General Knowledge of the Plumbing Code

Reference text for the general knowledge area. Each rule below is stated from the printed line of the

2024 International Plumbing Code (IPC 2024) that carries it, and names the section it comes from.

Numbers appear only where the printed line carries them.

What this area asks for is knowledge of the code itself rather than of one of its technical chapters,

and the corpus carries that knowledge in two places: Chapter 1, whose subject is the limits within

which the code applies and the way it is applied and enforced, and Chapter 2, whose subject is what

the code's terms mean for the purposes of the code (IPC 2024, Chapter 1 and Chapter 2). The map of

the chapters and appendices below is the one the corpus itself prints. The running of the code inside

a jurisdiction — its enforcing department, permits, fees, inspections and appeals — belongs to

Sections 103 through 115 of Chapter 1 and is deliberately outside this text; only the general

applicability of the code (Sections 101 and 102) and its definitions are treated here.

1. The code, its scope and its purpose

The code takes the name of the jurisdiction that adopts it and is called "this code" thereafter. The

printed line carries a placeholder where that name belongs, so no name appears here (IPC 2024, 101.1).

Scope covers a plumbing system's whole life inside the jurisdiction: putting it up, installing it,

altering it, repairing it, relocating it, replacing it, adding to it, using it, maintaining it

(IPC 2024, 101.2). Certain systems that are plumbing for this purpose are named in the same breath —

nonflammable medical gas, inhalation anesthetic, vacuum piping, nonmedical oxygen systems, and

sanitary and condensate vacuum collection systems. Two families of work are handed to another

document: fuel gas distribution piping and equipment, fuel-gas-fired water heaters, and water heater

venting systems belong to the International Fuel Gas Code (IPC 2024, 101.2). One exception opens a

second path for small residential work: detached one- and two-family dwellings and townhouses up to

three stories above grade plane with a separate means of egress, together with their accessory

structures of the same height limit, may comply with this code or with the International Residential

Code (IPC 2024, 101.2).

Appendices are inert until adopted: their provisions apply only where a jurisdiction adopts them

specifically (IPC 2024, 101.2.1).

Purpose is stated as a minimum standard — safety, health, protection of property and general welfare

at a reasonable level, obtained by regulating the design and the construction, the installation, the

quality of materials, the location, and the operation, maintenance or use of plumbing equipment and

systems (IPC 2024, 101.3).

Severability keeps the rest of the code standing if one part falls: holding a section, subsection,

sentence, clause or phrase unconstitutional does not touch the validity of the remaining portions

(IPC 2024, 101.4).

2. How the code applies

2.1 General against specific

Two requirements that disagree are settled by rank, not by preference: the specific requirement beats

the general one, and where different sections of the code offer different materials, methods of

construction or other requirements for one case, the most restrictive governs (IPC 2024, 102.1).

2.2 Work already in place

A plumbing system that was lawfully in existence when the code was adopted may keep being used and

maintained, on two conditions — its use, maintenance or repair follows the original design, and it

creates no hazard to life, health or property (IPC 2024, 102.2). Where the work concerns building or

structural matters rather than plumbing, additions, alterations, renovations and repairs fall to the

International Existing Building Code (IPC 2024, 102.2.1).

2.3 Maintenance is the owner's

Existing and new plumbing systems, their materials and appurtenances, and every part of them, are

kept working as designed and in a safe and sanitary condition; devices or safeguards the code requires

stay in compliance with the edition of the code they were installed under. Responsibility rests with

the owner or the owner's authorized agent, and the code official may require any plumbing system to be

reinspected to check that compliance (IPC 2024, 102.3).

2.4 Additions, alterations and repairs

New work on an existing system is held to the standard of a new plumbing system, but the code does not

use that to drag the whole existing system into compliance; what it forbids is leaving an existing

system unsafe, unsanitary or overloaded (IPC 2024, 102.4). Minor work is treated as new construction

unless it repeats the manner and arrangement already there, is not hazardous and is approved

(IPC 2024, 102.4).

2.5 Change of occupancy

A change of occupancy that brings a structure under a special provision of the code for the new

occupancy needs the code official's approval. Certification is then the official's: that the structure

meets the intent of the law governing building construction for the proposed occupancy, and that the

change creates no hazard to public health, safety or welfare (IPC 2024, 102.5).

