CPC · California Plumbing Code

How do I size roof drains, vertical leaders and horizontal storm drains?

Q: Are horizontal storm drains sized for full or partial flow?

Table 1103.2 values are based on the pipes flowing full ; design must consider slope and expected hydraulic grade.

You size roof drains and leaders by the roof area they serve using Table 1103.1, size horizontal storm pipes by area using Table 1103.2 (values assume full flow), and size gutters from Table 1103.3; use the local rainfall rate (see Appendix D) and interpolate if necessary, provide secondary drains or scuppers as required, and add cleanouts at leader bases.

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

The California Plumbing Code requires that vertical conductors and leaders be sized by the maximum projected roof area using Table 1103.1 and that horizontal storm drains (building storm drains and horizontal branches) be sized by the maximum projected roof or paved area using Table 1103.2. Gutters are sized from Table 1103.3. These basic rules are stated in § 1103.1, § 1103.2 and § 1103.3 of the CPC.

Size leaders/drains from the roof area they serve — use Table 1103.1 for vertical leaders, Table 1103.2 for horizontal piping (flowing full), and Table 1103.3 for gutters; interpolate between columns if your rainfall rate isn't listed.

Requirements in detail

Key rule (plain)

Vertical conductors and leaders — sized by the maximum projected roof area using Table 1103.1 (§ 1103.1) .
Horizontal storm drains and building storm sewers — sized by the maximum projected roof or paved area using Table 1103.2 (§ 1103.2) .
Gutters — sized by Table 1103.3 (§ 1103.3) .

What inputs you need

Maximum projected roof area served by each inlet (ft²).
Local design rainfall rate (inches/hour) — CPC typically uses a 60‑minute, 100‑year storm unless AHJ requires otherwise (see roof drainage sections and Appendix D for local rainfall rates).
For horizontal piping: assume the pipe is flowing full (Table 1103.2 values are based on full flow).

How to apply the tables (decision steps)

Determine the applicable rainfall rate (use Table D101.1 or the AHJ value).
For each roof drain / leader: find the allowable roof area for a chosen pipe/drain size at that rainfall rate in Table 1103.1. If your exact rainfall rate is not a column, you may interpolate between columns or use the 1 in/h column divided by the local rainfall rate per D103.1.
For horizontal runs: use Table 1103.2 (sizes assume flow full) to check required diameter at the computed gpm or area.
For gutters: use Table 1103.3, selecting the correct gutter slope (1/16" per ft or 1/8" per ft) column.

Quick decision table

Component	Key input(s)	What table to use	Quick rule	Code Reference
Vertical leaders / roof drains	Projected roof area; rainfall rate	Table 1103.1	Size leader so allowable area ≥ actual roof area (interpolate if needed)	§ 1103.1, Table 1103.1 file
Horizontal storm drains	Projected roof/paved area; rainfall rate; pipe slope	Table 1103.2	Table values assume pipe flows full — pick dia. that handles area	§ 1103.2, Table 1103.2 file
Gutters	Roof area and gutter slope	Table 1103.3	Choose gutter dia./profile and slope (1/16" or 1/8" per ft)	§ 1103.3, Table 1103.3 file

Exceptions & special cases

Secondary (emergency) drainage — roofs must have secondary roof drainage arranged per the CPC (scuppers or secondary drains). When combined with the primary, the combined system must be sized per § 1103.0 using double the rainfall rate for the local area (combined primary+secondary) when connected to the same vertical piping, or sized independently if separate. See § 1101.12.2 and the combined-system instruction.
Controlled‑flow roof drainage — the roof may be designed to store stormwater and use controlled-flow drains; in that case, design uses Table 1105.1 and the pre‑calibrated weir flow plus Table 1103.1/1103.2 for pipe sizing (alternative allowed under 1105.0). If you use controlled-flow, follow the controlled‑flow provisions rather than the straight table method.
Continuous or semi‑continuous discharges (pumps, A/C condensate) are converted to an equivalent roof area — 1 gpm = 24 ft² at 4 in/hr and must be considered when sizing (see § 1104.1).
Scuppers must be at least 4 inches high and have width equal to the circumference of the required roof drain per Table 1103.1.

Common mistakes

Using sanitary drainage sizing rules (different hydraulics) instead of CPC storm tables. The CPC explicitly separates storm sizing (Tables 1103.1–1103.3).
Forgetting to use the correct local rainfall rate (60‑minute, 100‑year is the usual default) or failing to interpolate between table columns per D103.1. file
Assuming horizontal piping capacities are for partially filled flow — Table 1103.2 is based on pipes flowing full; design must account for slope and expected flow conditions.
Omitting required cleanouts at the base of leaders before the horizontal drain or not providing traps where the leader connects to a combined sewer (see related sections).
Not providing required secondary/emergency drainage (scuppers or second drains) sized per the code.

