Isometric Analysis
Subject guide — a code subject several of the state exam outlines declare.
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).
| Size of the drain (inches) | Minimum slope (inch per foot) | Source |
|---|---|---|
| 2 1/2 or smaller | 1/4 | (IPC 2024, Table 704.1) |
| 3 through 6 | 1/8 | (IPC 2024, Table 704.1) |
| 8 or larger | 1/16 | (IPC 2024, Table 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).
| Type of fitting pattern | Horizontal to vertical | Vertical to horizontal | Horizontal to horizontal |
|---|---|---|---|
| Sixteenth bend | permitted | permitted | permitted |
| Eighth bend | permitted | permitted | permitted |
| Sixth bend | permitted | permitted | permitted |
| Quarter bend | permitted | permitted (2-inch fixture drain or smaller) | permitted (2-inch fixture drain or smaller) |
| Short sweep | permitted | permitted where 3 inches or larger (2-inch fixture drain or smaller) | permitted (2-inch fixture drain or smaller) |
| Long sweep | permitted | permitted | permitted |
| Sanitary tee | permitted (double sanitary tee limitation) | not permitted | not permitted |
| Wye | permitted | permitted | permitted |
| Combination wye and eighth bend | permitted | permitted | permitted |
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:
| Fixture | Drainage fixture units | Minimum trap size (inches) |
|---|---|---|
| Automatic clothes washer, residential | 2 | 2 |
| Bathroom group (1.6 gpf water closet) | 5 | — |
| Bathtub, with or without overhead shower or whirlpool | 2 | 1 1/2 |
| Dishwashing machine, domestic | 2 | 1 1/2 |
| Floor drain | 2 | 2 |
| Kitchen sink, domestic | 2 | 1 1/2 |
| Lavatory | 1 | 1 1/4 |
| Shower, 5.7 gpm or less | 2 | 1 1/2 |
| Shower, greater than 25.8 gpm to 55.6 gpm | 6 | 4 |
| Service sink | 2 | 1 1/2 |
| Urinal | 4 | consistent with the fixture outlet |
| Urinal, 1 gallon per flush or less | 2 | consistent with the fixture outlet |
| Water closet, private, 1.6 gpf | 3 | consistent with the fixture outlet |
| Water closet, public, 1.6 gpf | 4 | consistent with the fixture outlet |
| Water closet, public, flushing greater than 1.6 gpf | 6 | consistent with the fixture outlet |
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).
| Diameter (inches) | 1/8 inch per foot | 1/4 inch per foot | 1/2 inch per foot |
|---|---|---|---|
| 2 | — | 21 | 26 |
| 3 | 36 | 42 | 50 |
| 4 | 180 | 216 | 250 |
| 6 | 700 | 840 | 1,000 |
| 8 | 1,600 | 1,920 | 2,300 |
| 12 | 4,600 | 5,600 | 6,700 |
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.
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