2.6 Historic and moved buildings

Buildings identified and classified as historic by the state or local jurisdiction are released from

the code's construction, alteration, repair, enlargement, restoration, relocation and moving

provisions where the code official judges them safe and in the public interest of health, safety and

welfare for the work proposed (IPC 2024, 102.6). A building moved into or within the jurisdiction is a

different case: its plumbing systems must meet the code's provisions for new installations, except

where Section 102.2 decides otherwise (IPC 2024, 102.7).

3. The code and the documents it references

The referenced codes and standards are the ones listed in Chapter 15, and they form part of the code's

requirements to the extent each reference reaches, subject to the two rules that follow (IPC 2024,

102.8). Where the code and a referenced standard disagree, the code wins (IPC 2024, 102.8.1). Where a

reference reaches subject matter that the code itself covers, the code's provisions take precedence

over the referenced document's (IPC 2024, 102.8.2).

Three closing rules of this section settle what is not written down. Requirements for strength,

stability or proper operation of a system, or for public safety, health and general welfare, that the

code does not specifically cover are determined by the code official (IPC 2024, 102.9). No provision

of local, state or federal law is nullified by the code (IPC 2024, 102.10). And a reference to a

chapter or section number, or to a provision the code does not identify by number, is read as a

reference to that chapter, section or provision (IPC 2024, 102.11).

4. Reading the code through its definitions

Chapter 2 is where the terms used in the body of the code are defined, and those meanings govern

unless the code expressly states otherwise (IPC 2024, 201.1). The corpus gives the reason the chapter

exists: a code is technical writing, in which a single word, term or punctuation mark can enlarge or

narrow a requirement, and a shared understanding of each term is what holds the interpretation

together (IPC 2024, Chapter 2).

The chapter also carries rules for reading the text. Words in the present tense reach into the future;

masculine wording includes the feminine and the neuter; the singular includes the plural and the

plural the singular (IPC 2024, 201.2).

Terms the code leaves undefined are resolved in two steps. Where the term is defined in the

International Building Code, the International Fire Code, the International Fuel Gas Code or the

International Mechanical Code, it carries the meaning given there (IPC 2024, 201.3). Failing that,

the term means what it ordinarily means, as its context implies (IPC 2024, 201.4).

5. The working vocabulary of the code

The terms below are the ones the technical chapters lean on. They are given in the sense of Section

202 of the code (IPC 2024, 202).

