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Appendix D — SIZING STORM WATER DRAINAGE SYSTEMS

2025 California Plumbing Code (Title 24, Part 5) · 2025 edition · updated 2026-07-27 · California

Italicized text is a California amendment to the model code, as printed in the official publication.

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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

MAXIMUM RATES OF RAINFALL FOR VARIOUS CITIES*

STATES AND CITIES STORM DRAINAGE 60-MINUTE DURATION, 100-YEAR RETURN
STATES AND CITIES
 inches per hour
 gallons per minute per square foot
ALABAMA
 –
 –
Birmingham
 3.7
 0.038
Huntsville
 3.3
 0.034
Mobile
 4.5
 0.047
Montgomery 3.8 0.039
ALASKA
 –
 –
Aleutian Islands
 1.0
 0.010
Anchorage
 0.6
 0.006
Bethel
 0.8
 0.008
Fairbanks
 1.0
 0.010
Juneau 0.6 0.006
ARIZONA
 –
 –
Flagstaff
 2.3
 0.024
Phoenix
 2.2
 0.023
Tucson 3.0 0.031
ARKANSAS
 –
 –
Eudora
 3.8
 0.039
Ft. Smith
 3.9
 0.041
Jonesboro
 3.5
 0.036
Little Rock 3.7 0.038

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APPENDIX D

TABLE D 101.1 MAXIMUM RATES OF RAINFALL FOR VARIOUS CITIES (continued)*

STATES AND CITIES STORM DRAINAGE 60-MINUTE DURATION, 100-YEAR RETURN
STATES AND CITIES
 inches per hour
 gallons per minute per square foot
CALIFORNIA
 –
 –
Eureka
 1.5
 0.016
Lake Tahoe
 1.3
 0.014
Los Angeles
 2.0
 0.021
Lucerne Valley
 2.5
 0.026
Needles
 1.5
 0.016
Palmdale
 3.0
 0.031
Redding
 1.5
 0.016
San Diego
 1.5
 0.016
San Francisco
 1.5
 0.016
San Luis Obispo 1.5 0.016
COLORADO
 –
 –
Craig
 1.5
 0.016
Denver
 2.2
 0.023
Durango
 1.8
 0.019
Stratton 3.0 0.031
CONNECTICUT
 –
 –
Hartford
 2.8
 0.029
New Haven 3.0 0.031
DELAWARE
 –
 –
Dover
 3.5
 0.036
Rehobeth Beach 3.6 0.037
DISTRICT OF COLUMBIA
 –
 –
Washington 4.0 0.042
FLORIDA
 –
 –
Daytona Beach
 4.0
 0.042
Ft. Myers
 4.0
 0.042
Jacksonville
 4.3
 0.045
Melbourne
 4.0
 0.042
Miami
 4.5
 0.047
Palm Beach
 5.0
 0.052
Tampa
 4.2
 0.044
Tallahassee 4.1 0.043
GEORGIA
 –
 –
Atlanta
 3.5
 0.036
Brunswick
 4.0
 0.042
Macon
 3.7
 0.038
Savannah
 4.0
 0.042
Thomasville 4.0 0.042
HAWAII
 –
 –
Rainfall rates in the Hawaiian Islands vary from 11~~⁄2 inches per hour to 8 inches per hour, depending on location and elevation. Consult local data.~~
IDAHO
 –
 –
Boise
 1.0
 0.010
Idaho Falls
 1.2
 0.012
Lewiston
 1.0
 0.010
Twin Falls 1.1 0.011

