Plan Reading & Isometrics

Plan Reading

Reference text for the plan reading area. This text is not a chapter of the code: reading a plumbing

plan is a competence, and it is written here from the printed provisions that govern what the

document of a plumbing project must carry and what the figures drawn on it mean. Every rule below is

written from the printed line of the 2024 International Plumbing Code (IPC 2024) that carries it, and

each rule names the section it comes from. Where the printed line does not state a value, no value is

stated here; where a sizing table is printed under a number, the table is reproduced under that

number with the values read from the print. The text says what it does: it reads a plan through the

printed dispositions, and it does not add a method of drafting or a rule of any school that the code

read for this document does not print.

1. The document a plumbing plan belongs to

The printed code does not speak of a plan in the abstract; it speaks of the permit application and of

the construction documents filed with it. Work that the code regulates — to erect, install, enlarge,

alter, repair, remove, convert or replace any plumbing system — is not started, and no such work is

caused to be performed, before an application has been made to the code official and the required

permit obtained (IPC 2024, 105.1). A printed exception list removes a short set of work from the

permit requirement: the stopping of leaks in drains, water, soil, waste or vent pipe, with the

qualification that a concealed trap, drainpipe, water, soil, waste or vent pipe that has become

defective and must be removed and replaced with new material is treated as new work and requires a

permit and an inspection; the clearing of stoppages and the repair of leaks in pipes, valves or

fixtures; and the removal and reinstallation of water closets, provided such repairs do not involve

or require the replacement or rearrangement of valves, pipes or fixtures. Exemption from the permit

does not authorize work that violates the code (IPC 2024, 105.2).

Each application for a permit, with the required fee, is filed on the form the code official

furnishes and carries a general description of the proposed work and of its location, is signed by

the owner or the owner's authorized agent, and indicates the proposed occupancy of all parts of the

building and of that part of the site or lot not covered by the building or structure, with any other

information the code official requires (IPC 2024, 105.3). Before the permit is issued, the code

official is authorized to inspect and evaluate the systems, equipment, buildings, devices, premises

and spaces or areas to be used (IPC 2024, 105.3.1). An application is treated as abandoned 180 days

after filing unless it has been pursued in good faith or a permit has been issued, and the code

official may grant extensions of not more than 180 days each, on a written request showing

justifiable cause (IPC 2024, 105.3.2). The application is made by the person or agent who is to

install all or part of the plumbing system, that applicant meeting the qualifications established by

statute, rules, ordinance or resolution, and the full name and address of the applicant are stated in

the application (IPC 2024, 105.4).

The application and the construction documents filed with it are reviewed by the code official, and

where the proposed work is found to conform to the code and to the applicable laws and ordinances,

and the fees of Section 108.1 have been paid, the permit is issued (IPC 2024, 105.5). Where the code

official issues a permit for work for which construction documents are required, those documents are

endorsed in writing and stamped "APPROVED"; approved construction documents are not changed, modified

or altered without the authorization of the code official, and the work is done in accordance with

them. The code official may issue a permit for part of a plumbing system before the whole set of

documents has been submitted, on adequate information and detailed statements that comply with the

code, and the holder of such a permit proceeds at his or her own risk, without assurance that the

permit for the entire system will be granted (IPC 2024, 105.5.1).

A permit, or an approval of construction documents, is not a permit for or an approval of a violation

of the code or of any ordinance of the jurisdiction, and a permit purporting to give authority to

violate or cancel the code is not valid; issuing a permit on the basis of documents does not stop the

code official from later requiring errors in those documents to be corrected, or from preventing

building operations carried on under them where they violate the code (IPC 2024, 105.5.2). A permit

expires if the work authorized is not started within 180 days from its date, or if the work is

suspended or abandoned for a period of 180 days at any time after it was started; before the work can

be recommenced a new permit is obtained, at one-half the fee of a new permit, provided the original

documents are unchanged and the suspension or abandonment has not exceeded one year

(IPC 2024, 105.5.3). An unexpired permit may be extended for a period not exceeding 180 days where

there is reasonable cause, not more than once, on the same half-fee basis

(IPC 2024, 105.5.4). A permit issued in error, or on the basis of incorrect, inaccurate or incomplete

information, or in violation of any ordinance, regulation or provision of the code, may be suspended

or revoked (IPC 2024, 105.5.5). The permit, or a copy of it, is kept on the site of the work until

the project is completed (IPC 2024, 105.5.7).