Worked example — concrete scenario

Scenario: flat roof served by a single roof drain. Roof plan area served by that drain = 6,700 ft². Local design rainfall = 3.2 in/hr. Roof drain branch slope = 1/8" per ft.

Steps:

The CPC allows using the roof‑area method and interpolation; one convenient method is to use the 1 in/hr column from Table 1103.1 and divide by the actual rainfall rate (per D103.1).
The code example in the CPC (D103.1) shows a 6‑inch drain at 1/8" per ft slope has an allowable area of 21,400 ft² for 1 in/hr. Dividing by 3.2 in/hr gives 21,400 / 3.2 = 6,688 ft² allowable at 3.2 in/hr.
Your roof area is 6,700 ft², which is slightly above 6,688 ft² — so a 6‑inch vertical leader at that slope is marginal and by strict table arithmetic would be undersized by about 12 ft². The practical next steps are:
- Increase the leader/drain size (next size up per Table 1103.1), or
- Add a second drain (CPC requires at least two drains for certain large roofs per 1105.0 if controlled-flow is used) or redistribute roof area to two drains, or
- Use interpolation from the table to confirm exact allowable area for the next larger size, or
- Provide engineered design (siphonic/engineered storm system) per § 1106.0.

This worked example is taken from the CPC guidance on interpolation and the example calculation in D103.1; always state the exact table values you used on your plan submittal.

Related provisions (useful to read alongside § 1103.x)

§ 1101.12 — Primary and secondary roof drainage (scuppers, emergency drains).
§ 1101.13 — Cleanouts for building storm drains and leaders.
§ 1101.15 — Traps on storm drains and leaders (where required).
§ 1104.1 — Values for continuous flow (1 gpm = 24 ft² @ 4 in/hr).
§ 1105.0 — Controlled‑flow roof drainage (alternative sizing using storage and calibrated weirs).
§ 1106.0 — Engineered storm drainage systems (siphonic systems and special designs).
Appendix D (D101.1, D103.1) — rainfall rates and interpolation method; useful for selecting the rainfall intensity to use.

Code references

Grounded in the retrieved California Plumbing Code — click a citation to read the verbatim passage:

CPC § 1103.0 High relevance — show source text

1103.0 Size of Leaders, Conductors, and Storm Drains.

1103.1 Vertical Conductors and Leaders. Vertical conductors and leaders shall be sized by the maximum projected roof area and Table 1103.1.

1103.2 Size of Horizontal Storm Drains and Sewers. The size of building storm drains, or building storm sewers or their horizontal branches shall be based on the maximum projected roof or paved area to be handled and Table 1103.2.

1103.3 Size of Roof Gutters. The size of semi-circular gutters shall be based on the maximum projected roof area and Table 1103.3.

1103.4 Side Walls Draining onto a Roof. Where vertical walls project above a roof to permit storm water to drain into the roof area below, the adjacent roof area shall be permitted to be computed from Table 1103.1 as follows:

(1) For one wall – add 50 percent of the wall area to the roof area figures.

(2) For two adjacent walls of equal height – add 35 percent of the total wall areas.

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STORM DRAINAGE

Col1	Col2	TABLE 1103.1 SIZING ROOF DRAINS, LEADERS, AND VERTICAL RAINWATER PIPING2, 3	Col4	Col5	Col6	Col7	Col8	Col9	Col10	Col11	Col12	Col13	Col14
SIZE OF DRAIN, LEADER, OR PIPE	FLOW	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)	**MAXIMUM

CPC § 1101.16 High relevance — show source text
1101.16 Leaders, Conductors, and Connections. Leaders or conductors shall not be used as soil, waste, or vent pipes nor shall soil, waste, or vent pipes be used as leaders or conductors.

1101.16.1 Protection of Leaders. Leaders installed along alleyways, driveways, or other locations where exposed to damage shall be protected by metal guards, recessed into the wall, or constructed from the ferrous pipe. 1101.16.2 Combining Storm with Sanitary Drainage. The sanitary and storm drainage system of a building shall be entirely separate, except where a combined sewer is used, in which case the building storm drain shall be connected in the same horizontal plane through a single wye fitting to the combined building sewer not less than 10 feet (3048 mm) downstream from a soil stack.