Term	Sense in which the code uses it (IPC 2024, 202)
Approved	Acceptable to the code official.
Approved agency	An established, recognized organization that regularly conducts tests or furnishes inspection services, or furnishes product evaluation or certification, and is approved by the code official.
Code official	The officer or other designated authority charged with administering and enforcing the code, or a duly authorized representative.
Accepted engineering practice	That which conforms to principles, tests or standards accepted by nationally recognized technical or scientific authorities.
Plumbing	The practice, materials and fixtures used to install, maintain, extend and alter piping, fixtures, plumbing appliances and plumbing appurtenances inside or beside a structure, where they serve sanitary or storm drainage, venting, or a public or private water supply.
Plumbing fixture	A receptacle or device tied to a water supply system, to a drainage system, or to both: it needs a supply of water, or it discharges liquid or liquid-borne solid waste, or it does both.
Potable water	Water whose impurities stay below the amounts that cause disease or harmful physiological effects, and which meets the bacteriological and chemical quality requirements of the Public Health Service Drinking Water Standards or of the public health authority having jurisdiction.
Water supply system	The water service pipe, the water distribution pipes, and the connecting pipes, fittings, control valves and appurtenances inside or beside the structure or premises.
Water main	A water supply pipe, or a system of pipes, installed and maintained by a city, township, county, public utility company or other public entity, on public property, in the street, or in an approved dedicated easement of public or community use.
Cross connection	A physical connection or arrangement between two otherwise separate piping systems — one carrying potable water, the other carrying water of unknown or questionable safety, or steam, gas or chemical — where flow from one system into the other is possible, its direction being set by the pressure difference between them.
Backflow	Water or another liquid, mixture or substance entering the distribution pipes of a potable supply from a source other than the intended one. The code separates the condition into three parts. Low-head backpressure is a pressure of 4.33 psi (29.88 kPa) or less, which is the pressure a 10-foot (3048 mm) column of water exerts. Backsiphonage is contaminated water reaching the potable system because pressure there drops below atmospheric at the fixtures, pools, tanks or vats tied to the potable distribution piping. Backflow in drainage is a reversal of flow in the drainage system.
Building drain	The lowest piping of a drainage system, taking the discharge of the soil, waste and other drainage pipes inside the building and running 30 inches (762 mm) of developed length beyond the exterior walls on its way to the building sewer.
Building sewer	What continues from the end of the building drain and carries the discharge to a public sewer, a private sewer, an individual sewage disposal system or another point of disposal.
Soil pipe	A pipe carrying sewage that contains fecal matter to the building drain or building sewer.
Waste	Discharge from a fixture, appliance, area or appurtenance with no fecal matter in it.
Drainage fixture unit	A measure of the discharge a plumbing fixture is likely to put into the drainage system; the value for a given fixture follows from the rate at which it drains, how long one draining operation lasts, and the usual interval between one operation and the next.
Fixture drain	The pipe from a fixture's trap to the point where it joins another drain pipe.
Fixture branch	A drain that serves two or more fixtures and discharges into another drain or into a stack.
Stack	Any vertical run of soil, waste, vent or inside conductor piping passing through one story or more, with or without offsets.
Vertical pipe	A pipe or fitting set at 45 degrees (0.79 rad) or more from horizontal.
Trap	A fitting or device whose liquid seal keeps sewer gases from escaping, without getting in the way of the sewage or wastewater flowing through it.
Trap seal	The vertical distance from the weir to the top of the trap's dip.
Flood level rim	The edge of a receptacle over which water spills.
Cleanout	An access opening in the drainage system for removing obstructions; a removable plug or cap and a removable fixture or fixture trap are among its forms.
Air gap, drainage system	The vertical distance, unobstructed and through free atmosphere, from the waste pipe's outlet down to the flood level rim of the receptacle it discharges into.
Air gap, water distribution system	The vertical distance, unobstructed and through free atmosphere, from the lowest opening of any pipe or faucet supplying water to a tank, fixture or other device down to the flood level rim of the receptacle.
Air break, drainage system	An arrangement in which a fixture, appliance or device drain discharges indirectly into another fixture, receptacle or interceptor, below the flood level rim and above the trap seal.
Indirect waste pipe	A waste pipe that discharges into the drainage system through an air break or air gap into a trap, fixture, receptor or interceptor instead of connecting to it directly.
Developed length	Distance along the centerline of pipe and fittings.
Ready access	Reaching a fixture, appliance or equipment directly, with no panel or similar obstruction removed or moved and without a portable ladder, step stool or the like.
Individual vent	A pipe venting one fixture trap, connected to the vent system above the fixture it serves or ending in open air.
Common vent	A vent that joins at the junction of two fixture drains, or to a fixture branch, and vents both fixtures.
Stack vent	A soil or waste stack continued above the highest horizontal drain connected to it.
Vent stack	A vertical vent pipe whose main job is moving air to and from any part of the drainage system.
Relief vent	A vent whose main job is air circulation between the drainage and vent systems.
Combination waste and vent system	Waste piping designed as one system, wet-venting one or more sinks, lavatories, drinking fountains or floor drains horizontally through a common waste and vent pipe sized so air moves freely above the flow line of the drain.
Area drain	A receptacle built to collect surface or storm water from an open area.
Offset	Approved bends arranged so that one section of pipe leaves the line and returns parallel to it.

6. The reach of each chapter

The corpus prints, chapter by chapter, what each one is for. That description is the map of the code

(IPC 2024, Chapter 1).