2. What the construction documents must show

Construction documents, engineering calculations, diagrams and other such data are submitted in two

or more sets, or in a digital format where the code official allows it, with each application for a

permit. Where state law requires it, the code official requires those documents, computations and

specifications to be prepared and designed by a registered design professional. The printed

requirements on the drawing itself are these: the construction documents are drawn to scale, they are

of sufficient clarity to indicate the location, the nature and the extent of the work proposed, and

they show in detail that the work conforms to the provisions of the code. For buildings more than two

stories in height, the construction documents additionally indicate where penetrations will be made

for pipes, fittings and components, and indicate the materials and methods for maintaining the

required structural safety, fire-resistance rating and fireblocking (IPC 2024, 106.1).

The exception printed with that rule allows the code official to waive the submission of construction

documents, calculations or other data where the nature of the work applied for makes the review of

those documents unnecessary to determine compliance (IPC 2024, 106.1 and Exception).

One set of approved construction documents is retained by the code official for not less than

180 days from the date of completion of the permitted work, or as state or local laws require; one

set is returned to the applicant, and that returned set is kept on the site of the building or work

at all times while the work authorized by the permit is in progress (IPC 2024, 106.2).

3. Reading the drawing at the three printed inspection stages

Construction or work for which a permit is required is subject to inspection by the code official,

and that construction or work remains visible and able to be accessed for inspection purposes until

approved; it is the duty of the permit applicant to keep the work in that state. Approval following

an inspection is not an approval of a violation of the code (IPC 2024, 111.1).

The code prints three stages, each with its own scope, and a plan is read against them:

piping is installed, and before any backfill is put in place.

and bracing are in place and all sanitary, storm and water distribution piping is roughed-in, and

prior to the installation of wall or ceiling membranes.

properly connected, and the structure is ready for occupancy.

(IPC 2024, 111.2)

The holder of the permit, or an authorized agent, notifies the code official when the work is ready

for inspection and provides access and the means for the inspections the code requires

(IPC 2024, 111.2.2). Work is not done beyond the point indicated in each successive inspection

without first obtaining the approval of the code official; the code official indicates the portion of

the construction that is satisfactory as completed or notifies the permit holder where it fails to

comply, and any portion that does not comply is corrected and is not covered or concealed until the

code official authorizes it (IPC 2024, 111.2.3).

For an alternative engineered design, the registered design professional or designated inspector

periodically inspects and observes the installation to determine that it is in accordance with the

approved construction documents, discrepancies are brought to the immediate attention of the plumbing

contractor for correction, and records are kept of all inspections (IPC 2024, 111.3.1); a final

report in writing certifies that the installation conforms to the approved construction documents,

and a notice of approval is not issued until that written certification has been submitted

(IPC 2024, 111.3.2). The notice of approval itself is issued by the code official after the

prescribed tests and inspections indicate that the work complies in all respects with the code

(IPC 2024, 107.1), the permit holder making the applicable tests prescribed in Section 312 and giving

reasonable advance notice when the work is ready for them (IPC 2024, 312.1), and the plumbing work

and systems being tested as Section 312 requires, with the tests made by the permit holder and

observed by the code official (IPC 2024, 111.4).

4. The drainage figures a plan carries

Drainage fixture unit values designate the relative load weight of the different kinds of fixtures

and are used in estimating the total load carried by a soil or waste pipe, in connection with the

sizing tables for soil, waste and vent pipes in which the permissible load is given in fixture units

(IPC 2024, 709.1 and Table 709.1).

The maximum number of drainage fixture units that may be connected to a given size of building sewer,

building drain or horizontal branch of the building drain is determined from the sizing table for

buildings drains and sewers, and the maximum number connected to a given size of horizontal branch or

vertical soil or waste stack is determined from the sizing table for horizontal fixture branches and

stacks (IPC 2024, 710.1 and Table 710.1(1) and Table 710.1(2)).

A horizontal drainage pipe is drawn at a uniform slope, and the printed minimum slopes are these

(IPC 2024, 704.1 and Table 704.1):

Footnote a of that table sends the slope of piping draining to a grease interceptor back to the text

of the section, which requires not less than 1/4 inch per foot (2 percent slope) upstream of a grease

interceptor; and for SI: 1 inch = 25.4 mm, 1 inch per foot = 83.33 mm/m (IPC 2024, 704.1 and

Table 704.1). The control devices and the connections that follow from the slope are also read off

the drawing: fixture units establish the load, the load fixes the size, the size fixes the slope, and

the sizing tables of Section 710.1 give the permissible fixture-unit load for that size

(IPC 2024, 709.1, 710.1 and 704.1).

The size of the drainage piping is not reduced in the direction of flow; a 4-inch by 3-inch water

closet flange, a water closet bend fitting with a 4-inch inlet and a 3-inch outlet whose 4-inch leg

is upright and below the flange, and an offset closet flange are not treated as a reduction

(IPC 2024, 704.2). Horizontal branches connect to the bases of stacks at a point not less than

10 times the diameter of the drainage stack downstream from the stack, and connect to horizontal

stack offsets at a point not less than 10 times the diameter of the drainage stack downstream from

the upper stack (IPC 2024, 704.3). Drainage piping installed for future fixtures terminates with an

approved cap or plug, which is what the drawing shows for that rough-in (IPC 2024, 704.4).