1102.0 Roof Drains.

1102.1 Applications. Roof drains shall be constructed of aluminum, cast-iron, copper alloy of not more than 15 percent zinc, leaded nickel bronze, stainless steel, ABS, PVC, polypropylene, polyethylene, or nylon and shall comply with ASME A112.3.1 or ASME A112.6.4.

1102.2 Dome Strainers Required. Roof drains shall have domed strainers.

Exception: Roof drain strainers for use on sun decks, parking decks, and similar areas that are normally serviced and maintained, shall be permitted to be of the flat surface type. Such roof drain strainers shall be level with the deck.

1102.3 Roof Drain Flashings. The connection between the roof and roof drains that pass through the roof and into the interior of the building shall be made watertight by the use of proper flashing material. 1102.3.1 Lead Flashing. Where lead flashing material is used, it shall be not less than 4 pounds per square foot (lb/ft [2] ) (19 kg/m [2] ). 1102.3.2 Copper Flashing. Where copper flashing material is used, it shall be not less than 12 ounces per square foot (oz/ft [2] ) (3.7 kg/m [2] ).

1103.0 Size of Leaders, Conductors, and Storm Drains.

1103.1 Vertical Conductors and Leaders. Vertical conductors and leaders shall be sized by the maximum projected roof area and Table 1103.1.

1103.2 Size of Horizontal Storm Drains and Sewers. The size of building storm drains, or building storm sewers or their horizontal branches shall be based on the maximum projected roof or paved area to be handled and Table 1103.2.

1103.3 Size of Roof Gutters. The size of semi-circular gutters shall be based on the maximum projected roof area and Table 1103.3.

1103.4 Side Walls Draining onto a Roof. Where vertical walls project above a roof to permit storm water to drain into the roof area below, the adjacent roof area shall be permitted to be computed from Table 1103.1 as follows:

(1) For one wall – add 50 percent of the wall area to the roof area figures.

(2) For two adjacent walls of equal height – add 35 percent of the total wall areas.

2025 CALIFORNIA PLUMBING CODE 207
CPC § 1101.16 High relevance — show source text
1101.16 Leaders, Conductors, and

Connections . . . . . . . . . . . . . . . . .207

1102.0 Roof Drains . . . . . . . . . . . . . . . . . .207

1102.1 Applications . . . . . . . . . . . . . . . . .207

1102.2 Dome Strainers Required . . . . . . .207

1102.3 Roof Drain Flashings . . . . . . . . . .207

1103.0 Size of Leaders, Conductors,

and Storm Drains . . . . . . . . . . . . .207

1103.1 Vertical Conductors and

Leaders . . . . . . . . . . . . . . . . . . . . .207

1103.2 Size of Horizontal Storm

Drains and Sewers . . . . . . . . . . . .207

1103.3 Size of Roof Gutters . . . . . . . . . . .207

1103.4 Side Walls Draining onto a Roof . . . . . . . . . . . . . . . . . .207

Table 1103.1 Sizing Roof Drains, Leaders, and Vertical Rainwater Piping . . .208

1104.0 Values for Continuous Flow . . . . .208

1104.1 General . . . . . . . . . . . . . . . . . . . . .208

1105.0 Controlled-Flow Roof

Drainage . . . . . . . . . . . . . . . . . . . .208

1105.1 Application . . . . . . . . . . . . . . . . . .208

Table 1105.1(1) Controlled-Flow Maximum Roof Water Depth . . . . . . . . . . . . .209

Table 1105.1(2) Distance of Scupper Bottoms Above Roof . . . . . . . . . . . . . . . . . .209

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TABLE OF CONTENTS

1105.2 Setback Roofs . . . . . . . . . . . . . . .209

1106.0 Engineered Storm Drainage System . . . . . . . . . . . . . . . . . . . . .209

1106.1 General . . . . . . . . . . . . . . . . . . . . .209

1106.2 Siphonic Roof Drainage Systems . . . . . . . . . . . . . . . . . . . .209

1106.3 Siphonic Roof Drains . . . . . . . . . .209

1107.0 Testing . . . . . . . . . . . . . . . . . . . . .209

1107.1 Testing Required . . . . . . . . . . . . .209

1107.2 Methods of Testing Storm Drainage Systems . . . . . . . . . . . .209

Table 1103.2 Sizing of Horizontal Rainwater Piping . . . . . . . . . . . . .210

Table 1103.3 Size of Gutters . . . . . . . . . . . . . . .211
CPC § 1101.11 High relevance — show source text
1101.11 Paved Areas. Where the occupant creates surface water drainage, the sumps, gratings, or floor drains shall be piped to a storm drain or an approved water course. 1101.12 Roof Drainage. Roof drainage shall comply with Section 1101.12.1 and Section 1101.12.2.

