A Concrete Example, Not Another General Guide
The Plumbing System Design: A Complete Workflow Guide lays out the general six-step process any plumbing design project follows. This article is different: it's a worked sample project applying that process to one of the most common building types a plumbing engineer encounters — a mid-rise, multi-family apartment building. The goal here isn't to duplicate the fixture-unit or DWV methodology already covered elsewhere in this studio, but to show how those methods play out differently when the building consists of dozens of nearly identical dwelling units stacked vertically, rather than one large open occupancy.
Our example building: a 4-story apartment building with 40 units, 10 per floor, each a 2-bedroom / 2-bathroom unit with a kitchen, in-unit washer/dishwasher hookups, plus a ground-floor common laundry room and a small fitness/amenity space. We'll walk through it conceptually — the point is to illustrate how the pieces fit together, not to substitute for an actual code-table calculation on a real project with a specific adopted code edition.
1. Fixture Unit Calculation With a Diversity Factor
The naive approach — multiply one unit's fixture unit total by 40 — dramatically overstates real demand. Fixture unit tables (both WSFU for supply and DFU for drainage) are themselves probabilistic, already accounting for some non-simultaneity of use within a single unit. But across 40 identical units, an additional layer of statistical behavior applies: at any given moment, only a fraction of the 40 units have a fixture running at all. This is the diversity factor (sometimes called a demand factor) — it recognizes that not every tenant is showering, flushing, and running the dishwasher at the same instant, even during a morning peak.
In practice, this diversity is already partially built into how the WSFU/DFU curves flatten at high total fixture-unit counts — the Hunter curve, for example, produces a demand flow rate that grows much more slowly than fixture count once totals get large, which is exactly the diversity effect expressed as a curve rather than a separate multiplier. For a building like our 40-unit example, the practical output is that the riser and building supply main are sized for a peak simultaneous demand far below "40 × one unit's demand" — but each individual unit branch is still sized as if that one unit could see its own full fixture demand, since a single unit's occupants absolutely can run all their own fixtures at once. The diversity discount applies at the aggregation point (the riser and main), not at the branch serving one unit. This is the same WSFU/DFU methodology detailed in Domestic Water Supply System Design and Sanitary Drainage, Traps, and Venting Systems — the technique doesn't change for apartments, but the fixture count it's applied to is much larger and the payoff from correctly applying diversity is much bigger.
2. Riser Design for a Multi-Story Building
Rather than running independent supply and drainage piping from the street/sewer to every unit, a multi-story apartment building centralizes vertical distribution in risers (supply) and stacks (drainage/vent) — typically located in a chase or shaft that stacks vertically through all four floors, often shared between two units per floor for efficiency. Each floor's units branch off the riser horizontally at that level. In our 40-unit example, a common layout might use a small number of riser/stack chases (perhaps 4-5, one per pair or cluster of units) rather than one riser per unit, balancing pipe cost against branch length and floor penetration count.
Riser sizing works from the top down conceptually but is calculated bottom-up: each riser carries the accumulated diversified demand of every unit connected above a given floor. The riser segment serving the ground floor carries the combined demand of all four floors' worth of connected units; the top-floor segment carries only its own floor's demand. This is analogous to stack sizing per branch interval described in the DWV article, but for supply risers the same principle drives increasing pipe size as more floors' demand accumulates lower in the building.
3. Upper-Floor Water Pressure: A Real Design Concern
Every foot of vertical rise costs approximately 0.433 psi of static pressure (the hydrostatic head relationship also used in the general water supply sizing methodology). Across 4 stories — roughly 40-45 feet from the street-level service to the top-floor fixtures — that's on the order of 17-20 psi lost to elevation alone, before any friction losses in the riser and branch piping are even considered. If the street/municipal pressure is moderate (say, 55-65 psi, common in many areas) and top-floor fixtures need a legally required minimum residual pressure, the top floor of a 4-story building can end up marginal or under-pressured even though the ground floor tests fine.
Two standard remedies apply, and the choice between them depends on how tight the margin is: a pressure-reducing valve (PRV) at the service entrance manages excessive pressure at the lower floors so the system isn't over-pressured there, while a booster pump system (typically a duplex or triplex set with a variable frequency drive) adds pressure specifically to serve upper floors when the available street pressure isn't enough to overcome elevation and friction losses and still deliver the minimum required residual pressure at the top. A 4-story building is often right at the boundary where a booster pump becomes necessary rather than optional — a taller building makes booster pumps close to mandatory, while a 1-2 story building rarely needs one. This is exactly the pressure-zone and booster-pump analysis described in Domestic Water Supply System Design, applied here to a specific, common building height where the decision is genuinely close either way and worth checking with an actual pressure budget calculation rather than assuming.