Chapter	What the corpus says it governs (IPC 2024, Chapter 1)
1	The limits of the code's applicability and the manner of its application and enforcement, in two parts: scope and application (Sections 101 and 102) and administration and enforcement (Sections 103 through 115).
2	The definitions of the terms used in the body of the code — read in the light of the reason the corpus gives, that the definitions are essential to interpreting the code and that a reader may not realize a term is defined at all.
3	The safety requirements for installing plumbing and nonplumbing fixtures of every kind.
4	The materials, design and installation of plumbing fixtures, faucets and fixture fittings according to the occupancy, with the minimum number of fixtures for each type of occupancy.
5	The design, approval and installation of water heaters and of the safety devices that go with them, so as to reduce the hazards of their installation and operation.
6	Bringing potable water from public and individual sources to every fixture and outlet without losing its potability, and designing the distribution system so fixtures work properly and backflow is prevented.
7	The materials, design and installation of sanitary drainage piping and the connections made to it, so that drainage systems work reliably, are neither too small nor larger than needed, and are built of the materials, fittings and connections the code prescribes.
8	Drainage installations that need an indirect connection to the sanitary drainage system — fixtures and appliances tied to food preparation and handling, to health care facilities and to potable liquids, which must be shielded from the contamination a direct connection to drainage can bring.
9	Vents and venting, whose purpose is to keep every trap from losing its seal.
10	The design requirements and installation limits for traps, and the design and location of the several kinds of interceptors and separators.
11	Removing storm water, done properly so that a flat roof cannot collapse structurally and footings and foundations are not damaged.
12	Designing, installing, storing, handling and using nonflammable medical gas systems, including inhalation anesthetic and vacuum piping, bulk oxygen storage, and oxygen-fuel gas systems for welding and cutting.
13	Designing and installing nonpotable water systems, rainwater harvesting among them.
14	Designing and installing subsurface graywater soil absorption systems that dispose of on-site nonpotable water such as graywater.
15	The product and installation standards and codes referenced in Chapters 1 through 14, with the bodies that promulgate them and the sections that reference them — and, as Section 102.8 provides, those documents become enforceable as part of the code to the extent each reference reaches, as though printed in the body of the code.

Two rules from Section 101 close the map. What the code requires is a minimum and a reasonable level

of safety, health, property protection and general welfare — not a guarantee (IPC 2024, 101.3). And

among requirements that disagree, the more specific, and then the most restrictive, governs

(IPC 2024, 102.1).

Trade Mathematics of the Plumbing Code

Reference text for the trade math area. Each rule below is stated from the printed line of the 2024

International Plumbing Code (IPC 2024) that carries it, and names the section it comes from. Numbers

appear only where the printed line carries them.

Trade math is the side of the trade the code writes down as numbers: sizes, slopes, loads,

conversions, pressures, depths and test values. What follows is a reference text and not a workbook —

for each printed line it gives the rule and the numbers that line carries, and it shows how the code's

tables are read. Sections 301, 305, 306, 312, 608, 704, 709, 713, 714 and 802 are the sections the

trade math questions of this area already cite, and the text is built on them; Sections 604 and 710

are added where the numbers they carry belong to the same subject. Two things often mistaken for

trade math are left out: the arithmetic of the code's own administration (fees, valuations, permit

time limits) and the engineering calculation itself, which the code hands to accepted engineering

practice and to approved design methods.

1. What a number in the code is

Sizes in this code are designations, not measurements. Unless the code says otherwise, the pipe, tube

and fitting sizes it states are nominal or standard sizes as the referenced material standards

designate them (IPC 2024, 301.5).

Every inch-pound value the code prints comes with a metric equivalent, and the conversion factors

stand under the tables: an inch is 25.4 mm and a foot 304.8 mm; an inch per foot is 83.33 mm/m; a

gallon is 3.785 L (and one gallon per minute works out at 3.785 L/m); a pound per square inch is

6.895 kPa; and a foot per second is 0.305 m/s (IPC 2024, Table 704.1, Table 604.4, Table 604.5 and

Table 604.10.1).

Some lines carry no number at all, and then none may be supplied from elsewhere: the code has left

that value to the jurisdiction. Building sewer depth is the clearest case in this text. Section

305.4.1 needs two depths in inches — one below finished grade at the point of septic tank connection,

one below grade — and prints placeholders where both belong, so neither number can be read off the

line and neither is given here (IPC 2024, 305.4.1). The name of the jurisdiction in the code's title

(IPC 2024, 101.1) and the department enforcing the code (IPC 2024, 103.1) are withheld in the same

way.

2. Counting occupants, and rounding

The code's fixture count shows how it does arithmetic. The total occupant load is split in half to

give the occupant load of each sex; each fixture type's ratio is applied to that half; and the

fractional figure that comes out is rounded up to the next whole number. More than one occupancy in

the same calculation changes the order of the steps, not the rounding: each occupancy's fractional

figures are added first, and the sum is rounded up afterwards (IPC 2024, 403.1.1).

