Storm Drainage

Storm Drainage

Reference text for the storm drainage area. 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 whole of Chapter 11 of the code is read for this text, together with the sections it refers to.

1. General

The chapter governs the materials, design, construction and installation of storm drainage

(IPC 2024, 1101.1). Rainwater falling on roofs, and storm water from paved areas, yards, courts and

courtyards, must drain to an approved place of disposal; for one- and two-family dwellings, and where

the code official approves it, such water is permitted to discharge onto flat areas such as streets or

lawns as long as the flow is directed away from the building (IPC 2024, 1101.2). The purpose stated by

the code for the whole chapter: rainfall onto buildings must be removed and directed to a location

that can accommodate storm water, and the chapter fixes the design rainfall event for the geographic

area and the sizing methods for piping and gutter systems, together with the piping materials and the

subsoil drainage rules (IPC 2024, Chapter 11, About this chapter).

Storm water must not be drained into a sewer intended for sewage only (IPC 2024, 1101.3). That

prohibition is the mirror of the rule printed for sanitary drainage: where the public sewer is a

combined system carrying both sanitary sewage and storm water, the sanitary sewer is connected

independently to that public sewer (IPC 2024, 703.6), and the storm sewer is likewise connected

independently to it (IPC 2024, 1109.1).

The conductors and the building storm drain are tested in accordance with Section 312

(IPC 2024, 1101.4). The size of a drainage pipe in the storm system must not be reduced in the

direction of flow (IPC 2024, 1101.5). Connections and changes of direction in the storm drainage

system must be made with approved drainage-type fittings in accordance with Table 706.3, and those

fittings must not obstruct or retard the flow in the system (IPC 2024, 1101.6).

Roof design carries a structural rule: roofs must be designed for the maximum possible depth of water

that will pond on them, as determined by the relative levels of the roof deck and of the overflow

weirs, scuppers, edges or serviceable drains, combined with the deflected structural elements. In

fixing that maximum depth, all primary roof drainage means are assumed to be blocked, and the depth

must include the height of water required above the inlet of the secondary roof drainage means for

that secondary means to carry the design rainfall rate required by Section 1106 (IPC 2024, 1101.7).

Cleanouts must be installed in the storm drainage system and must comply with the provisions of the

code for sanitary drainage pipe cleanouts; the exception printed there is the subsurface drainage

system (IPC 2024, 1101.8 and Exception). Storm drainage systems must be provided with backwater

valves as required for sanitary drainage systems in accordance with Section 714 (IPC 2024, 1101.9).

2. Materials

The materials and methods used to construct and install storm drainage systems must comply with this

section and with the applicable provisions of Chapter 7 (IPC 2024, 1102.1).

Inside storm drainage conductors installed above ground conform to one of the standards listed in

Table 702.1 (IPC 2024, 1102.2). Underground building storm drain pipe conforms to one of the standards

listed in Table 702.2 (IPC 2024, 1102.3). Building storm sewer pipe conforms to one of the standards

listed in Table 1102.4 (IPC 2024, 1102.4).

TABLE 1102.4 BUILDING STORM SEWER PIPE

Subsoil drains are open-jointed, horizontally split or perforated pipe conforming to one of the

standards listed in Table 1102.5 (IPC 2024, 1102.5).

TABLE 1102.5 SUBSOIL DRAIN PIPE

Roof drains conform to ASME A112.3.1 or ASME A112.6.4; roof drains other than siphonic roof drains

are tested and rated in accordance with ASME A112.6.4 or ASPE/IAPMO Z1034 (IPC 2024, 1102.6).

Pipe fittings must be approved for installation with the piping material installed and must conform

to the respective pipe standards or to one of the standards listed in Table 1102.7. The fittings must

not have ledges, shoulders or reductions capable of retarding or obstructing the flow in the piping,

and threaded drainage pipe fittings must be of the recessed drainage type (IPC 2024, 1102.7).

TABLE 1102.7 PIPE FITTINGS

3. Traps

Leaders and storm drains that connect to a combined sewer must be trapped. The trap is installed

either individually, on the storm water drain branch serving each conductor, or as a single trap in

the main storm drain immediately before its connection with the combined building sewer or with the

public sewer. Leaders and storm drains that connect to a building storm sewer are not required to be

trapped (IPC 2024, 1103.1). A storm water trap must be of the same material as the piping system it

is attached to (IPC 2024, 1103.2), and the trap for an individual conductor must be the same size as

the horizontal drain it is connected to (IPC 2024, 1103.3). A cleanout is installed on the building

side of the trap and is provided with access (IPC 2024, 1103.4).