1101.12.1 Primary Roof Drainage. Roof areas of a building shall be drained by roof drains or gutters. The location and sizing of drains and gutters shall be coordinated with the structural design and pitch of the roof. Unless otherwise required by the Authority Having Jurisdiction, roof drains, gutters, vertical conductors or leaders, and horizontal storm drains for primary drainage shall be sized based on a storm of 60 minutes duration and 100 year return period. Refer to Table D 101.1 (in Appendix D) for 100 years, 60-minute storms at various locations. 1101.12.2 Secondary Drainage. Secondary (emergency) roof drainage shall be provided by one of the methods specified in Section 1101.12.2.1 or Section 1101.12.2.2.

1101.12.2.1 Roof Scuppers or Open Side. Secondary roof drainage shall be provided by an opensided roof or scuppers where the roof perimeter construction extends above the roof in such a manner that water will be entrapped. An open-sided roof or scuppers shall be sized to prevent the depth of ponding water from exceeding that for which the roof was designed as determined by Section 1101.12.1. Scupper openings shall be not less than 4 inches (102 mm) high and have a width equal to the circumference of the roof drain required for the area served, sized in accordance with Table 1103.1.

1101.12.2.2 Secondary Roof Drain. Secondary roof drains shall be provided. The secondary roof drains shall be located not less than 2 inches (51 mm) above the roof surface. The maximum height of the roof drains shall be a height to prevent the depth of ponding water from exceeding that for which the roof was designed as determined by Section 1101.12.1. The secondary roof drains shall connect to a piping system in accordance with Section 1101.12.2.2.1 or Section 1101.12.2.2.2.

1101.12.2.2.1 Separate Piping System. The secondary roof drainage system shall be a

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STORM DRAINAGE

separate system of piping, independent of the primary roof drainage system. The discharge shall be above grade, in a location observable by the building occupants or maintenance personnel. Secondary roof drain systems shall be sized in accordance with Section 1101.12.1

based on the rainfall rate for which the primary system is sized. 1101.12.2.2.2 Combined System. The secondary roof drains shall connect to the vertical piping of the primary storm drainage conductor downstream of the last horizontal offset located below the roof. The primary storm drainage system shall connect to the building storm water that connects to an underground public storm sewer. The combined secondary and primary roof drain systems shall be sized in accordance with Section 1103.0 based on double the rain fall rate for the local area.
CPC § 1101.1 High relevance — show source text
1101.1 Applicability . . . . . . . . . . . . . . . . . .205

1101.2 Where Required . . . . . . . . . . . . . .205

1101.3 Storm Water Drainage to Sanitary Sewer Prohibited . . . . . .205

1101.4 Material Uses . . . . . . . . . . . . . . . .205

Table 1101.4.6 Materials for Subsoil Drainpipe and Fittings . . . . . . . . . . . . . . . . . .205

1101.5 Expansion Joints Required . . . . . .205

1101.6 Subsoil Drains . . . . . . . . . . . . . . .205

1101.7 Building Subdrains . . . . . . . . . . . .206

1101.8 Areaway Drains . . . . . . . . . . . . . .206

1101.9 Window Areaway Drains . . . . . . .206

1101.10 Filling Stations and Motor Vehicle Washing Establishments . . . . . . . . . . . . . . .206

1101.11 Paved Areas . . . . . . . . . . . . . . . . .206

1101.12 Roof Drainage . . . . . . . . . . . . . . .206

1101.13 Cleanouts . . . . . . . . . . . . . . . . . . .207

1101.14 Rainwater Sumps . . . . . . . . . . . . .207

1101.15 Traps on Storm Drains and Leaders . . . . . . . . . . . . . . . . . . . . .207

1101.16 Leaders, Conductors, and

Connections . . . . . . . . . . . . . . . . .207

1102.0 Roof Drains . . . . . . . . . . . . . . . . . .207

1102.1 Applications . . . . . . . . . . . . . . . . .207

1102.2 Dome Strainers Required . . . . . . .207

1102.3 Roof Drain Flashings . . . . . . . . . .207

1103.0 Size of Leaders, Conductors,

and Storm Drains . . . . . . . . . . . . .207

1103.1 Vertical Conductors and

Leaders . . . . . . . . . . . . . . . . . . . . .207

1103.2 Size of Horizontal Storm

Drains and Sewers . . . . . . . . . . . .207

1103.3 Size of Roof Gutters . . . . . . . . . . .207

1103.4 Side Walls Draining onto a Roof . . . . . . . . . . . . . . . . . .207

Table 1103.1 Sizing Roof Drains, Leaders, and Vertical Rainwater Piping . . .208

1104.0 Values for Continuous Flow . . . . .208

1104.1 General . . . . . . . . . . . . . . . . . . . . .208

1105.0 Controlled-Flow Roof

Drainage . . . . . . . . . . . . . . . . . . . .208
CPC § 408.8.3 High relevance — show source text
Sheet . . . . . . . . . . . . . . . . . . . . . . .408.8.3, 701.5, 906.6

