Plumbing Strategy

Table 1 maps the parts to the three components. Subsections that cover one component carry an Applies to tag. An Informs tag marks the subsections behind each decision above.

1 · Demand and pressure: how much water, at what pressure

Bottom line. Design flow is a probability: fixtures rarely run together, so the code converts weighted fixture counts to a flow, and the pipe is sized so that the pressure left at the farthest fixture after elevation, meter and friction stays above a minimum.

Design flow comes from weighted fixture units through a probability curve

SupplyHot waterDrainage

Informs D1

The CPC assigns each fixture a water supply fixture unit (WSFU) weight that combines its flow and how long and how often it runs (Table 610.3, private use: a 1.6 gpf tank water closet 2.5, a lavatory 1.0, a bathtub 4.0, a shower head 2.0, a clothes washer 4.0). The sum converts to a design flow through Hunter's curve, derived in the 1940s from the probability that a given number of fixtures run at once (Figure 1). The reference set of Table 2, two and a half baths with a kitchen, laundry and two hose bibbs, totals 29.5 WSFU and about 23 gpm, against 35 gpm if every fixture ran at once.

WSFU from CPC Table 610.3 (private), DFU from Table 702.1, as reproduced from the 2022 edition by secondary sources; 2025 values under Verify.

Hunter's data came from fixtures that used several times today's water. The CPC's Appendix M (the IAPMO Water Demand Calculator), available in California as an alternate method since July 2024, estimates peak demand from present fixture flows and use probabilities; in IAPMO's published single-family example it returned about 20 gpm where the fixture-unit method gave about 30, one pipe size smaller. The method is chosen with the building department; a calculator run on the actual fixture schedule replaces the illustrative ratio used below.

Pressure at the farthest fixture is what elevation, meter and friction leave

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Informs D1

P_static street pressure at the meter, psi · Δz height of the highest fixture above the meter, ft · ΔP_meter meter loss at design flow · P_min residual pressure at the fixture, 15 psi minimum (CPC 608.1) or the fixture maker's figure

street pressure, less what lifting water to the top fixture and pushing it through the meter cost, less what the fixture needs, is what the pipe may lose to friction.

The CPC offers two routes. Within its range, Table 610.4 sizes the meter and building supply directly from the total WSFU, the static pressure band and the developed length: at 46–60 psi and 100 ft, a ¾ in meter with a 1 in building supply serves up to 33 WSFU, so the 29.5 WSFU reference set takes 1 in. Appendix A works from friction instead: it divides the pressure available for friction by the developed length to the farthest fixture plus a fitting allowance, and sizes each segment for the flow it carries at that one friction rate per 100 ft.

Above 80 psi static the code requires a pressure regulator (608.2). The friction budget moves with static pressure one for one, so a 45 psi street leaves less than half the budget of a 60 psi street (Table 3).

Reference elevation, meter and fixture minimum at 23 gpm; developed length 100 ft with fittings +50%; treatment 8 psi.

Hazen–Williams, C = 150, PEX inside diameters. Budgets 19 and 22 psi/100 ft; shaded cells exceed them. 15 gpm is the illustrative two-thirds point.

Table 4 applies the friction method to the service alone, a simplification since the full design flow runs only that far. At the Hunter flow a 1 in PEX service loses 29 psi per 100 ft, over the 19 psi budget, so the method gives 1¼ in where Table 610.4 gives 1 in; the difference comes from the assumed meter loss and fitting allowance, which the table builds in differently. At the lower flow 1 in fits. Velocity is a second criterion: the copper limit is 8 ft/s, and PEX follows the maker's published limit. Friction rises with about the 1.85 power of flow and falls with the 4.87 power of diameter, so one nominal size roughly triples or thirds it.

2 · Distribution: layout, velocity, hammer and material

Bottom line. Layout decides how much flow each pipe sees at once; velocity in that pipe sets noise, erosion and hammer; material sets how violent a hammer is and how the pipe can run through framing.

