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Plumbing

Primary standard: IPC 2024 / IRC 2024

Most US states adopt the International Plumbing Code (roughly 35–38 states) or the Uniform Plumbing Code (roughly 9–12 states). The International Residential Code has its own plumbing chapters (25–33) for one- and two-family dwellings. This page uses the IPC / IRC 2024 editions; local jurisdictions frequently amend both, so verify with your building department.

Photo · Kelsey Todd on Unsplash

Water pressure

Where the static water pressure entering the house exceeds 80 psi, a pressure-reducing valve is required — IPC 2024 §604.8 / IRC 2024 §P2903.3.1. The valve must conform to ASSE 1003 and be installed with a strainer downstream of the meter.

Minimum flow pressures at fixtures come from IPC 2024 §Table 604.3 / IRC 2024 §Table P2903.1:

FixtureFlow pressureFlow
Lavatory8 psi0.5 gpm
Shower8 psi2.5 gpm
Water closet (tank)20 psi—
Water closet (flushometer)35 psi—
Hose bibb (sillcock)8 psi—

Cheap ($10) gauge that screws onto a hose bibb reads static pressure at any outdoor faucet. Over 80 psi is a red flag; over 100 psi will eventually rupture washing-machine hoses and fatigue joints.

Traps & vents

Every fixture has a trap — the U-shaped bend under it. The trap holds a water seal that blocks sewer gas from entering the house. Per IPC 2024 §1002.4 / IRC 2024 §P3201.3, the seal depth must be ≥ 2 inches and ≤ 4 inches. Deeper is only permitted where required by design (e.g. certain accessible fixtures).

Every trap must be vented. Without a vent, draining one fixture pulls the seal out of another and lets sewer gas in. Common ways this fails in an older house: an S-trap (no vent above the trap), a "cheater" (unlisted) vent, or a vent stack that's been cut for a remodel and never reconnected.

fixture tailseal depth2″ min · 4″ maxIPC 1002.4IRC P3201.3to vented drain →water seal (blocks sewer gas)P-trap water sealSeal deeper than 4″ is only permitted where required by fixture design (e.g. accessible fixtures).SNOT an S-trapS-traps self-siphon — the drain vent must break the negative pressure.
P-trap seal depthWater sits in the low point of the U-bend. Its vertical depth is what blocks sewer gas — deeper than 4 in. is only allowed when required by fixture design.Source · IPC 1002.4 · IRC P3201.3

Water heater safety

Every storage water heater has a temperature-and-pressure (T&P) relief valve. If either temperature or pressure runs away, the valve opens and dumps water instead of the tank exploding. T&P valves are listed under ANSI Z21.22 / CSA 4.4 to open at or before 210 °F and 150 psi — that's the valve's listed setting, not a code section number.

What is enforceable is the discharge piping — IPC 2024 §504.6 / IRC 2024 §P2804.6.1:

  • · Full size of the valve outlet.
  • · Drains by gravity to a safe location.
  • · Terminates between 2 pipe diameters and 6 inches above the floor or receptor.
  • · Not directly connected to drainage — must discharge through an air gap.
  • · No valves or threaded end on the discharge.

If your T&P discharge is capped, plugged, or trapped, that's a life-safety defect — replace or reconnect it before doing anything else on the heater.

water heatertankT&P valveANSI Z21.22210 °F / 150 psi listedcold inhot outfloor drain / receptorair gap2× dia. to 6″full-size dischargesame diameter as valve outletno valves, no plugsnothing that can restrict flowno threaded endprevents anyone capping itnever solid-connectto drainageT&P relief valve dischargeIPC 504.6 · IRC P2804.6.1
T&P relief valve discharge — code-correct routingFull-size discharge line from the valve, gravity-drained, terminating between 2 pipe diameters and 6 inches above a floor drain or receptor through an air gap. No valves, no caps, no threaded end.Source · IPC 504.6 · IRC P2804.6.1

Cross-connection & backflow

A cross-connection is any point where non-potable water could siphon back into the drinking-water supply. IPC 2024 §608 / IRC 2024 §P2902 prohibits them unless protected by an approved backflow preventer or air gap. The most common household examples:

  • Hose bibbs without a vacuum breaker — a hose in a bucket of soapy water can back-siphon during a supply pressure drop.
  • Handheld shower wands that can reach into the tub water without a check valve.
  • Boiler / hydronic system fill lines without a reduced-pressure backflow preventer.
  • Irrigation systems without an atmospheric or pressure vacuum breaker.