Water supply . . . . . . . . . . . . . . . . . . . . . .604.2, 604.2.1

LEADERS

Cleanouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1101.13

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .214.0

Installed outside . . . . . . . . . . . . . . . . . . . . . . . . .1101.4.3

Leaders, conductors, and connections . . . . . . . .1101.16 Roof drainage . . . . . . . . . . . . . . . . . . . . . . . . . . .1101.12 Size of leaders, conductors, and

storm drains . . . . . . . . . . . . . . . . . . . . . . . . . . .1103.0

Sizing roof drains, leaders, and vertical rainwater piping . . . . . . . . . . . . .Table 1103.1 Traps on storm drains and leaders . . . . . . . . . . .1101.15

LIMITED-DENSITY OWNER-BUILT

RURAL DWELLING

Connections to plumbing system . . . . . . . . . . . . . .304.1

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .214.0

Disposal of liquid waste . . . . . . . . . . . . . . . . . . . . .303.1 Water supply and flushing . . . . . . . . . . . . . . . . . . .601.2 LIQUEFIED PETROLEUM

GAS (LP-GAS) . . . . . . . . . . . . . . . 214.0, 1202.1, 1208.4.1,

1208.5.6.3, 1208.5.10.4,

1212.11, E 401.2,

E 403.1, E 403.10

LIQUID WASTES . . . . . . . . . . . . . . . . .214.0, 303.0, L 302.0

LISTED . . . . . . . . . . . . . . . . . . . . . . . .214.0, 301.2, 402.6.2,

501.1

LISTING AGENCY . . . . . . . . . . . . . . . . . . . . . . .214.0, 301.2

LOCATION OF

Backwater valves . . . . . . . . .710.1, 710.3, 710.4, 710.6,

1006.0, 1101.6.4, 1503.2.3,

1503.9.1, 1503.9.7
CPC § 1101.12.2.2 High relevance — show source text
1101.12.2.2 Secondary Roof Drain. Secondary roof drains shall be provided. The secondary roof drains shall be located not less than 2 inches (51 mm) above the roof surface. The maximum height of the roof drains shall be a height to prevent the depth of ponding water from exceeding that for which the roof was designed as determined by Section 1101.12.1. The secondary roof drains shall connect to a piping system in accordance with Section 1101.12.2.2.1 or Section 1101.12.2.2.2.

1101.12.2.2.1 Separate Piping System. The secondary roof drainage system shall be a

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STORM DRAINAGE

separate system of piping, independent of the primary roof drainage system. The discharge shall be above grade, in a location observable by the building occupants or maintenance personnel. Secondary roof drain systems shall be sized in accordance with Section 1101.12.1

based on the rainfall rate for which the primary system is sized. 1101.12.2.2.2 Combined System. The secondary roof drains shall connect to the vertical piping of the primary storm drainage conductor downstream of the last horizontal offset located below the roof. The primary storm drainage system shall connect to the building storm water that connects to an underground public storm sewer. The combined secondary and primary roof drain systems shall be sized in accordance with Section 1103.0 based on double the rain fall rate for the local area.

1101.13 Cleanouts. Cleanouts for building storm drains shall comply with the requirements of Section 719.0 of this code.

1101.13.1 Rain Leaders and Conductors. Rain leaders and conductors connected to a building storm sewer shall have a cleanout installed at the base of the

leader or conductor before it connects to the horizontal

drain.

1101.14 Rainwater Sumps. Rainwater sumps serving “public use” occupancy buildings shall be provided with dual pumps arranged to function alternately in the case of overload or mechanical failure. Pumps rated 600 V or less shall comply with UL 778 and shall be installed in accordance with the manufacturer’s installation instructions.

1101.15 Traps on Storm Drains and Leaders. Leaders and storm drains, where connected to a combined sewer, shall be trapped. Floor and area drains connected to a storm drain shall be trapped.

Exception: Traps shall not be required where roof drains, rain leaders, and other inlets are at locations permitted under Section 906.0, Vent Termination.