Cleanout locations are drawn from the printed intervals and geometries: horizontal drainage pipes in

buildings have cleanouts at intervals of not more than 100 feet (30 480 mm), and building drains also

at not more than 100 feet, except that manholes used in place of cleanouts are at not more than

400 feet (122 m), the interval being measured from the cleanout or manhole opening along the

developed length of the piping to the next drainage fitting that provides access for cleaning, to the

end of the horizontal drain or to the end of the building drain (IPC 2024, 708.1.1). The junction of

the building drain and the building sewer is served by a cleanout located at the junction or within

10 feet (3048 mm) of the developed length of piping upstream of it (IPC 2024, 708.1.3). A change of

horizontal direction greater than 45 degrees (0.79 rad) in a horizontal drainage pipe, a building

drain or a building sewer carries a cleanout at that change, and where more than one such change

occurs within 40 feet (12 192 mm) of developed length, the cleanout installed for the first change

serves all the changes within that length (IPC 2024, 708.1.4). Cleanouts are the same size as the

piping they serve, except that cleanouts for piping larger than 4 inches (102 mm) need not be larger

than 4 inches (102 mm) (IPC 2024, 708.1.5). Clearance is drawn too: not less than 18 inches (457 mm)

from the face of the opening for cleanouts of 6-inch (153 mm) and smaller piping, and not less than

36 inches (914 mm) for cleanouts of 8-inch (203 mm) and larger piping, in both cases perpendicular to

the face of the opening and measured to any obstruction (IPC 2024, 708.1.10).

5. The vent geometry a plan carries

The minimum diameter of a stack vent or a vent stack is read from the developed length and the total

drainage fixture units connected to it in the sizing table for stack vents and vent stacks, and the

diameter is in no case less than one-half the diameter of the drain served or less than 1 1/4 inches

(32 mm) (IPC 2024, 906.1 and Table 906.1). The developed length that goes into that reading is

measured from the farthest point of vent connection to the drainage system to the point of connection

to the vent stack, the stack vent or the termination outside of the building (IPC 2024, 906.3).

The protecting vent of each fixture trap is located so that the slope and the developed length in the

fixture drain from the trap weir to the vent fitting are within the printed limits, which are read

from the table of maximum distance of fixture trap from vent (IPC 2024, 909.1 and Table 909.1):

Where a plan shows a sidewall vent terminal, the dimensions it carries come from the printed

clearances: 10 feet (3048 mm) or more from the lot line, and 10 feet (3048 mm) or more above the

highest adjacent grade where that grade lies within 10 feet (3048 mm) of the terminal horizontally,

with no termination under the overhang of a structure that has soffit vents, and protection against

birds and rodents entering or blocking the opening (IPC 2024, 903.1.4). No open vent terminal is drawn

directly below a door, an openable window or any other air intake of that building or of a

neighbouring one, and it stays outside 10 feet (3048 mm) horizontally of such an opening unless it

rises 3 feet (914 mm) or more above the top of that opening (IPC 2024, 903.5). Vent and branch vent

pipes are graded and connected so as to drain back to the drainage pipe by gravity (IPC 2024, 905.2),

every dry vent connecting to a

horizontal drain connects above the centerline of that drain (IPC 2024, 905.3), and every dry vent

rises vertically to not less than 6 inches (152 mm) above the flood level rim of the highest trap or

trapped fixture vented (IPC 2024, 905.4). Where the print read for this document carries no number on

a line — the height above the roof for an unprotected roof extension is printed as a bracketed

placeholder rather than as a figure — the drawing cannot take a value from that line, and none is

stated here (IPC 2024, 903.1.1).

6. The water supply figures a plan carries

The water distribution system is designed, and the pipe sizes are selected, so that under peak demand

the capacity at each fixture supply pipe outlet is not less than the printed value for that outlet;

where a fixture or appliance is not listed, the minimum flow rate and flow pressure come from the

manufacturer's installation instructions (IPC 2024, 604.3 and Table 604.3).

The maximum flow rates and water consumption for plumbing fixtures and fixture fittings are read from

the printed table, with the printed exceptions for blowout design water closets, vegetable sprays,

clinical sinks, service sinks and emergency showers (IPC 2024, 604.4 and Table 604.4).