Two exceptions replace the halving rather than the rounding. Approved statistical data showing a

distribution of the sexes other than 50 percent of each may replace the half-and-half split, and where

multiple-user facilities serve all genders the count is calculated at 100 percent of the total

occupant load (IPC 2024, 403.1.1).

3. Slope

Horizontal drainage piping runs in uniform alignment at uniform slopes, never flatter than Table

704.1 allows — with one exception stated in the same line: piping upstream of a grease interceptor

slopes at 1/4 inch per foot at least, which the line also gives as a 2 percent slope (IPC 2024,

704.1).

Pipe size (inches)	Minimum slope (inch per foot) (IPC 2024, Table 704.1)
2 1/2 or less	1/4
3 to 6	1/8
8 or larger	1/16

The table's footnote hands the slope of piping draining to a grease interceptor back to Section 704.1

(IPC 2024, Table 704.1). Design by computer changes nothing here: horizontal drainage piping is still

laid at Table 704.1 slopes (IPC 2024, 713.3.2).

4. Load, in drainage fixture units

Every drainage design adds up drainage fixture units. Table 709.1's fixture unit values give each kind

of fixture its relative load weight, for use in estimating what a soil or waste pipe carries and for

use with Tables 710.1(1) and 710.1(2), which give the permissible load of soil, waste and vent pipes

in fixture units (IPC 2024, 709.1).

Where only a flow rate is known — gallons per minute, or liters per second — the code converts it at a

printed rate: 1 gpm (0.06 L/s) of flow counts as two drainage fixture units (IPC 2024, 709.3). An

indirect waste receptor's load is the sum of the fixture unit values of the fixtures discharging into

it, and never less than the value Table 709.1 or 709.2 gives that receptor (IPC 2024, 709.4). A

receptor that takes nothing but clear-water waste from the sources the line names — display cases,

refrigerated display cases, ice bins, coolers and freezers — has a fixture unit value of one-half;

floor drains, floor sinks and hub drains are the receptors in question (IPC 2024, 709.4.1).

Two table rules tell the reader which table to use. A fixture absent from the tables takes its load

from the size of its drain or trap under Table 709.2, whose values are 1 for a 1 1/4-inch trap, 2 for

1 1/2-inch, 3 for 2-inch, 4 for 2 1/2-inch, 5 for 3-inch and 6 for 4-inch (IPC 2024, 709.2); and a

trap larger than 3 inches is sized from Table 709.2 rather than 709.1 (IPC 2024, Table 709.1). A

third rule protects the load figure on building drains and sewers: a water closet or urinal is not

rated at a lower fixture unit value than the table gives unless testing confirms the lower value

(IPC 2024, Table 709.1).

5. Reading the sizing tables

Table 710.1(1) answers for building sewers, building drains and horizontal branches of the building

drain; Table 710.1(2) answers for horizontal branches and for vertical soil or waste stacks

(IPC 2024, 710.1).

Table 710.1(1) is read across the slope, because the capacity of a pipe is a capacity at a slope. Each

row is a diameter, each column a slope:

Diameter (inches)	1/16 inch slope	1/8 inch slope	1/4 inch slope	1/2 inch slope
1 1/4	—	—	1	1
1 1/2	—	—	3	3
2	—	—	21	26
2 1/2	—	—	24	31
3	—	36	42	50
4	—	180	216	250
5	—	390	480	575
6	—	700	840	1,000
8	1,400	1,600	1,920	2,300
10	2,500	2,900	3,500	4,200
12	3,900	4,600	5,600	6,700
15	7,000	8,300	10,000	12,000

The figures are drainage fixture units (IPC 2024, Table 710.1(1)). A footnote of the same table sets

a floor of its own: a building drain serving a water closet is at least 3 inches (IPC 2024,

Table 710.1(1)).

Table 710.1(2) is read across the component rather than the slope. Its columns run in the order the

table prints them: the total for a horizontal branch; and then, for stacks, the total discharge into

one branch interval, the total for a stack of three branch intervals or less, and the total for a

stack of greater than three branch intervals.