1101.15.1 Where Not Required. No trap shall be required for leaders or conductors that are connected to a sewer carrying storm water exclusively. 1101.15.2 Trap Size. Traps, where installed for individual conductors, shall be the same size as the horizontal drain to which they are connected.

1101.15.3 Method of Installation of Combined Sewer. Individual storm-water traps shall be installed on the stormwater drain branch serving each storm-water inlet, or a single trap shall be installed in the main storm drain just before its connection with the combined building sewer. Such traps shall be provided with an accessible cleanout on the outlet side of the trap.
CPC § 0.019 High relevance — show source text
35|0.019|–|–|–|–| |0.71|0.061|–|–|–|–| |1.06|0.120|–|–|–|–| |1.41|0.194|–|–|–|–| |1.77|0.284|–|–|–|–| |2.12|0.387|–|–|–|–| |2.47|0.504 –|–|–|–|| |2.82|0.634|–|–|–|–| |3.18|0.777|–|–|–|–| |3.53|0.932|0.238|–|–|–| |4.24|1.277|0.325|–|–|–| |4.94|1.669|0.424|–|–|–| |5.65|2.106|0.534|–|–|–| |6.36|2.586|0.655|–|–|–| |7.06|3.110|0.787|0.272|–|–| |7.77|3.674|0.929|0.321|–|–| |8.47|4.280|1.081|0.373|–|–| |9.18|4.927|1.243|0.429|–|–| |9.89|–|1.416|0.488|–|–| |10.59|–|1.597|0.551|0.242|–| |11.30|–|1.789|0.616|0.270|–| |12.01|–|1.990|0.685|0.300|–| |12.71|–|2.200|0.757|0.332|–| |13.42|–|2.419|0.832|0.365|–| |14.12|–|2.648|0.911|0.399|–| |14.83|–|2.886|0.992|0.435|–| |15.54|–|3.132|1.077|0.471|–| |16.24
|–
|3.388
|1.164
|0.510
|–
| |16.95|–|3.652|1.254|0.549|–| |17.66|–|3.925|1.348|0.590|–| |18.36|–|4.207|1.444|0.632|0.167| |19.07|–|4.498|1.543|0.675|0.179| |19.77|–|4.797|1.646|0.720|0.190| |20.48|–|–|1.751|0.766|0.202| |21.19|–|–|1.859|0.813|0.214| |24.72|–|–|2.441|1.066|0.281| |28.25|–|–|3.092|1.350|0.356| |31.78|–|–|3.811|1.662|0.438| |35.31|–|–|4.596|2.004|0.527| |38.84|–|–|–|2.373|0.
CPC § 1101.7 High relevance — show source text
(1) They do not serve continuously flowing springs or groundwater.

(2) The point of discharge is not less than 10 feet (3048 mm) from a property line.

(3) It is impracticable to discharge such drains to a storm drain, to an approved water course, to the front street curb or gutter, or to an alley. 1101.7 Building Subdrains. Building subdrains located below the public sewer level shall discharge into a sump or receiving tank, the contents of which shall be automatically lifted and discharged into the drainage system as required for building sumps. 1101.8 Areaway Drains. Open subsurface space adjacent to a building, serving as an entrance to the basement or cellar of a building, shall be provided with a drain or drains. The areaway drains shall be not less than 2 inches (50 mm) in diameter for areaways at a maximum of 100 square feet (9.29 m [2] ) in area, and shall be discharged in the manner provided for subsoil drains not serving continuously flowing springs or groundwater (see Section 1101.6.1). Areaways exceeding 100 square feet (9.29 m [2] ) shall not drain into subsoil drains.

The drains for areaways exceeding 100 square feet (9.29 m [2] ) shall be sized in accordance with Table 1103.2.

1101.9 Window Areaway Drains. Window areaways at a maximum of 10 square feet (0.93 m [2] ) in area shall be permitted to discharge to the subsoil drains through a 2 inch (50 mm) diameter pipe. However, window areaways exceeding 10 square feet (0.93 m [2] ) in area shall be handled in the manner provided for entrance areaways (see Section 1101.8). 1101.10 Filling Stations and Motor Vehicle Washing Establishments. Public filling stations and motor vehicle washing establishments shall have the paved area sloped toward sumps or gratings within the property lines. Curbs not less than 6 inches (152 mm) high shall be placed where required to direct water to gratings or sumps.

1101.11 Paved Areas. Where the occupant creates surface water drainage, the sumps, gratings, or floor drains shall be piped to a storm drain or an approved water course. 1101.12 Roof Drainage. Roof drainage shall comply with Section 1101.12.1 and Section 1101.12.2.