Hot water and cold water manifolds installed with gridded or parallel-connected individual

distribution lines to each fixture or fitting are sized from the table of manifold sizing, in which

the total gallons per minute is the demand of all outlets supplied; the table reads a maximum demand

against a nominal size at a stated velocity (IPC 2024, 604.10.1 and Table 604.10.1). Where the street

water main pressures fluctuate, the building distribution system is designed for the minimum

available pressure (IPC 2024, 604.6); where the pressure from the street main or other source is not

enough to give the flow pressures at fixture outlets required by the table of design criteria, a

booster system conforming to Section 606.5 is installed (IPC 2024, 604.7); and where the pressure

within a building exceeds 80 psi (552 kPa) static, an approved water pressure-reducing valve

conforming to ASSE 1003 or CSA B356 with strainer is installed, to reduce the pressure in the

building water distribution piping to not more than 80 psi (552 kPa) static (IPC 2024, 604.8).

For the hot water side, the developed length of the hot or tempered water piping, measured from the

source of hot water to the fixtures that require hot or tempered water, is not more than 50 feet

(15 240 mm), with recirculating system piping and heat-traced piping counting as sources

(IPC 2024, 607.2).

7. The storm figures a plan carries

The size of vertical conductors and leaders, of building storm drains and building storm sewers, and

of any horizontal branches of such drains or sewers, is based on the 100-year hourly rainfall rate of

the geographic area as mapped in the figures published with Section 1106.1, or on other rainfall

rates determined from approved local weather data (IPC 2024, 1106.1).

Storm drain piping is sized on the flow rate through the roof drain, that flow rate being converted

from the rainfall rate by Equation 11-1 — the rainfall intensity in inches per hour multiplied by

the roof area in square feet, multiplied by 0.0104 — and then checked against the roof drain

manufacturer's published flow rate for the model and size selected; the flow rate in storm drain

piping does not exceed the printed values of Table 1106.2, which are read against the pipe size for a

vertical drain and for a horizontal drain at a slope of 1/16, 1/8, 1/4 or 1/2 inch per foot

(IPC 2024, 1106.2, 1106.2.1 and Table 1106.2). Vertical leaders are sized on the flow rate from

horizontal gutters or the maximum flow rate through the roof drains, and the flow rate through them

does not exceed the printed values of Table 1106.3, which pairs a leader size with its capacity in

gallons per minute (IPC 2024, 1106.3 and Table 1106.3). Horizontal gutters are sized on the flow rate

from the roof surface, and the flow rate in them does not exceed the printed values of Table 1106.6,

which reads a capacity against the gutter dimensions and the gutter slope (IPC 2024, 1106.6 and

Table 1106.6).

Two geometry rules change the areas and the openings drawn on the plan: one-half of the area of any

vertical wall that diverts rainwater onto the roof is added to the projected roof area, and that

combined area is what enters the calculation of the required size of the vertical conductors, the

leaders and the horizontal storm drainage piping (IPC 2024, 1106.4); and a scupper opening is not

less than 4 inches (102 mm) in height, with a width equal to or greater than the circumference of a

roof drain sized for the same roof area (IPC 2024, 1106.5).

Secondary (emergency) roof drains or scuppers are drawn where the roof perimeter construction extends

above the roof in a manner that would entrap water if the primary drains allow a buildup

(IPC 2024, 1108.1), their discharge end point is separate from the primary system, above grade, at a

location normally observed by the building occupants or by maintenance personnel (IPC 2024, 1108.2),

and they are sized in accordance with Section 1106 on the rainfall rate for which the primary system

is sized, with the flow through the primary system not counted (IPC 2024, 1108.3). Subsoil drains are

drawn at not less than 4 inches (102 mm) in diameter, from open-jointed, horizontally split or

perforated pipe, protected by an accessibly located backwater valve where the building is subject to

backwater (IPC 2024, 1111.1), and a sump pit is drawn at not less than 18 inches (457 mm) in

diameter and not less than 24 inches (610 mm) in depth, located so that all drainage flows into it by

gravity (IPC 2024, 1113.1.2).

8. The units the printed figures arrive in

The sizing figures a plan carries are printed with their units, and the same units are what a reader

takes off the drawing: pipe and trap diameters in inches with millimetre equivalents, slopes in inch

per foot with a millimetre-per-metre equivalent (IPC 2024, Table 704.1 and Table 909.1), capacities in

gallons per minute with a litre-per-minute equivalent (IPC 2024, Table 604.3, Table 1106.2,

Table 1106.3 and Table 1106.6), loads in drainage fixture units (IPC 2024, 709.1 and Table 709.1),

lengths in feet with millimetre equivalents (IPC 2024, 607.2, 708.1.1 and 909.1), pressures in psi

with kilopascal equivalents (IPC 2024, 604.8 and 504.5), and temperatures in degrees Fahrenheit with

degree Celsius equivalents (IPC 2024, 501.2, 501.6 and 607.1.2). The conversions printed with the

tables for SI are part of the reading: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon per minute =

3.785 L/m, 1 inch per foot = 83.3 mm/m (IPC 2024, Table 1106.2, Table 1106.3 and Table 1106.6).

Where a printed table is a grid, the drawing carries the conditions of the row and column, not a

single number: the sizing tables of the code are read at the intersection of those conditions

(IPC 2024, Table 710.1(1), Table 710.1(2), Table 1106.2 and Table 1106.3).