4. Conductors and connections

Conductor pipes must not be used as soil, waste or vent pipes, and soil, waste or vent pipes must not

be used as conductors (IPC 2024, 1104.1). Floor drains must not be connected to a storm drain

(IPC 2024, 1104.2).

5. Roof drains

Roof drains are installed in accordance with the manufacturer's instructions, and the inside opening

of the roof drain must not be obstructed by the roofing membrane material (IPC 2024, 1105.1). The

published flow rate of the roof drain, based on the head of water above the drain, is the figure used

to size the storm drainage system in accordance with Section 1106; the flow rate used for sizing the

storm drainage piping is based on the maximum anticipated ponding at the roof drain

(IPC 2024, 1105.2).

6. Size of conductors, leaders and storm drains

6.1 Design rainfall

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 the section, or on other rainfall rates

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

6.2 Storm drain piping

Vertical and horizontal storm drain piping is sized on the flow rate through the roof drain. That

flow rate is calculated in accordance with Section 1106.2.1 and is then checked against the roof

drain manufacturer's published flow rate for the specific model and size of the drain selected, to

verify that the drain will handle the anticipated flow. The flow rate in storm drain piping must not

exceed the values printed in Table 1106.2 (IPC 2024, 1106.2).

TABLE 1106.2 STORM DRAIN PIPE SIZING

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 gallon per minute = 3.785 L/m

(IPC 2024, Table 1106.2).

6.3 Rainfall rate converted to a flow rate

The rainfall rate falling on a roof surface is converted to a gallon-per-minute flow rate by

Equation 11-1: the flow rate equals the rainfall intensity in inches per hour, multiplied by the roof

area in square feet, multiplied by 0.0104 (IPC 2024, 1106.2.1).

6.4 Vertical leaders

Vertical leaders are sized on the flow rate delivered from horizontal gutters or on the maximum flow

rate through the roof drains. The flow rate through vertical leaders must not exceed the values of

Table 1106.3 (IPC 2024, 1106.3).

TABLE 1106.3 VERTICAL LEADER SIZING

For SI: 1 inch = 25.4 mm, 1 gallon per minute = 3.785 L/m (IPC 2024, Table 1106.3).

6.5 Vertical walls

When roof drains and storm drainage piping are sized, 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 the

area used in calculating the required size of the vertical conductors, the leaders and the horizontal

storm drainage piping (IPC 2024, 1106.4).

6.6 Parapet wall scuppers

Where scuppers serve as primary roof drainage, as secondary (emergency overflow) roof drainage, or as

both, their quantity, size, location and inlet elevation are chosen so that the depth of ponding

water on the roof does not exceed the maximum water depth for which the roof was designed as

determined by the International Building Code. Scupper openings are not less than 4 inches (102 mm)

in height and have a width equal to or greater than the circumference of a roof drain sized for the

same roof area. The flow through the primary system is not counted when the secondary scuppers are

located and sized (IPC 2024, 1106.5).

6.7 Roof gutters

Horizontal gutters are sized on the flow rate from the roof surface, and the flow rate in horizontal

gutters must not exceed the values of Table 1106.6 (IPC 2024, 1106.6).

TABLE 1106.6 HORIZONTAL GUTTER SIZING

Footnote a of the table: dimensions are width by depth for rectangular shapes, and a single dimension

is the diameter of a semicircle. For SI: 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 1106.6).

7. Siphonic roof drainage systems

Siphonic roof drains and siphonic drainage systems are designed in accordance with ASME A112.6.9 and

ASPE 45 (IPC 2024, 1107.1).

8. Secondary (emergency) roof drains

Where roof drains are required, secondary (emergency overflow) roof drains or scuppers are provided

where the roof perimeter construction extends above the roof in a manner that would entrap water if

the primary drains allow a buildup of water for any reason. Where primary and secondary roof drains

are manufactured as a single assembly, the inlet and the outlet of each drain are independent

(IPC 2024, 1108.1).

The secondary roof drain system discharges at an end point separate from the primary system, above

grade, at a location normally observed by the building occupants or by maintenance personnel

(IPC 2024, 1108.2).

Secondary (emergency) roof drain systems are sized in accordance with Section 1106, on the rainfall

rate for which the primary system is sized. Scuppers are sized so that the depth of ponding water does

not exceed the depth for which the roof was designed as determined by Section 1101.7; a scupper has

an opening dimension of not less than 4 inches (102 mm) in height, and an opening width equal to the

circumference of the roof drain required for the area served. The flow through the primary system is

not counted when the secondary roof drain system is sized (IPC 2024, 1108.3).