Air gap for a common lavatory (effective opening ≤ ½ in.) is a minimum of 1 in. from the flood-level rim, or 1½ in. if it's near a wall — IPC 2024 §Table 608.15.1.

flood-level rimair gap1″ minimumif opening ≤ ½″ and away from wall1½″ minimumif within 3× the opening diameter of a wallIPC Table 608.15.1IRC P2902 (backflow protection)Air gap — the simplest backflow preventerA vertical, physical break between the potable-water outlet and any point where non-potable water could accumulate.
Air gap — a physical break in the water pathVertical distance between the potable-water outlet and the flood-level rim of the fixture. Because no water can bridge the gap, no back-siphonage can occur — even during a supply pressure drop.Source · IPC Table 608.15.1 · IRC P2902

Fixture flow limits

Federal maximum flow rates were set by the Energy Policy Act of 1992 (as amended). They cap what a fixture can be sold at in the US, regardless of what your local plumbing code says. WaterSense is the EPA's voluntary label with tighter limits.

FixtureFederal maxWaterSense
Toilet1.6 gpf≤ 1.28 gpf
Showerhead2.5 gpm @ 80 psi≤ 2.0 gpm @ 80 psi
Lavatory faucet2.2 gpm @ 60 psi≤ 1.5 gpm @ 60 psi
Kitchen faucet2.2 gpm @ 60 psi— (not labeled)

Sources: 10 CFR 430 (EPAct 1992 baseline flows); EPA WaterSense specifications for tank-type toilets, showerheads, and lavatory faucets.

Self-inspection checklist

Walk-through for the whole house. Anything failing → licensed plumber.

Household plumbing walk-through

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This checklist tracks your progress locally — nothing is saved to a server.

Household water-use calculator

What replacing legacy fixtures with WaterSense equivalents actually saves. Uses AWWA REU2016 use-frequency assumptions and EPA WaterSense / federal flow limits.

Fixture water-use calculator

Compares three fixture generations across toilet, shower, and lavatory faucet. Federal max flows are set by EPAct 1992 / EISA 2007; WaterSense limits are the EPA voluntary specification (linked on the Plumbing page).

SetToilet (gpf)Shower (gpm)Faucet (gpm)Annual (gal)
Pre-1992 legacy (conservative)3.53.52.7599,973
Current federal max (EPAct 1992)1.62.52.267,627
EPA WaterSense1.2821.551,027

48,946 gal/year saved

Difference between a full WaterSense fixture set and a pre-1992 house for a household of 4. Doesn't include leaks — the average US household loses about 9,400 gal/year to leaks according to the EPA.

Use-frequency assumptions from AWWA Research Foundation, "Residential End Uses of Water, Version 2" (2016), summarizing national household data.

Codes & standards referenced on this page

StandardSectionWhat it covers
IPC 2024§604.880 psi maximum static; PRV required above
IPC 2024§Table 604.3Minimum flow pressures at fixtures
IPC 2024§1002.4Trap seal depth 2–4 in.
IPC 2024§708Cleanouts on horizontal drains
IPC 2024§504.6Water heater T&P discharge piping
IPC 2024§608Cross-connection control
IPC 2024§Table 608.15.1Minimum air gap sizes
IRC 2024§Chapters 25–33Residential plumbing (1- and 2-family)
IRC 2024§P2903.3.1Water pressure — PRV threshold
IRC 2024§P2801.6Hot water temperature at fixtures
ASSE 1003§—PRV product standard
ANSI Z21.22 / CSA 4.4§—T&P relief valve listing (210 °F / 150 psi)
EPAct 1992§10 CFR 430Federal maximum fixture flow rates
EPA WaterSense§—Voluntary efficiency specifications

Reminder

This page is guidance, not a permit.

Model codes are adopted (and amended) by each state and municipality. Always confirm the edition in force where you live and pull permits when the work requires it. When in doubt, hire a licensed professional.