Three layouts differ in what one pipe carries at a time

Informs D2 D3

A trunk-and-branch layout runs a large main past the fixtures with short branches off it; a home-run layout gives each fixture its own small line from a manifold; a hybrid serves a fixture group (a bathroom) from a manifold port. Home runs use more pipe and fewer fittings, give each fixture a shutoff at the manifold, and keep one fixture's flow out of every other fixture's pipe (Figure 3). A trunk shares its flow, which is economical in pipe and is where the velocity of a busy branch rises.

Code velocity limits protect fittings; quiet needs less

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Informs D2

V velocity, ft/s · Q flow, gpm · d inside diameter, in

velocity rises with flow and falls with the square of the bore, so a pipe one size smaller runs about twice as fast.

The CPC limits copper systems, and other tubing joined with copper fittings, to 8 ft/s cold and 5 ft/s hot (610.12), to keep erosion-corrosion out of fittings and hammer moderate; PEX velocities follow the maker's installation guide. Flow noise in the pipe and at valves becomes audible well below 8 ft/s, and the 4–5 ft/s often used for quiet operation is practice, not code (Figure 4). A ½ in home run to a 1.8 gpm shower runs 3.3 ft/s whatever else is open. A ¾ in PEX branch serving two baths with both showers and both lavatories running carries 6.0 gpm at 5.4 ft/s, above the quiet range, and above the 5 ft/s that would apply to a copper hot line; the same draw on a ½ in branch would run 10.9 ft/s.

Pipe elasticity sets the size of a hammer surge

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Informs D2

ρ water density · a pressure-wave speed · Δv velocity stopped · K bulk modulus of water · D diameter · E pipe wall modulus · e wall thickness

a valve that stops flow suddenly raises pressure by the momentum stopped times the speed at which the pipe carries the wave; a stretchier pipe carries it slower.

A quick-closing valve, a washer or dishwasher solenoid or a single-lever faucet, stops the column faster than a pressure wave can travel the pipe and back. Stopping 5 ft/s raises pressure by about 283 psi in copper, where the wave runs near 4,206 ft/s, and about 68 psi in PEX, near 1,015 ft/s (Table 5). The surge travels as structure-borne noise through straps and studs (Soundproofing · Rigid connections). The CPC requires an arrestor at quick-closing valves (609.10); lower velocity reduces the surge in proportion.

Wave speeds from water bulk modulus 2.2 GPa with copper E 117 GPa, D/e 18, and PEX E 0.7 GPa, D/e 7; class-typical.

Pipe materials compared

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Informs D2

Table 6 compares the common materials. Potable PEX needs no oxygen barrier: potable water is replaced with oxygenated water at every draw, so the barrier would change nothing, while in a closed hydronic loop it keeps oxygen from reaching ferrous parts (HVAC · The hydronic loop). Brass fittings in some PEX systems have failed by dezincification in aggressive water, which the fitting alloy, not the barrier, addresses. All fittings and fixtures carrying drinking water in California meet the lead-free limit of 0.25% weighted average on wetted surfaces (Health and Safety Code §116875).

3 · Hot water delivery: the wait, the waste and recirculation

Bottom line. Hot water arrives only after the cooled water in the pipe has run out, so the wait is the pipe's volume divided by the fixture's flow; recirculation removes the wait by keeping the pipe hot, which costs a standing loss that its control decides.

The wait is the pipe volume divided by the draw rate

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Informs D3

L length of each segment, ft · d inside diameter, in · Q fixture flow, gpm · f warm-up factor for the pipe wall, about 1.2–1.5

the wait is how long the fixture takes to empty the pipe between it and the heater, stretched a little by the pipe itself warming up.

½ in PEX holds 0.0092 gal per ft and ¾ in 0.0184; ¾ in copper holds 0.0251. Diameter matters more than distance: a lavatory at 1.2 gpm on a home run of 50 ft of ⅜ in waits about 12 s, on a trunk of 40 ft of ¾ in and 10 ft of ½ in about 41 s, and the water run to drain is the same volume every time (Figure 5, Table 7). Low-flow fixtures lengthen the wait in proportion. The EPA WaterSense labeled-home specification caps the stored volume between the heater or loop and a fixture near half a gallon.

Pipe volume only; the warm-up of the pipe wall adds 20–50%.