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APPENDIX D

TABLE D 101.1 MAXIMUM RATES OF RAINFALL FOR VARIOUS CITIES (continued)*

STATES AND CITIES STORM DRAINAGE 60-MINUTE DURATION, 100-YEAR RETURN
STATES AND CITIES
 inches per hour
 gallons per minute per square foot
ILLINOIS
 –
 –
Chicago
 2.7
 0.028
Harrisburg
 3.1
 0.032
Peoria
 2.9
 0.030
Springfield 3.0 0.031
INDIANA
 –
 –
Evansville
 3.0
 0.031
Indianapolis
 2.8
 0.029
Richmond
 2.7
 0.028
South Bend 2.7 0.028
IOWA
 –
 –
Council Bluffs
 3.7
 0.038
Davenport
 3.0
 0.031
Des Moines
 3.4
 0.035
Sioux City 3.6 0.037
KANSAS
 –
 –
Goodland
 3.5
 0.036
Salina
 3.8
 0.039
Topeka
 3.8
 0.039
Wichita 3.9 0.041
KENTUCKY
 –
 –
Bowling Green
 2.9
 0.030
Lexington
 2.9
 0.030
Louisville
 2.8
 0.029
Paducah 3.0 0.031
LOUISIANA
 –
 –
Monroe
 3.8
 0.039
New Orleans
 4.5
 0.047
Shreveport 4.0 0.042
MAINE
 –
 –
Bangor
 2.2
 0.023
Kittery
 2.4
 0.025
Millinocket 2.0 0.021
MARYLAND
 –
 –
Baltimore
 3.6
 0.037
Frostburg
 2.9
 0.030
Ocean City 3.7 0.038
MASSACHUSETTS
 –
 –
Adams
 2.6
 0.027
Boston
 2.7
 0.028
Springfield 2.7 0.028
MICHIGAN
 –
 –
Detroit
 2.5
 0.026
Grand Rapids
 2.6
 0.027
Kalamazoo 2.7 0.028

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APPENDIX D

TABLE D 101.1 MAXIMUM RATES OF RAINFALL FOR VARIOUS CITIES (continued)*

STATES AND CITIES STORM DRAINAGE 60-MINUTE DURATION, 100-YEAR RETURN
STATES AND CITIES
 inches per hour
 gallons per minute per square foot
Sheboygan
 2.1
 0.022
Traverse City 2.2 0.023
MINNESOTA
 –
 –
Duluth
 2.6
 0.027
Grand Forks
 2.5
 0.026
Minneapolis
 3.0
 0.031
Worthington 3.4 0.035
MISSISSIPPI
 –
 –
Biloxi
 4.5
 0.047
Columbus
 3.5
 0.036
Jackson 3.8 0.039
MISSOURI
 –
 –
Independence
 3.7
 0.038
Jefferson City
 3.4
 0.035
St. Louis
 3.2
 0.033
Springfield 3.7 0.038
MONTANA
 –
 –
Billings
 1.8
 0.019
Glendive
 2.5
 0.026
Great Falls
 1.8
 0.019
Missoula 1.3 0.014
NEBRASKA
 –
 –
Omaha
 3.6
 0.037
North Platte
 3.5
 0.036
Scotts Bluff 2.8 0.029
NEVADA
 –
 –
Las Vegas
 1.5
 0.016
Reno
 1.2
 0.012
Winnemucca 1.0 0.010
NEW HAMPSHIRE
 –
 –
Berlin
 2.2
 0.023
Manchester 2.5 0.026
NEW JERSEY
 –
 –
Atlantic City
 3.4
 0.035
Paterson
 3.0
 0.031
Trenton 3.2 0.033
NEW MEXICO
 –
 –
Albuquerque
 2.0
 0.021
Carlsbad
 2.6
 0.027
Gallup 2.1 0.022
NEW YORK
 –
 –
Binghamton
 2.4
 0.025
Buffalo 2.3 0.024