Diameter (inches)	Horizontal branch	One branch interval	Stack, 3 intervals or less	Stack, greater than 3 intervals
1 1/2	3	2	4	8
2	6	6	10	24
2 1/2	12	9	20	42
3	20	20	48	72
4	160	90	240	500
5	360	200	540	1,100
6	620	350	960	1,900
8	1,400	600	2,200	3,600
10	2,500	1,000	3,800	5,600
12	3,900	1,500	6,000	8,400

The figures are drainage fixture units, and a horizontal branch figure leaves out branches of the

building drain, which go back to Table 710.1(1) (IPC 2024, Table 710.1(2)). Three rules of the table

say how its figures are applied. Stacks follow the total accumulated load at each story or branch

interval, so a stack may be reduced in size as that load falls, with a floor: no reduction to less

than half the diameter of the largest stack size required (IPC 2024, Table 710.1(2)). A horizontal

stack offset is sized as a building drain under Table 710.1(1), except where Section 711.3 provides

otherwise (IPC 2024, 710.1.1). A vertical stack offset is sized as a straight stack under Table

710.1(2), except where Section 711.1.1 requires a building drain size (IPC 2024, 710.1.2).

Fixtures planned for later count now: where provision is made for future fixtures, those fixtures are

included in determining the required drain pipe sizes (IPC 2024, 710.2).

A drainage system designed by approved computer methods works from discharge conditions rather than

from a single load figure. The load is computed from simultaneous or sequential discharges of

fixtures, appurtenances and appliances, or from the peak usage design condition; discharge profiles

of flow rate against time are taken from the manufacturer's specifications; sizing keeps the pipe

from running full-bore; and the size calculation uses the manufacturer's pipe wall roughness factor,

modified for the roughness that deposits and corrosion add with age (IPC 2024, 713.1, 713.2, 713.2.1,

713.3 and 713.3.1).

6. Water supply: capacity, pressure, size

Under peak demand, the capacity at each fixture supply pipe outlet must reach the figures Table 604.3

sets out; where a fixture or appliance is not in that table, its minimum flow rate and flow pressure

come from the manufacturer's installation instructions (IPC 2024, 604.3). The system itself is

designed on accepted engineering practice and on pipe-sizing methods that have been approved

(IPC 2024, 604.1).

Pressure is designed from the low side and controlled on the high side. Fluctuating street main

pressures are met by designing the building distribution system for the minimum pressure available

(IPC 2024, 604.6). Where the street main or another source cannot deliver the flow pressures Table

604.3 requires at the outlets, a water pressure booster system complying with Section 606.5 is

installed (IPC 2024, 604.7).

Above 80 psi (552 kPa) static inside a building, an approved water pressure-reducing valve complying

with ASSE 1003 or CSA B356 with strainer brings the pressure in the building distribution piping down

to 80 psi (552 kPa) static or less. Two cases are exempt from the valve: service lines running to

sill cocks or outside hydrants, and main supply risers where the mains deliver 80 psi (552 kPa) or

less at individual fixtures (IPC 2024, 604.8). The valve stays open on failure, so water keeps flowing (IPC 2024, 604.8.1), and

valves, regulators and strainers are built and installed so that parts can be repaired or taken out

without breaking a pipeline or taking the assembly off the pipe (IPC 2024, 604.8.2).

Water hammer is controlled at the source of the problem: the flow velocity of the distribution system

is kept down, and where quick-closing valves are used a water-hammer arrestor complying with ASSE 1010

is installed (IPC 2024, 604.9).

6.1 Manifold sizing and the fixture supply

Manifolds serving gridded or parallel distribution lines, hot and cold, are sized from Table 604.10.1,

in which the total gallons per minute is the demand of all outlets supplied (IPC 2024, 604.10.1). The

demand the table allows at each of its two velocities:

Nominal size, internal diameter (inches)	Maximum demand, velocity of 4 feet per second (gpm)	Maximum demand, velocity of 8 feet per second (gpm)
1/2	2	5
3/4	6	11
1	10	20
1 1/4	15	31
1 1/2	22	44

(IPC 2024, Table 604.10.1). Fixture supply pipe size is set by Table 604.5, and a fixture supply pipe

ends no more than 30 inches (762 mm) from where it connects to the fixture (IPC 2024, 604.5).