9. What the print read for this text does not carry

The word "isometric" is not printed anywhere in the corpus read for this document: the code's

instrument of communication with the code official is the permit application and the construction

documents of Sections 105 and 106, and the printed rule about what the drawing must make visible is

that construction documents are drawn to scale and are of sufficient clarity to indicate the

location, nature and extent of the work proposed and to show in detail that the work conforms

(IPC 2024, 106.1). No projection convention, no drafting method and no rule of a trade school is

taken from this code, because it prints none. What the code does print, and what a reader of a plan

therefore finds in it, is the pair of things this text has set out: the document rules of Sections 105

and 106, and the printed dispositions whose figures a plan carries — fixture units and loads

(IPC 2024, 709.1 and 710.1), sizes (IPC 2024, 710.1 and 604.3), slopes (IPC 2024, 704.1),

developed lengths (IPC 2024, 607.2, 906.3 and 909.1), clearances (IPC 2024, 903.1.4, 903.5, 708.1.10

and 1106.5), rainfall rates and capacities (IPC 2024, 1106.1, 1106.2 and 1106.3), and the inspection

stages at which each part of the drawn work becomes visible (IPC 2024, 111.2).

10. What the approved drawing binds

The approved construction documents are not a description of intent: the work is done in accordance

with them (IPC 2024, 105.5.1), a returned set is kept on the site while the work is in progress

(IPC 2024, 106.2), the permit or its copy is kept on the site until the project is completed

(IPC 2024, 105.5.7), the work remains visible and able to be accessed for inspection until approved

(IPC 2024, 111.1), and no portion not in compliance is covered or concealed until the code official

authorizes it (IPC 2024, 111.2.3). A drawing whose figures are read off the printed tables is what

the code official reviews against the code, and the same tables are what the installer works to; the

notice of approval follows the tests and inspections, and those tests are the permit holder's to make

and the code official's to observe (IPC 2024, 107.1, 111.4 and 312.1).

Isometric Analysis

This subject is a competence, not a chapter of the code. The 2024 International Plumbing Code (IPC 2024) carries no chapter called "isometric analysis"; what it carries is the set of printed dispositions that an isometric of a drain, waste and vent installation has to show and to agree with. This text is therefore written from those dispositions alone — the slope of a horizontal drain, the fittings placed at a change of direction, the fixture units behind every pipe size, the vent that protects each trap, the cleanouts, the supports, and the elevations that decide whether a backwater valve belongs on the drawing. Every rule below names the address it was read from. Where the printed line states no value, no value is stated here.

1. What the drawing has to answer for

An isometric is a line drawing that shows, without a plan view, where each pipe runs, how big it is, which way it slopes, where it connects, and what is connected to it. The code fixes the scope of the systems those lines belong to. The chapter on sanitary drainage regulates the methods and piping systems that remove water which has already served a purpose, beginning at the receiving fixtures and ending where the liquid waste is disposed of, and it sets out a design method for a gravity system of vertical and horizontal piping based on the probability of flow from specific fixtures (IPC 2024, Chapter 7). The chapter on vents regulates the locations of connections, the arrangements of venting systems and the sizing of vent piping, because a gravity drainage system only works while an air path is kept open through it (IPC 2024, Chapter 9). The chapter on traps, interceptors and separators regulates the design of fixture traps, the ways of keeping a trap seal from evaporating, and where interceptors and separators are required (IPC 2024, Chapter 10).

A drawing that answers to those three chapters is a drawing a plan reviewer can check line by line. The sections that follow name what is checked.

2. Slope — the first quantity the drawing fixes

A horizontal drain is drawn with one alignment and one grade. It is not permitted to be laid on a slope that varies, and the grade it is laid on must be at least the value the table gives for its size (IPC 2024, 704.1).

The same table carries a note that any pipe draining to a grease interceptor takes its slope from Section 704.1 rather than from the column above (IPC 2024, Table 704.1). Section 704.1 states that value directly: upstream of a grease interceptor a horizontal drain runs at not less than 1/4 inch per foot, which is a 2 percent slope (IPC 2024, 704.1). On an isometric this is drawn as a fixture branch that steepens before it reaches the interceptor, and it is a common place for a drawing to be wrong.