1101.12.1 Primary Roof Drainage. Roof areas of a building shall be drained by roof drains or gutters. The location and sizing of drains and gutters shall be coordinated with the structural design and pitch of the roof. Unless otherwise required by the Authority Having Jurisdiction, roof drains, gutters, vertical conductors or leaders, and horizontal storm drains for primary drainage shall be sized based on a storm of 60 minutes duration and 100 year return period. Refer to Table D 101.1 (in Appendix D) for 100 years, 60-minute storms at various locations. 1101.12.2 Secondary Drainage. Secondary (emergency) roof drainage shall be provided by one of the methods specified in Section 1101.12.2.1 or Section 1101.12.2.2.
CPC § 1.11.0. High relevance — show source text
This state agency does not adopt sections identified with the following symbol: The Office of the State Fire Marshal’s adoption of this chapter or individual sections is applicable to structures regulated by other state agencies pursuant to Section 1.11.0.

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APPENDIX D

SIZING STORM WATER DRAINAGE SYSTEMS

The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.

D 101.0 General.

D 101.1 Applicability. This appendix provides general guidelines for the sizing of storm water drainage systems based on maximum rates of rainfall for various cities. The rainfall rates in Table D 101.1 shall be permitted to be used for design unless higher values are established locally.

D 102.0 Sizing by Flow Rate. D 102.1 General. Storm drainage systems shall be permitted to be sized by storm water flow rates, using the gallons per minute per square foot [(L/s)/m [2] ] of rainfall listed in Table D 101.1 for the local area. Multiplying the listed gallons per minute per square foot [(L/s)/m [2] ] by the roof area being drained (in square feet) (m [2] ) by each inlet produces the gallons per minute (gpm) (L/s) of required flow for sizing each drain inlet. The flow rates shall be permitted to be added to determine the flows in each of the drainage systems. Required pipe sizes for various flow rates are listed in Table 1103.1 and Table 1103.2.

D 103.0 Sizing by Roof Area. D 103.1 General. Storm drainage systems shall be permitted to be sized using the roof area served by each of the

drainage systems. Maximum allowable roof areas with various rainfall rates are listed in Table 1103.1 and Table 1103.2, along with the required pipe sizes. By using this method, it shall be permitted to interpolate between two listed rainfall rate columns (inches per hour) (mm/h). To determine the allowable roof area for a listed pipe size at a listed slope, divide the allowable square feet (m [2] ) of the roof for a 1 inch per hour (in/h) (25.4 mm/h) rainfall rate by the listed rainfall rate for the local area. For example, the allowable roof area for a 6 inch (150 mm) drain at [1] ⁄ 8 inch per foot (10.4 mm/m) slope with a rainfall rate of 3.2 in/h (81 mm/h) is 21 400/3.2 = 6688 square feet (621.3 m [2] ).

D 104.0 Capacity of Rectangular Scuppers. D 104.1 General. Table D 104.1 lists the discharge capacity of rectangular roof scuppers of various widths with various heads of water. The maximum allowable level of water on the roof shall be obtained from the registered design professional, based on the design of the roof.

TABLE D 101.1
CPC § 407.2.3 Medium relevance — show source text
LAVATORY FAUCETS

Common and public use areas . . . . . . . . . . . . . .407.2.3 Maximum flow rate . . . . . . . . . . . . . . . . .407.2.1, 407.2.2

Metering faucet . . . . . . . . . . . . . . . . . . . . . . . . . .407.2.4

Public . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .407.2.1

Residential . . . . . . . . . . . . . . . . . . . . . . . . . . . . .407.2.2

LAWN SPRINKLERS . . . . . . . . .Table 610.3, Table A 103.1,

A 103.2, M 102.5

LEAD

Allowable lead contents . . . . . . . . . . . . . .604.2, 604.2.1

Bends and traps . . . . . . . . . . . . . . . . . . . . .701.5, 906.6 Closet rings . . . . . . . . . . . . . . . . . . . . . . . . . . . .402.6.1

Flashing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1102.3.1

Roof drains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1102.1

Sheet . . . . . . . . . . . . . . . . . . . . . . .408.8.3, 701.5, 906.6

Water supply . . . . . . . . . . . . . . . . . . . . . .604.2, 604.2.1

LEADERS

Cleanouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1101.13

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .214.0

Installed outside . . . . . . . . . . . . . . . . . . . . . . . . .1101.4.3

Leaders, conductors, and connections . . . . . . . .1101.16 Roof drainage . . . . . . . . . . . . . . . . . . . . . . . . . . .1101.12 Size of leaders, conductors, and

storm drains . . . . . . . . . . . . . . . . . . . . . . . . . . .1103.0

Sizing roof drains, leaders, and vertical rainwater piping . . . . . . . . . . . . .Table 1103.1 Traps on storm drains and leaders . . . . . . . . . . .1101.15