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APPENDIX D

TABLE D 101.1 MAXIMUM RATES OF RAINFALL FOR VARIOUS CITIES (continued)*

STATES AND CITIES STORM DRAINAGE 60-MINUTE DURATION, 100-YEAR RETURN
STATES AND CITIES inches per hour gallons per minute per square foot
New York City 3.1 0.032
Schenectady 2.5 0.026
Syracuse 2.4 0.025
NORTH CAROLINA – –
Asheville 3.2 0.033
Charlotte 3.4 0.035
Raleigh 4.0 0.042
Wilmington 4.4 0.046
NORTH DAKOTA – –
Bismarck 2.7 0.028
Fargo 2.9 0.030
Minot 2.6 0.027
OHIO – –
Cincinnati 2.8 0.029
Cleveland 2.4 0.025
Columbus 2.7 0.028
Toledo 2.6 0.027
Youngstown 2.4 0.025
OKLAHOMA – –
Boise City 3.4 0.035
Muskogee 4.0 0.042
Oklahoma City 4.1 0.043
OREGON – –
Medford 1.3 0.014
Ontario 1.0 0.010
Portland 1.3 0.014
PENNSYLVANIA – –
Erie 2.4 0.025
Harrisburg 2.9 0.030
Philadelphia 3.2 0.033
Pittsburgh 2.5 0.026
Scranton 2.8 0.029
RHODE ISLAND – –
Newport 3.0 0.031
Providence 2.9 0.030
SOUTH CAROLINA – –
Charleston 4.1 0.043
Columbia 3.5 0.036
Greenville 3.3 0.034
SOUTH DAKOTA – –
Lemmon 2.7 0.028
Rapid City 2.7 0.028
Sioux Falls 3.4 0.035

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APPENDIX D

TABLE D 101.1 MAXIMUM RATES OF RAINFALL FOR VARIOUS CITIES (continued)*

STATES AND CITIES STORM DRAINAGE 60-MINUTE DURATION, 100-YEAR RETURN
STATES AND CITIES
 inches per hour
 gallons per minute per square foot
TENNESSEE
 –
 –
Knoxville
 3.1
 0.032
Memphis
 3.5
 0.036
Nashville 3.0 0.031
TEXAS
 –
 –
Corpus Christi
 4.6
 0.048
Dallas
 4.2
 0.044
El Paso
 2.0
 0.021
Houston
 4.6
 0.048
Lubbock
 3.3
 0.034
San Antonio 4.4 0.046
UTAH
 –
 –
Bluff
 2.0
 0.021
Cedar City
 1.5
 0.016
Salt Lake City 1.3 0.014
VERMONT
 –
 –
Bennington
 2.5
 0.026
Burlington
 2.3
 0.024
Rutland 2.4 0.025
VIRGINIA
 –
 –
Charlottesville
 3.4
 0.035
Norfolk
 4.0
 0.042
Richmond
 4.0
 0.042
Roanoke 3.3 0.034
WASHINGTON
 –
 –
Seattle
 1.0
 0.010
Spokane
 1.0
 0.010
Walla Walla 1.0 0.010
WEST VIRGINIA
 –
 –
Charleston
 2.9
 0.030
Martinsburg
 3.0
 0.031
Morgantown 2.7 0.028
WISCONSIN
 –
 –
Green Bay
 2.5
 0.026
Lacrosse
 2.9
 0.030
Milwaukee
 2.7
 0.028
Wausau 2.5 0.026
WYOMING
 –
 –
Casper
 1.9
 0.020
Cheyenne
 2.5
 0.026
Evanston
 1.3
 0.014
Rock Springs 1.4 0.015

For SI units: 1 inch per hour = 25.4 mm/h, 1 gallon per minute per square foot = 0.679 [(L/s)/m [2] ]

  • The rainfall rates in this table are based on U.S. Weather Bureau Technical Paper No. 40, Chart 14: 100-Year 60-Minute Rainfall (inches).