The section on secondary roof drains carries no sizing table of its own in the print read for this

document; the sizing of secondary drains is by reference to Section 1106 (IPC 2024, 1108.3), and the

only sizing tables printed in Chapter 11 are Table 1106.2, Table 1106.3 and Table 1106.6

(IPC 2024, Table 1106.2, Table 1106.3, Table 1106.6).

9. Combined sanitary and storm public sewer

Where the public sewer is a combined system serving both sanitary sewage and storm water, the storm

sewer is connected independently to that public sewer (IPC 2024, 1109.1).

10. Controlled flow roof drain systems

Where the storm drainage system is engineered for controlled flow, the roof of the structure is

designed for the storage of water. The controlled flow roof drain system is an engineered system

under this section and under the design, submittal, approval, inspection and testing requirements of

Section 316.1, and it is designed on the required rainfall rate of Section 1106.1 (IPC 2024, 1110.1).

The control devices are installed so that the discharge rate of water per minute does not exceed the

values for continuous flow indicated in Section 1110.1 (IPC 2024, 1110.2). Runoff control is by those

control devices, and the control devices are protected by strainers (IPC 2024, 1110.3). The minimum

number of roof drains in such a roof is fixed: not fewer than two roof drains in roof areas of

10,000 square feet (929 m²) or less, and not fewer than four roof drains in roofs of more than

10,000 square feet (929 m²) (IPC 2024, 1110.4).

11. Subsoil drains

Subsoil drains are open-jointed, horizontally split or perforated pipe conforming to one of the

standards listed in Table 1102.5, and they are not less than 4 inches (102 mm) in diameter. Where the

building is subject to backwater, the subsoil drain is protected by a backwater valve located with

access. Subsoil drains discharge to a trapped area drain, to a sump, to a dry well or to an approved

location above ground. A subsoil sump is not required to have a gastight cover or a vent, and the sump

and the pumping system comply with Section 1113.1 (IPC 2024, 1111.1).

12. Building subdrains

Building subdrains located below the level of the public sewer discharge into a sump or a receiving

tank whose contents are lifted automatically and discharged into the drainage system as required for

building sumps; the sump and the pumping equipment comply with Section 1113.1 (IPC 2024, 1112.1).

13. Sumps and pumping systems

The sump pump, the pit and the discharge piping conform to Sections 1113.1.1 through 1113.1.4

(IPC 2024, 1113.1).

The sump pump is of a capacity and a head appropriate to the anticipated use requirements

(IPC 2024, 1113.1.1). The sump pit is not less than 18 inches (457 mm) in diameter and not less than

24 inches (610 mm) in depth, unless otherwise approved; it is provided with access and is located so

that all drainage flows into the pit by gravity. The pit is constructed of tile, steel, plastic, cast

iron, concrete or another approved material, with a removable cover adequate to support the

anticipated loads in the area of use, and the pit floor is solid and provides permanent support for

the pump (IPC 2024, 1113.1.2). Electrical service outlets, where they are required, meet the

requirements of NFPA 70 (IPC 2024, 1113.1.3).

The discharge piping meets the requirements of Section 1102.2, 1102.3 or 1102.4 and includes a gate

valve and a full flow check valve; the pipe and the fittings are the same size as, or larger than,

the pump discharge tapping (IPC 2024, 1113.1.4). The exception printed with that rule: in one- and

two-family dwellings only a check valve is required, located on the discharge piping from the pump or

the ejector (IPC 2024, 1113.1.4 and Exception).

14. Where the rules of the chapter meet the other chapters

Storm drainage borrows from the rest of the code by printed reference, and those references fix what

the storm installer shares with the sanitary installer: the tests of Section 312 (IPC 2024, 1101.4),

the drainage-type fittings and changes of direction of Table 706.3 (IPC 2024, 1101.6), the pipe

cleanout provisions of Chapter 7 through Section 1101.8 (IPC 2024, 1101.8), the backwater valves of

Section 714 (IPC 2024, 1101.9), the materials of Chapter 7 through Section 1102.1 (IPC 2024, 1102.1),

and the above-ground conductor materials of Table 702.1 and the underground building drain materials

of Table 702.2 (IPC 2024, 1102.2 and 1102.3). The combination of storm and sanitary flow in one

building drain does not merge the two systems: each is connected independently to a combined public

sewer (IPC 2024, 703.6 and 1109.1).