LIMITED-DENSITY OWNER-BUILT

RURAL DWELLING

Connections to plumbing system . . . . . . . . . . . . . .304.1
CPC § 1103.1. Medium relevance — show source text
(9) Scupper openings shall be not less than 4 inches (102 mm) high and have a width equal to the circumference of

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STORM DRAINAGE

the roof drain required for the area served, sized in accordance with Table 1103.1.

(10)Flashings shall extend above the top of the scuppers.

(11)At a wall or parapet, 45 degree (0.79 rad) cants shall be installed.

(12)Separate storm and sanitary drainage systems shall be provided within the building.

(13)Calculations for the roof drainage system shall be submitted along with the plans to the Authority Having Jurisdiction for approval.

TABLE 1105.1(1) CONTROLLED-FLOW MAXIMUM ROOF WATER DEPTH

ROOF RISE*
(inches) MAXIMUM WATER DEPTH AT
DRAIN
(inches)

Flat 3

2 4

4 5

6 6

For SI units: 1 inch = 25.4 mm
- Vertical measurement from the roof surface at the drain to the highest point of the roof surface served by the drain, ignoring a local depression immediately adjacent to the drain.
TABLE 1105.1(2) DISTANCE OF SCUPPER BOTTOMS ABOVE ROOF

ROOF RISE*
(inches) ABOVE ROOF LEVEL
AT DRAIN
(inches)

Flat 3

2 4

4 5

6 6

For SI units: 1 inch = 25.4 mm
- Vertical measurement from the roof surface at the drain to the highest point of the roof surface served by the drain, ignoring a local depression immediately adjacent to the drain.
1105.2 Setback Roofs. Drains on setback roofs shall be permitted to be connected to the controlled-flow drainage systems provided:

(1) The setback is designed for storing water, or

(2) The square footage of the setback drainage area is converted as outlined in Section 1105.0 to gpm, and the storm-water pipe sizes in the controlled-flow system are based on the sum of the loads.

(3) The branch from each of the roof drains that are not provided with controlled flow shall be sized in accordance

with Table 1103.1.

1106.0 Engineered Storm Drainage System. 1106.1 General. The design and sizing of a storm drainage system shall be permitted to be determined by accepted engineering practices. The system shall be designed by a regis

TABLE 1105.1(1) CONTROLLED-FLOW MAXIMUM ROOF WATER DEPTH

For SI units: 1 inch = 25.4 mm
- Vertical measurement from the roof surface at the drain to the highest point of the roof surface served by the drain, ignoring a local depression immediately adjacent to the drain.
TABLE 1105.1(2) DISTANCE OF SCUPPER BOTTOMS ABOVE ROOF

For SI units: 1 inch = 25.4 mm

ROOF RISE* (inches)	MAXIMUM WATER DEPTH AT DRAIN (inches)
Flat	3
2	4
4	5
6	6

ROOF RISE* (inches)	ABOVE ROOF LEVEL AT DRAIN (inches)
Flat	3
2	4
4	5
6	6

Frequently asked questions

How do I pick the correct rainfall rate for my site?

Use the local design rainfall in Table D101.1 (or the AHJ‑specified value). The code typically uses 60‑minute, 100‑year rates unless the Authority Having Jurisdiction directs otherwise. file

If my rainfall rate is not shown in the table columns, can I interpolate?

Yes. The CPC permits interpolation between listed rainfall columns or use the 1 in/hr value divided by your rainfall rate per D103.1 to determine allowable roof area.

Are horizontal storm drains sized for full or partial flow?

Table 1103.2 values are based on the pipes flowing full; design must consider slope and expected hydraulic grade.

Do I need a cleanout on a roof leader?

Yes — rain leaders and conductors connected to a building storm sewer require a cleanout at the base before joining the horizontal drain.

What about secondary (emergency) drains?

Secondary roof drainage (scuppers or a secondary drain) is required where water could pond; if tied into the primary vertical, size combined piping for double the local rainfall rate or provide separate piping sized per the primary rainfall rate as required in § 1101.12.

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Related in the CPC

What are the rules for controlled‑flow roof drainage (precalibrated weirs, ponding depths)?

When can I use engineered sizing or Appendix D sizing methods?

Sizing of leaders, storm drains, gutters and controlled-flow systems

California Plumbing Code