4. Individual Unit Metering and Sub-Metering
Multi-family buildings commonly need to allocate water (and sometimes water heating) cost to individual tenants rather than absorbing it entirely into building operating expense. Three approaches are typical: a single building master meter with cost allocated to tenants by lease terms or square footage (no per-unit metering hardware, simplest but least precise); individual unit meters tied into each unit's branch off the riser, allowing direct tenant billing for actual consumption; or sub-metering, where a master meter still serves the utility relationship but individual unit sub-meters (often utility-owned or landlord-installed, sometimes remotely read) allocate consumption internally without each unit having its own utility account.
The metering approach chosen affects riser and branch layout: individually metered or sub-metered units need the meter installed at an accessible point on each unit's branch (commonly in a utility closet or corridor chase) before the branch enters the dwelling unit, which affects riser chase sizing and unit branch routing decisions made back in Step 2. This decision is typically made early, alongside the ownership/rental model for the building, since retrofitting individual metering after the riser and branch piping is roughed in is disruptive and costly.
5. Common-Area Plumbing Sized Separately
The ground-floor laundry room and fitness/amenity space in our example are not part of any individual unit's fixture count and must be sized as their own separate load. A common laundry room with, say, 6-8 commercial or residential-grade washers has a fixture unit demand (both supply and drainage) calculated the same way as any commercial fixture group — using the applicable code table for that fixture type and quantity — and is added to the building's total demand as its own line item, not folded into the diversified per-unit calculation used for the dwelling units. The same applies to any common restroom, service sink, or drinking fountain required in amenity or lobby spaces by code occupancy tables.
Common-area fixtures typically connect to their own dedicated branch or a branch near the building's point of entry, rather than tying into a unit riser, both because their demand pattern differs from residential use and because it simplifies any unit-level metering scheme described above — common-area consumption stays on the building's account rather than being attributed to any tenant.
6. DWV Stack Venting Strategy for a Multi-Story Residential Building
For a 4-story apartment building, the most common approach — where each floor's fixture layout stacks directly above the floor below (bathrooms and kitchens aligned vertically, which is typical and cost-driven in multi-family design) — is a combined stack vent arrangement: the drainage stack itself extends up through the roof and functions as its own vent for the building, provided it falls within the branch-interval and fixture-count limits described in Sanitary Drainage, Traps, and Venting Systems. Each floor's connection into the stack constitutes one branch interval; a 4-story building has 4 branch intervals feeding the stack, which is within reach of the low-rise stack-venting limits for many fixture counts but should always be checked against the specific code table values, since the fixture count on each floor (10 units' worth of bathroom groups) can push a 4-story building past the simple stack-vent threshold and into requiring a separate vent stack alongside the drainage stack.
Individual unit bathroom groups within each floor commonly use wet venting internally (a bathroom group's lavatory, tub/shower, and water closet sharing a single oversized wet-vented pipe) to minimize the number of vent penetrations needed per floor, with the wet-vented branch then connecting into the shared vertical stack. This two-level venting strategy — wet venting inside each unit, stack venting (or full vent-stack pairing, if fixture counts require it) for the building as a whole — is standard for stacked residential floor plans and is worth confirming unit-by-unit during design, since even small differences in bathroom layout between unit types can change which venting method applies.
Takeaways From This Example
The core lesson of a multi-family sample project is that none of the underlying calculation methods change from what's described elsewhere in this studio — the same WSFU/DFU tables, the same Hazen-Williams pressure-loss method, the same stack-vent code limits all apply. What changes is where diversity and aggregation enter the picture: at the riser and stack level rather than within a single fixture group, and in the added considerations — metering strategy, common-area loads, upper-floor pressure margin — that only become relevant once a building stacks many similar units vertically. A 40-unit, 4-story building sits at a scale where several of these considerations (especially booster pump necessity and stack-vent vs. vent-stack choice) are genuinely close calls, which is exactly why they deserve deliberate calculation rather than assumption on a real project.