7. Pressure, gauge and time in the tests

The permit holder makes whichever of the tests of Sections 312.2 through 312.11 apply, and system

piping is tested with water, or with air where the piping is not plastic (IPC 2024, 312.1). The gauge

is chosen to match the pressure under test: 10 pounds per square inch (psi) (69 kPa) or less calls for

a gauge graduated in increments of 0.10 psi (0.69 kPa) or less; above 10 psi (69 kPa) and up to

100 psi (689 kPa), increments of 1 psi (6.9 kPa) or less; above 100 psi (689 kPa), increments of

2 psi (14 kPa) or less (IPC 2024, 312.1.1).

The pressure, gauge reading and holding time of each test:

Test	What the printed line requires (IPC 2024)
Drainage and vent water test (312.2)	Tested in sections, no section gets less than a 10-foot (3048 mm) head of water; in testing successive sections, the upper 10 feet (3048 mm) of the preceding section is retested so that nothing except the uppermost 10 feet (3048 mm) of the system is tested with less than that head; the pressure is held for at least 15 minutes, and the system must then be tight everywhere.
Drainage and vent air test (312.3)	Plastic piping is not tested with air. Air is forced in until a uniform gauge pressure of 5 psi (34.5 kPa) — or a pressure able to balance a 10-inch (254 mm) column of mercury — is reached, held for at least 15 minutes; adjustments for ambient temperature or for gaskets seating are made before the test period starts.
Drainage and vent vacuum test (312.4)	Evacuated to a uniform gauge pressure of negative 5 pounds per square inch, or a negative 10-inch column of mercury (-34 kPa), held 15 minutes without drawing further air.
Drainage and vent final test (312.5)	Visual and detailed enough to judge compliance. Under a smoke test, all traps are filled with water and a pungent, thick smoke is introduced into the whole system; when smoke shows at the roof stack openings, those openings are closed and a pressure equivalent to a 1-inch water column (248.8 Pa) is held for at least 15 minutes.
Water supply system test (312.6)	Tested and proved tight at a water pressure no lower than the system's working pressure or, for piping other than plastic, by an air test of at least 50 psi (344 kPa); held at least 15 minutes; test water from a potable source.
Gravity sewer test (312.7)	The building sewer's end at the public sewer is plugged, the sewer filled with water, and the test run with a head of at least 10 feet (3048 mm) of water held for 15 minutes.
Forced sewer test (312.8)	The same plug at the public sewer, with a pressure 5 psi (34.5 kPa) above the pump rating applied and held for 15 minutes.
Storm drainage system test (312.9)	Storm drain systems inside a building are tested by water or air under the drainage and vent water test or air test.
Shower liner test (312.10)	With the shower drain pipe plugged watertight, the floor and receptor area is filled with potable water to a depth of at least 2 inches (51 mm) measured at the threshold — a temporary threshold being built where none of 2 inches (51 mm) or more exists — and held at least 15 minutes with no leakage.

Backflow prevention assemblies and air gaps are inspected once a year, to establish that the

assemblies still work and that the air gaps are still there (IPC 2024, 312.11.1). The listed assembly

types are tested when installed, straight after repair or relocation, and at least annually, with test

gauges complying with ASSE 1064 (IPC 2024, 312.11.2).

8. Geometry the code fixes by number

8.1 Depth, sleeves, shields

Exterior water supply piping is buried at least 6 inches (152 mm) under the frost line and at least

12 inches (305 mm) under grade (IPC 2024, 305.4). A pipe crossing a foundation wall is relieved by an

arch or by a sleeve built into the wall, the sleeve being two pipe sizes larger than the pipe it

receives (IPC 2024, 305.3). Concealed piping that is not cast iron or galvanized steel, crossing a

stud, joist, rafter or similar member within 1 1/4 inches (32 mm) of the member's edge, is covered by a

steel shield plate spanning the notch or bore and reaching at least 2 inches (51 mm) above sole plates

and below top plates; the plate is steel of at least 0.0575 inch (1.463 mm), printed as No. 16 gage

(IPC 2024, 305.6 and 305.6.1). Sheathing used to separate metallic piping from structure is at least

0.008 inch (8 mil) (0.203 mm) thick (IPC 2024, 305.1).