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APPENDIX D

TABLE D 104.1 DISCHARGE FROM RECTANGULAR SCUPPERS (gallons per minute)1, 2, 3, 4
WATER HEAD
(inches)		 WIDTH OF SCUPPER (inches) WIDTH OF SCUPPER (inches) WIDTH OF SCUPPER (inches) WIDTH OF SCUPPER (inches) WIDTH OF SCUPPER (inches) WIDTH OF SCUPPER (inches)
WATER HEAD
(inches)		 6 12 18 24 30 36
1⁄2 6 13 19 25 32 38
1 17 35 53 71 89 107
11⁄2 31 64 97 130 163 196
2 – 98 149 200 251 302
21⁄2 – 136 207 278 349 420
3 – 177 271 364 458 551
31⁄2 – – 339 457 575 693
4 – – 412 556 700 844

For SI units: 1 inch = 25.4 mm, 1 gallon per minute = 0.06 L/s

Notes: 1 Table D 104.1 is based on discharge over a rectangular weir with end contractions. 2 The head is the depth of water above the bottom of the scupper opening. 3 The height of the scupper opening shall be not less than two times the design head. 4 Coordinate the allowable head of water with the structural design of the roof.

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▸ Contents — 2025 California Plumbing Code (Title 24, Part 5)
2025 California Plumbing Code (Title 24, Part 5)
  1. Chapter 1 — GENERAL CODE PROVISIONS
  2. Chapter 2 — DEFINITIONS
  3. Chapter 3 — GENERAL REGULATIONS
  4. Chapter 4 — PLUMBING FIXTURES AND FIXTURE FITTINGS
  5. Chapter 5 — WATER HEATERS
  6. Chapter 6 — WATER SUPPLY AND DISTRIBUTION
  7. Chapter 7 — SANITARY DRAINAGE
  8. Chapter 8 — INDIRECT WASTES
  9. Chapter 9 — VENTS
  10. Chapter 10 — TRAPS AND INTERCEPTORS
  11. Chapter 11 — STORM DRAINAGE
  12. Chapter 12 — FUEL GAS PIPING
  13. Chapter 13 — HEALTH CARE FACILITIES AND MEDICAL GAS AND MEDICA…
  14. Chapter 14 — FIRESTOP PROTECTION
  15. Chapter 15 — ALTERNATE WATER SOURCES FOR NONPOTABLE APPLICATIONS
  16. Chapter 16 — NONPOTABLE RAINWATER CATCHMENT SYSTEMS
  17. Chapter 17 — REFERENCED STANDARDS
  18. Appendix A — RECOMMENDED RULES FOR SIZING THE WATER SUPPLY SYS…
  19. Appendix B — EXPLANATORY NOTES ON COMBINATION WASTE AND VENT S…
  20. Appendix C — ALTERNATE PLUMBING SYSTEMS
  21. Appendix D — SIZING STORM WATER DRAINAGE SYSTEMS
  22. Appendix E — MANUFACTURED/MOBILE HOME PARKS AND RECREATIONAL V…
  23. Appendix F — FIREFIGHTER BREATHING AIR REPLENISHMENT SYSTEMS
  24. Appendix G — SIZING OF VENTING SYSTEMS
  25. Appendix H — PRIVATE SEWAGE DISPOSAL SYSTEMS
  26. Appendix I — INSTALLATION STANDARDS
  27. Appendix J — COMBINATION OF INDOOR AND OUTDOOR COMBUSTION AND …
  28. Appendix K — POTABLE RAINWATER CATCHMENT SYSTEMS
  29. Appendix L — SUSTAINABLE PRACTICES
  30. Appendix M — PEAK WATER DEMAND CALCULATOR
  31. Appendix N — IMPACT OF WATER TEMPERATURE ON THE POTENTIAL FOR …
  32. Appendix O — NON-SEWERED SANITATION SYSTEMS
  33. Appendix P — PROFESSIONAL QUALIFICATIONS
  34. Appendix Q — INDOOR HORTICULTURAL FACILITIES
  35. Appendix R — TINY HOUSES
  36. Appendix S — ONSITE STORMWATER TREATMENT SYSTEMS

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