A recirculation loop trades the wait for a standing heat loss

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Informs D3

UA_loop loss per ft per °F · C_loop heat capacity of water and pipe per ft · ΔT hot water above surrounding air

a loop kept hot loses heat all day; a loop heated only on demand costs one loop-full of heat per event; demand wins until the events outnumber the ratio of the two.

A loop brings hot water close to every fixture and returns it to the heater. Kept hot continuously, 120 ft of ¾ in PEX at 120 °F in 65 °F space loses about 665 W bare and 254 W with ¾ in insulation, 16 and 6.1 kWh of heat a day; a heat pump water heater supplies that at about a third of the electricity. Reheating the cold loop costs about 0.39 kWh a time, so demand control, a pump started by a call and stopped when the loop is hot, uses less heat than an insulated continuous loop until about 15 events a day, and less than a bare one until about 41 (Figure 6). The insulated loop's time constant is about 1.6 h, so a loop reheated within an hour of its last use costs less than a full reheat.

Table 8 compares the arrangements.

Storage temperature and delivery temperature are separate limits

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Informs D3

Storing at 140 °F with a mixing valve stretches the tank and keeps it above the range where Legionella grows (HVAC · Peak-hour draw). The CPC limits what reaches people: shower and tub-shower valves are pressure-balancing or thermostatic, set to a maximum of 120 °F (408.3), and bathtub fillers have a limiting device to 120 °F (409.4). A thermostatic mixing valve at the heater sets the distribution temperature; the fixture limits remain. Dead legs between a loop and a fixture, and branches that carry water only occasionally, are where water sits in the growth range.

4 · Drain, waste and vent: seals, slope and noise

Bottom line. Every fixture drain holds a water seal against sewer gas; vents admit air so draining water cannot pull that seal out, and gravity drains need slope that framing depth and direction limit.

Vents protect trap seals from siphonage

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Informs D4

A trap holds 2–4 in of water (CPC 1005). A slug of water leaving a fixture pulls a partial vacuum behind it; without air admitted nearby, that vacuum siphons the seal (Figure 7). A vent connected to the trap arm within a set distance relieves it. The allowed distance grows with the trap arm's size because a larger pipe runs partly full and draws less vacuum (Table 9). An S-trap, whose only air path is the drain itself, siphons its own seal and is prohibited (1004), as are drum and bell traps and double trapping. Air admittance valves are limited in the CPC to uses the building department approves. Fixtures in a bathroom group can share vents: a common vent serves two traps at one connection, and wet venting lets a lavatory's drain serve as the vent for a water closet or tub downstream (CPC 908), which is how most bathroom groups are laid out.

CPC Table 1002.2; the minimum is two pipe diameters.

Drains are sized by drainage fixture units and slope

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Informs D4

Drainage fixture units (DFU, Table 702.1) weight each fixture's discharge; the reference set totals 26 DFU. Table 703.2 gives each size a capacity at ¼ in per ft: a 3 in horizontal drain carries 35 DFU and no more than three water closets, a 3 in stack 48 and four; a 2 in drain carries 8 DFU horizontal and no water closet. A 4 in building drain is typical for a house. Horizontal drains slope ¼ in per ft (2%, CPC 708.1); larger drains may run flatter where the building department approves. Cleanouts are required at the upper end of each horizontal drain, every 100 ft, and where horizontal changes of direction total more than 135° (707.4), so a line can be rodded without opening a finish (Table 2 lists the reference DFU).

Slope and joist depth limit a horizontal drain in the floor

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Informs D4 D6

h_joist joist depth, in · D_outside pipe outside diameter, in · s slope, in per ft

a drain running along a joist bay can drop only as far as the joist is deep, so the slope sets how far it can go before it leaves the floor.

Between floors a drain runs either along a joist bay or across joists. Along a bay, a 3 in drain (3.5 in outside) at ¼ in per ft with ½ in clearance top and bottom runs about 19 ft in a 2×10 and 27 ft in a 2×12 before it reaches the ceiling below (Figure 8). Across sawn joists a bored hole may not exceed a third of the depth (Framing · Notching and boring): 3.08 in in a 2×10, which a 3 in drain cannot pass, and 3.75 in in a 2×12, which it passes at the limit; each hole steps down ¼ in per ft, and with holes kept 2 in from each edge a 3 in drain can cross 2×12s for about 15 ft. Engineered I-joists take holes only where the maker's chart allows, and no truss member is cut (Framing · Truss members). On a slab, ground-floor drains run below the slab and a new one means a saw cut and a trench; pad footings occupy the same ground (Framing · Footings on a slab).