Three further rules about the shape of the run are read from the same area of the code:

3. Changes of direction and the fittings drawn at them

Every turn on the drawing is a fitting, and the fitting is chosen by the direction of the turn. Connections and changes of direction are made with approved drainage fittings (IPC 2024, 706.1). The fitting may not have a ledge, shoulder or reduction that can hold up or block the flow, and threaded drainage fittings must be of the recessed drainage type (IPC 2024, 706.2).

The table below is the one an isometric is marked against. It reads across the type of fitting pattern and down the three possible changes of direction (IPC 2024, Table 706.3).

The notes that ride with that table state that the restricted fittings are only allowed on a fixture drain of 2 inches or smaller, that the short sweep is allowed on 3 inches and larger, and that the limitation on double sanitary tees is found in Section 706.3 (IPC 2024, Table 706.3). Section 706.3 adds that fittings are installed so as to guide the waste in the direction of flow, that combination fittings, side inlets and increasers are read by the pattern of flow they create, and that a double sanitary tee pattern does not receive the discharge of back-to-back water closets or of fixtures and appliances that discharge by pumping. One exception is printed with it: back-to-back water closets are permitted on a double sanitary tee where the horizontal developed length between the water closet outlet and the connection to the tee pattern is at least 18 inches (IPC 2024, 706.3).

Two fittings have their own rules. A heel-inlet quarter bend is acceptable except where the quarter bend serves a water closet, and a low-heel inlet is not used as a wet-vented connection, while side-inlet quarter bends are acceptable for drainage, wet venting and stack venting arrangements (IPC 2024, 706.4).

4. The load behind every size: fixture units

A size is not drawn out of habit; it is drawn from a load. The drainage fixture unit values in the first table of Section 709 state the relative load weight of different kinds of fixtures, and those values are used in estimating the total load carried by a soil or waste pipe and with the tables of sizes for soil, waste and vent pipes in Section 710 (IPC 2024, 709.1). The values read from that table, with the trap size printed beside them, include:

All of those values are read from the printed table (IPC 2024, Table 709.1). The shower values step with the total flow through the showerheads and body sprays: 2 units up to 5.7 gpm, 3 above that to 12.3 gpm, 5 above that to 25.8 gpm and 6 from there to 55.6 gpm (IPC 2024, Table 709.1). The table also notes that for the purpose of computing loads on building drains and sewers a water closet or urinal is not rated lower unless the lower values are confirmed by testing, and that a trap larger than 3 inches is read from the second table instead (IPC 2024, Table 709.1).

A fixture that is not in that table is not left without a value. Where the outlet size of the fixture is known, the load comes from the table for fixture drains or traps, and the smallest trap for such a fixture is its drainage outlet size but never less than 1 1/4 inches (IPC 2024, 709.2). That second table reads 1 1/4 inches: 1 unit; 1 1/2 inches: 2; 2 inches: 3; 2 1/2 inches: 4; 3 inches: 5; 4 inches: 6 (IPC 2024, Table 709.2).

Where the only thing known about a discharge is its flow rate in gallons per minute, the conversion is fixed: 1 gpm is taken as two drainage fixture units (IPC 2024, 709.3). This is the arithmetic an isometric does when it carries a pump, a process tank or a piece of equipment whose outlet is described by flow rather than by fixture.

5. Sizing what the drawing shows

With the loads totalled, two tables do the sizing. The first gives the greatest number of drainage fixture units that may connect to a building sewer, a building drain, or a horizontal branch of a building drain, at each of four slopes; the second gives the greatest number that may connect to a horizontal fixture branch, to a stack of three branch intervals or less, or to a stack of more than three branch intervals (IPC 2024, 710.1).

Those rows are read from the printed table for building drains and sewers (IPC 2024, Table 710.1(1)), which also carries the rule that the smallest building drain serving a water closet is 3 inches (IPC 2024, Table 710.1(1)). The companion table for horizontal fixture branches and stacks reads, for the same diameters, a branch total, a per-branch-interval total, a stack total for three branch intervals or less and a stack total above three branch intervals: 1 1/2 inches 3 / 2 / 4 / 8; 2 inches 6 / 6 / 10 / 24; 3 inches 20 / 20 / 48 / 72; 4 inches 160 / 90 / 240 / 500 (IPC 2024, Table 710.1(2)). Its notes state that a stack is sized on the accumulated connected load at each story or branch interval, that a stack may be reduced as that load falls, and that a stack is never reduced below one half the diameter of the largest stack size required (IPC 2024, Table 710.1(2)).

