the end of a project — undetected leaks, cross-connections, or contamination discovered after occupancy are far more costly than the time spent testing before it. See Plumbing System Testing, Disinfection & Commissioning for the full checklist by system type.

Putting the Sequence to Work

On a real project, these steps overlap in practice — DWV layout and supply sizing typically proceed together once fixture counts are locked, and special systems are scoped in parallel with the main design once the building program is understood. But the dependency direction never reverses: fixture counts feed supply sizing and DWV design, supply sizing feeds water heater sizing, and everything feeds into the final testing and commissioning step. If you're new to plumbing design and unsure where to start on a project, start at Step 1 — fixture counting and code selection — every other step in this guide assumes that number is already known.