Drain noise follows pipe mass and how the pipe touches the frame

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Informs D4

Water falling in a stack and turning at the base radiates from the pipe wall as airborne sound and passes into studs and joists at every strap. Mass per foot governs the airborne part: cast iron no-hub runs about three times the weight of Schedule 40 PVC of the same size, and its makers report about 10 dB lower levels at the same flow; independent test data are sparse, so the figure is typical. The structure-borne part is controlled where the pipe meets the frame, with resilient clamps, sleeves at penetrations and no contact with drywall (Soundproofing · Rigid connections). A stack in a chase beside a bedroom is a flanking path a wall rating does not include (Soundproofing · Flanking). Table 10 compares the materials.

Weights typical by class.

5 · Water quality: what each treatment stage removes

Bottom line. Treatment stages act on different contaminant classes: carbon on disinfectant taste and organics, ion exchange on hardness, reverse osmosis on dissolved salts and trace contaminants at one tap; the water quality report says which problems exist.

Each stage acts on a different class of contaminant

Informs D5

The utility's annual Consumer Confidence Report gives hardness, disinfectant type, and any detected regulated contaminants for the system, free. Point-of-entry stages treat all the water at whole-house flow; point-of-use stages treat one tap (Figure 9). Chloramine, used by many California systems for a stable residual, needs catalytic carbon and longer contact than free chlorine. Hardness scales heat exchangers and water heater elements, faster at higher temperature and with the volume heated. Reverse osmosis removes dissolved salts, nitrate, arsenic and most PFAS but rejects water to drain and runs at a few gallons an hour, which confines it to a drinking-water tap; its drain discharges through an air gap (CPC 611.2). Table 11 gives the water it rejects.

1.5 gal per person a day for drinking and cooking, typical. Units with a permeate pump or a recirculating design run near 1:1.

A salt-regenerated softener discharges brine to the sewer at each regeneration. California allows local agencies to restrict self-regenerating softeners where brine affects water recycling (Health and Safety Code §116775 onward), and several have, so the sewer agency's rule decides whether a softener is an option. A softened supply also raises sodium at the drinking tap, which a reverse osmosis stage removes.

6 · Routing and the repipe

Bottom line. Pipes compete with ducts, wiring and structure for the same framing; replacing a metal water service changes the grounding system, and transitions between metals and the water heater's installation have their own rules.

Routes follow the same three zones as other services

Informs D6

The attic, the floor framing between stories and the slab are the routing zones described in HVAC · Routing zones. Flexible supply lines fit all three; a drain fits only where slope and depth allow (Part 4), which makes it the least flexible occupant of a shared chase. Supply in an attic sits outside the thermal boundary and is insulated; in a mild climate freezing is rare, and PEX tolerates an occasional freeze where copper splits. Pipes in an exterior wall sit on the warm side of the insulation (Insulation · Air barrier).

Existing materials decide whether a repipe is optional

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Informs D6

Houses of 1985–2005 carry one of four supply materials, each with its own failure mode (Table 12). Polybutylene, used until about 1995, degrades in chlorinated water and fails at fittings without warning. PEX-AL-PEX sold as Kitec, about 1995–2007, fails at its brass fittings by dezincification. Type M copper, the thinnest wall, pinholes in some waters after decades. Type L copper and plain PEX have no class-wide failure. The material is identified where the pipe is visible, at the water heater, under sinks or at the main shutoff.

A plastic water service can remove a grounding electrode

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Informs D6

A metal water pipe in direct contact with earth for 10 ft or more is a grounding electrode (NEC 250.52(A)(1)) and must be bonded into the electrode system. Replacing the buried service with PEX or other plastic removes it; the remaining electrodes then carry the system, and a single rod needs a second unless it measures 25 Ω or less (Electrical · Grounding). A repipe that keeps a metal service and replaces the interior changes nothing at the electrode. Galvanized-to-copper joints corrode galvanically without a dielectric fitting, often inside a wall, and a dielectric union also interrupts the bonding path, which a jumper restores.