8.2 Trench, bedding, backfill

Where a trench is dug below the pipe's installation level, sand or fine gravel brings it back up in

layers of 6 inches (152 mm) or less, each compacted as placed (IPC 2024, 306.2.1). Rock is taken out to

at least 3 inches (76 mm) below that level (IPC 2024, 306.2.2). Poor load-bearing material is handled

by overexcavating at least two pipe diameters and backfilling with fine gravel, crushed stone or a

concrete foundation (IPC 2024, 306.2.3). The tracer wire required alongside plastic sewer piping is at

least 14 American Wire Gauge (2.5 mm2), with insulation listed for direct burial (IPC 2024, 306.2.4).

Backfill goes in 6-inch (152 mm) layers, tamped, until 12 inches (305 mm) of tamped earth covers the

crown of the pipe (IPC 2024, 306.3). Trenching parallel to a footing or wall stays clear of the

footing's bearing plane, whose upper boundary the code draws as a line descending at 45 degrees

(0.79 rad) from horizontal from the outside bottom edge of the footing or wall (IPC 2024, 307.6).

8.3 The air gap, measured from the effective opening

An air gap is sized from the effective opening of whatever it protects. Table 608.16.1 gives the

minimums:

Fixture	Minimum air gap, away from a wall (inches)	Minimum air gap, close to a wall (inches)
Lavatories and other fixtures whose effective openings are 1/2 inch or less in diameter	1	1 1/2
Sinks, laundry trays, gooseneck back faucets and other fixtures whose effective openings are 3/4 inch or less in diameter	1 1/2	2 1/2
Over-rim bath fillers and other fixtures whose effective openings are 1 inch or less in diameter	2	3
Drinking water fountains with a single orifice of 7/16 inch or less in diameter, or with multiple orifices totalling 0.150 square inch (the area of a circle 7/16 inch in diameter)	1	1 1/2
Effective openings larger than 1 inch	Two times the diameter of the effective opening	Three times the diameter of the effective opening

Values as printed (IPC 2024, Table 608.16.1). The larger column applies where a wall or obstruction

sits more than three times the effective-opening diameter from the nearest inside edge of the spout

opening for a single wall, or more than four times that diameter for two intersecting walls

(IPC 2024, Table 608.16.1). Where the protection is a vacuum breaker rather than an air gap, the

breaker's critical level sits at least 6 inches (152 mm) above the fixture's or device's flood level

rim, and a pipe-applied breaker is installed at least 6 inches (152 mm) above that rim; a

deck-mounted or equipment-mounted breaker, or an integral atmospheric breaker, has its critical level

at least 1 inch (25 mm) above the rim, installed as the manufacturer instructs (IPC 2024, 608.16.4

and 608.16.4.1).

An indirect waste connection measures its gap against the waste pipe rather than the fixture: the

distance from the indirect waste pipe down to the flood level rim of the waste receptor is at least

twice the effective opening of that pipe (IPC 2024, 802.3.1), while an air break places the pipe

between the flood level rim and the trap seal (IPC 2024, 802.3.2). Indirect waste piping longer than

30 inches (762 mm) measured horizontally, or longer than 54 inches (1372 mm) in total developed

length, needs a trap of its own (IPC 2024, 802.3). A hub drain is a hub, or a pipe rising at least

1 inch (25 mm) above a water-impervious floor (IPC 2024, 802.4.2). A standpipe rises between 18 inches

(457 mm) and 42 inches (1067 mm) above the trap weir (IPC 2024, 802.4.3). Where a laundry tray waste

line runs to an automatic clothes washer standpipe, the standpipe rises at least 30 inches (762 mm)

above the weir of the standpipe trap, and the laundry tray outlet sits no more than 30 inches (762 mm)

horizontally from the side of the standpipe (IPC 2024, 802.4.3.1).

9. The elevation that decides a backwater valve

What decides whether fixtures need a backwater valve is an elevation comparison, not a pressure

reading. Where the finished floor carrying fixtures sits lower than the cover of the next upstream

manhole in the public sewer, a backwater valve is installed in the building drain, or in the

horizontal branch serving those fixtures, to protect them. Where that floor sits higher, the fixtures

must not discharge through a backwater valve, except that existing buildings are not prohibited from

doing so (IPC 2024, 714.1). The valve itself complies with ASME A112.14.1, CSA B181.1

or CSA B181.2 (IPC 2024, 714.2), and is installed with access to its working parts (IPC 2024, 714.3).

General Knowledge & Plumbing Math