The two odd cases on a drawing are the offsets:

In a building five stories or more in height, an offset carries venting obligations. A horizontal branch that connects to a stack within 2 feet above or below a vertical stack offset, where the offset sits more than four branch intervals below the top of the stack, requires the offset to be vented as Section 907 requires (IPC 2024, 711.1), and that vent is not required where the stack and its offset are sized as a building drain (IPC 2024, 711.1.1). A horizontal stack offset more than four branch intervals below the top of the stack is vented as Section 907 requires and is sized in three parts: above the offset as a vertical stack for the load above it, the offset itself as a building drain, and below the offset as the greater of the offset size and the size for the whole stack (IPC 2024, 711.2); that vent is not required where the stack and its offset are one pipe size larger than a building drain would need and the whole assembly is at least as large in cross-sectional area as a straight stack plus the offset vent of Section 907 (IPC 2024, 711.2.1). A vertical offset below the lowest horizontal branch does not force a change of diameter, but a horizontal offset below the lowest horizontal branch is sized as a building drain (IPC 2024, 711.3).

6. Cleanouts the drawing has to place

Cleanouts are a drawing problem: each one has a location, a size and a clearance, and each one has to be reachable. The rules are read one by one from Section 708.

7. The vent side of the drawing

Every trap on the drawing is protected by a vent, and the vent system is drawn as a path, not as a decoration. The system admits or emits air so that no fixture trap seal sees a pressure difference greater than 1 inch of water column (IPC 2024, 901.2), and traps and trapped fixtures are vented by one of the methods the chapter sets out (IPC 2024, 901.2.1).

How the vent is drawn:

How the vent is sized:

Distance from trap to vent is its own drawing measurement. Each fixture trap has a protecting vent placed so that the slope and the developed length of the fixture drain, from the trap weir to the vent fitting, fall inside the table (IPC 2024, 909.1). The table reads: 1 1/4-inch trap at 1/4 inch per foot, 5 feet; 1 1/2-inch trap at 1/4 inch per foot, 6 feet; 2-inch trap at 1/4 inch per foot, 8 feet; 3-inch trap at 1/8 inch per foot, 12 feet; 4-inch trap at 1/8 inch per foot, 16 feet (IPC 2024, Table 909.1). Fixtures that siphon themselves, such as water closets, are not limited by it (IPC 2024, 909.1). Two more rules apply to the same measurement: the total fall of a fixture drain from its slope may not exceed the drain's own diameter, and the vent connection to a fixture drain, other than a water closet, may not sit below the trap weir (IPC 2024, 909.2); and a vent is not placed within two pipe diameters of the trap weir, which is the crown vent prohibition (IPC 2024, 909.3).

Where two fixtures share a vent, the drawing shows which arrangement it is. An individual vent may serve two traps or trapped fixtures as a common vent only where both are on the same floor level (IPC 2024, 911.1). Where the two fixture drains connect at the same level, the vent connects at their interconnection or downstream of it (IPC 2024, 911.2). Where they connect at different levels, the vent is a vertical extension of the vertical drain, and that vertical length is the vent for the lower fixture drain, sized from the table of common vent sizes — 1 1/2 inches for a maximum discharge of 1 unit from the upper fixture drain, 2 inches for 4 units, 2 1/2 to 3 inches for 6 units — and the upper fixture may not be a water closet (IPC 2024, 911.3).

Section 907 governs the offsets that the drawing may carry. A horizontal offset of a drainage stack is vented where five or more branch intervals sit above the offset, by venting the upper and the lower section of the stack (IPC 2024, 907.1). The upper section is vented as a separate stack with a vent stack connection at its base, the offset being treated as the base of the stack (IPC 2024, 907.2). The lower section is vented by a yoke vent running between the offset and the next lower horizontal branch, which may be a vertical extension of the drainage stack, and the yoke vent and its connection are at least as large as the vent stack required for the stack (IPC 2024, 907.3).

The chapter also offers arrangements in which the drain itself does part of the venting, and an isometric that uses one of them must say so: wet venting, horizontal or vertical (IPC 2024, 912.1, 912.1.1), the waste stack vent (IPC 2024, 913.1), circuit venting (IPC 2024, 914.1), the combination waste and vent system (IPC 2024, 915.1), island fixture venting (IPC 2024, 916.1) and the single-stack vent system (IPC 2024, 917.1). Each of those carries its own limits on fixture type, size, slope and distance, and each is read from its own section before it is drawn.

8. Supports, anchors and the elevations the drawing fixes

An isometric that shows where pipe runs also shows what holds it. Piping is supported as Section 308 requires (IPC 2024, 308.1), and the interval of support comes from the table of hanger spacing (IPC 2024, 308.5). Read from it: ABS pipe, 4 feet horizontal, 10 feet vertical; cast-iron pipe, 5 feet horizontal (10 feet where 10-foot lengths are installed), 15 feet vertical; copper or copper-alloy pipe, 12 feet horizontal, 10 feet vertical; PVC pipe, 4 feet horizontal, 10 feet vertical; steel pipe, 12 feet horizontal, 15 feet vertical; cross-linked polyethylene pipe 1 inch and smaller, 2.67 feet (32 inches) horizontal, 10 feet vertical (IPC 2024, Table 308.5). The notes to that table add that for sizes 2 inches and smaller a guide is installed midway between the required vertical supports, to stop movement across the axis of the pipe, and that the cast-iron interval rises to 10 feet horizontally where 10-foot lengths are used (IPC 2024, Table 308.5). The exception sends piping designed for expansion and contraction to its engineered design under Section 316.1 (IPC 2024, 308.5).

