Why can't you just multiply one apartment unit's fixture units by the number of units?

That overstates real demand because not every unit uses maximum fixtures simultaneously. A diversity (demand) factor — largely expressed through how the WSFU/DFU demand curves flatten at high total fixture-unit counts — accounts for the statistical unlikelihood of all units peaking at once. The diversity discount applies at the riser/stack aggregation point, not to the branch serving an individual unit, which still needs to handle that one unit's full potential demand.

Why is water pressure a bigger concern on upper floors of an apartment building?

Every foot of elevation costs about 0.433 psi of static pressure. Across a multi-story building this adds up quickly — roughly 17-20 psi over 4 stories — on top of friction losses in the riser and branch piping. If street pressure is moderate, upper floors can end up under the minimum required residual pressure even when the ground floor tests fine, which is why pressure-reducing valves or booster pumps are common design responses.

What is the difference between individual unit metering and sub-metering?

Individual metering gives each unit its own connection to the water utility with a dedicated utility account. Sub-metering keeps a single master meter and utility account for the building, with internal meters allocating consumption to each unit for billing purposes without each tenant having a separate utility relationship. Both differ from simple cost allocation off a single master meter with no per-unit measurement at all.

Do common areas like a laundry room count toward the per-unit fixture calculation?

No. Common-area fixtures (a shared laundry room, fitness/amenity restrooms, lobby drinking fountains) are sized as their own separate fixture-unit load using the applicable code table for that fixture type, then added to the building total as their own line item — they are not folded into the diversified per-unit dwelling calculation.

What DWV venting strategy is typical for a multi-story apartment building?

A common approach is a combined stack vent, where the drainage stack itself extends through the roof and serves as the vent for the building, provided the fixture count and number of branch intervals stay within the code's low-rise stack-vent limits. Individual unit bathroom groups often use wet venting internally. Taller buildings or higher per-floor fixture counts may need a separate vent stack instead of relying on a simple stack vent.

At what building height does a booster pump become necessary rather than optional?

There is no universal threshold — it depends on street pressure, minimum required fixture pressure, and friction losses in the specific system — but a 4-story building is often close to the boundary where the answer could go either way, making it worth an actual pressure-budget calculation rather than an assumption. Taller buildings make booster pumps close to mandatory, while 1-2 story buildings rarely need one.

Engineering·10 min read·July 22, 2026

🏢 Apartment Plumbing Design: A Multi-Family Sample Project

A worked sample-project walkthrough of multi-family apartment plumbing design: diversified fixture unit demand across many units, riser design and upper-floor pressure, unit metering, common-area loads, and stack venting strategy.

📖Plumbing Systems — Illustrated Guide (Full Access)

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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.

Topics covered

apartment plumbing designmulti-family plumbingriser designplumbing diversity factorwater pressure upper floorssub-metering plumbing