The water heater installation has its own plumbing rules

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Informs D3 D6

The CPC requires a storage water heater to be strapped against earthquake motion in its upper and lower thirds, the lower strap at least 4 in above the controls (507.2); a temperature and pressure relief valve whose discharge runs full size to a safe point outside or to an indirect waste (504.6); and a thermal expansion tank where a check valve, backflow preventer or pressure regulator closes the system (608.3). A heat pump water heater adds a condensate drain and an air volume or ducting requirement, and an Energy Code backup-heat rule where its inlet air is unconditioned; that rule and the choice between a dedicated heater and a shared heat pump belong to HVAC · Shared heat pump or dedicated; its location sets every hot-water run in Part 3.

7 · Code, permits and documentation

Which codes apply

The 2025 California Plumbing Code (Title 24 Part 5, based on the 2024 UPC) applies to permits from 1 January 2026, with CALGreen (Part 11) for fixture flow rates and the Energy Code (Part 6) for pipe insulation and recirculation. AB 130 (2025) holds residential standards at this cycle until 1 June 2031. For houses built on or before 1 January 1994, Civil Code §1101.1 onward requires noncompliant fixtures (toilets over 1.6 gpf, showerheads over 2.5 gpm, faucets over 2.2 gpm) to be replaced as a condition of final approval of a permitted alteration; houses built later are outside it. Table 13 lists the provisions a plumbing remodel engages.

Permits and inspection

A repipe, a water heater replacement and any drain or vent change need a plumbing permit; Figure 10 draws which provisions each engages. Rough inspection sees the supply under test, at working pressure or as the code's test requires (609.4), and the drain and vent system under a water test of 10 ft of head or an air test (712), before walls close. The water heater's straps, relief discharge and expansion tank are seen at final, with the Energy Code forms for an alteration (CF1R at permit; CF2R from the installer).

Where it goes wrong

1. A shared branch sized on fixture units alone runs fast and loud when the fixtures it serves open together; fixture units describe the house, not one branch (Part 2).
2. No arrestor at a quick-closing valve. Washer and dishwasher solenoids hammer, and in copper the surge is several times the static pressure (Part 2).
3. Insert fittings that choke a small line. PEX-B insert fittings reduce the bore at every joint, raising velocity and pressure loss in a ⅜ or ½ in home run (Part 2).
4. A recirculation pump on a timer in an alteration. It delivers hot water and does not meet the prescriptive rule (Part 3).
5. Large hot-water mains to small fixtures. A ¾ in trunk to a lavatory doubles the wait of a ½ in line and the water run to drain (Part 3).
6. An S-trap to save a vent. It passes a visual check and siphons its seal in use (Part 4).
7. A drain drawn across joists it cannot pass, or a horizontal run longer than the bay depth allows at ¼ in per ft (Part 4).
8. A stack strapped rigidly in a bedroom wall. The wall's rating does not include the pipe (Part 4).
9. A softener installed where brine discharge is restricted, or carbon chosen for chlorine on a chloraminated supply (Part 5).
10. A plastic water service without the electrode replaced, found at the electrical final or not at all (Part 6).
11. A dielectric union with no bonding jumper, which isolates a section of metal piping from the bond (Part 6).
12. Relief valve discharge reduced or run uphill, which the final inspection checks (Part 6).
13. An existing Kitec or polybutylene system left in place behind new finishes, where its fitting failures then occur (Part 6).

Reference

Glossary

Appendix M

The CPC appendix adopting IAPMO's Water Demand Calculator for peak demand in dwellings, an alternate to fixture units.

Developed length

Length of a pipe run along its centerline including fittings, as distinct from straight-line distance.

DFU

Drainage fixture unit: the weight of a fixture's discharge for sizing drains (Table 702.1).

Demand recirculation

A recirculation pump started by a call (button or sensor) and stopped by a temperature or time limit.