Around that interval sit the rest of the support rules:

The elevations on the drawing decide whether a backwater valve belongs on it. Where fixtures sit on a floor whose finished elevation is below the elevation of the manhole cover of the next upstream manhole in the public sewer, those fixtures are protected by a backwater valve in the building drain or in the horizontal branch that serves them; fixtures on a floor above that elevation are not discharged through a backwater valve, and in existing buildings the higher fixtures are not prohibited from doing so (IPC 2024, 714.1). The valve complies with ASME A112.14.1, CSA B181.1 or CSA B181.2 (IPC 2024, 714.2) and is installed so that its working parts can be reached (IPC 2024, 714.3).

9. When the drawing departs from the tables

An isometric that cannot be built from the tables is drawn as an alternative engineered design, and Section 316 states what that submission carries. Such a design conforms to the intent of the code and delivers an equivalent level of quality, strength, effectiveness, fire resistance, durability and safety (IPC 2024, 316.1.1). The registered design professional marks the permit application so that the permit and the permanent permit records show an alternative engineered design was part of the approved installation (IPC 2024, 316.1.2), submits enough technical data to substantiate the design and to prove that its performance meets the code's intent (IPC 2024, 316.1.3), and submits two complete sets of signed and sealed construction documents (IPC 2024, 316.1.4). Those construction documents are specified precisely: they include floor plans and a riser diagram of the work, and where appropriate they show the direction of flow, every pipe size, the grade of the horizontal piping, the loading, and the location of the fixtures and appliances (IPC 2024, 316.1.4). That list is, in effect, the definition of a complete isometric. Approval follows where the code official finds the design conforms to the intent of the code, and a refusal is given in writing with reasons (IPC 2024, 316.1.5), with testing and inspection under Sections 111 and 312 (IPC 2024, 316.1.6).

A drainage system may also be sized and laid out by approved computer design methods (IPC 2024, 713.1). The load is then computed from the simultaneous or sequential discharge conditions of the fixtures, appurtenances and appliances, or from the peak usage condition (IPC 2024, 713.2); the discharge profiles of flow against time come from the manufacturer's specifications (IPC 2024, 713.2.1); pipe is sized so that full-bore flow does not occur (IPC 2024, 713.3); the calculation uses the pipe wall roughness factor from the manufacturer's specifications, modified for the roughness that aging, deposits and corrosion bring (IPC 2024, 713.3.1); and the horizontal piping is still designed and installed at the slopes of the slope table (IPC 2024, 713.3.2). An engineered vent system follows the same pattern: it complies with Section 316 as well as with its own section (IPC 2024, 919.1), and the maximum developed length of individual fixture vents to vent branches and headers is determined from the table for the minimum diameters at the indicated vent airflow rates, with the airflow rate computed from the number of fixtures per header against the total number of fixtures connected to the vent stack and the total vent stack airflow rate (IPC 2024, 919.2).

10. The order in which the drawing is read

An isometric is checked in a fixed order, because each step feeds the next. The slope comes first, for each horizontal run, against the slope table and against the steepening required upstream of a grease interceptor (IPC 2024, 704.1, IPC 2024, Table 704.1). Then the size, which may not fall in the direction of flow (IPC 2024, 704.2). Then each turn, which must be a fitting the table allows for that direction (IPC 2024, Table 706.3). Then the loads, from the fixture unit values or from the outlet sizes of unlisted fixtures or from the flow-rate conversion (IPC 2024, 709.1, IPC 2024, 709.2, IPC 2024, 709.3), and the sizes that follow from those loads (IPC 2024, 710.1, IPC 2024, Table 710.1(1), IPC 2024, Table 710.1(2)). Then the cleanouts, one by one, against interval, position, size and access (IPC 2024, 708.1.1, IPC 2024, 708.1.11). Then the vent path: where each vent connects, how far the trap sits from it, how big it is, and how long it develops (IPC 2024, 905.1, IPC 2024, 909.1, IPC 2024, 906.2). Then the support intervals and the anchors (IPC 2024, 308.5, IPC 2024, 308.7). And last the elevations, which decide the backwater valve (IPC 2024, 714.1). A drawing that answers all ten steps in that order answers the code on every point this text was written from.