Home run

A dedicated supply line from a manifold to one fixture.

Hunter curve

The probability curve converting supply fixture units to a design flow.

Point of entry, point of use

Treatment of all water entering the house, or of one tap.

Reject ratio

Water a reverse osmosis unit sends to drain per unit of treated water.

Trap arm

The drain between a trap's outlet and its vent.

Water hammer

The pressure surge when a valve stops flowing water faster than a pressure wave can travel the pipe and back.

WSFU

Water supply fixture unit: a fixture's weight for sizing supply (Table 610.3).

Formula sheet

Pipe size and what it supports

Table 14 gives each PEX size's volume per foot and the flow it carries at three velocities.

SDR 9 copper-tube-size PEX; inside diameters typical by maker.

Figure color legend

Red: deficient, or fails the stated criterionAmber: marginal or conditionalGreen: meets the criterionGray: existing or baselineTeal: neutral quantity, reference line or annotation; never a grade

Blue marks water piping and water in traps, as in the HVAC guide. In Figure 7 green and red grade whether the trap seal survives.

Code points and inputs to confirm

• CPC Tables 610.3, 610.4, 702.1 and 703.2 values as adopted; Appendix A chart values; Appendix M status
• Street static pressure and the meter size and model from the water purveyor
• CALGreen 4.303.1 values in the 2025 text; whether any local amendment preceded the AB 130 freeze
• RA4.4.9 and §150.2(b)1H wording for recirculation in alterations
• CPC position on air admittance valves
• The sewer agency's rules on self-regenerating softeners
• Test pressures for supply and drainage under 609.4 and 712

Sources

• California Plumbing Code (2022 and 2025): Table 610.3 as reproduced by Marina Coast Water District; 610.12 velocity text via GoCodeBook (2025); Tables 702.1, 703.2 and 1002.2 as summarized from the UPC by King County and open-exam-prep study guides; 408.3, 409.4, 504.6, 507.2, 603, 608, 609, 707, 712, 1004, 1005.
• IAPMO, California approves Water Demand Calculator as alternate standard (2024); JLC, IAPMO's Water Demand Calculator for right-size plumbing — 20 gpm against about 30 gpm in a single-family example.
• International Plumbing Code Table E103.3(3), estimating demand, flush-tank systems (Hunter curve values).
• BayREN, Residential Water Heater Replacements: Permit Guide — §150.2(b)1H and RA4.4.9; Energy Code Ace, 2025 water heating measures (RA4.4.9, RA4.4.10); CEC, 2025 Residential Mandatory Measures Summary (§150.0(j)1); CPC 609.12.2, 611.2 and 707.4 via GoCodeBook.
• US EPA, WaterSense Labeled Homes specification — hot water delivery volume.
• Open Exam Prep, UPC 610.3/610.4/610.8 — Table 610.4, 33 WSFU at 100 ft for a ¾ in meter and 1 in supply, 46–60 psi.
• CALGreen §4.303.1 residential fixture flow rates; California Health and Safety Code §116875 (lead-free) and §116775 onward (softener restrictions); Civil Code §1101.1–1101.8.
• Cast Iron Soil Pipe Institute, Sound attenuation — manufacturer comparison of cast iron and PVC drain noise.
• NFPA 70 (2023) 250.52(A)(1), 250.53, 250.104(A).
• Companion guides: HVAC, Framing, Soundproofing, Electrical, Insulation, Solar & Battery (set v2).

Set v2 draft 1, 2026-10-23: rewritten as a generic theory guide to spec set v2 from the v1 guide (archived). Fixture-unit weights corrected to CPC Table 610.3 (v1 used values about 30% low) and the Hunter reading corrected; Table 610.4 and Appendix M added beside the friction method; existing supply materials added; recirculation controls split into RA4.4.9 and RA4.4.10; cleanout, insulation and air-gap citations corrected after review; the 8 ft/s limit restated as the copper rule with 5 ft/s for hot water; pressure budget, hammer, loop-loss, drain-in-framing and reverse osmosis arithmetic added; the Energy Code's alteration rule for recirculation corrected to RA4.4.9; house values moved to the House Project Guide. Values from one script.

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