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Appendix D — SIZING STORM WATER DRAINAGE SYSTEMS

California Plumbing Code (Title 24, Part 5) · 2019 edition · updated 2026-09-10 · California

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

(Matrix Adoption Tables are non-regulatory, intended only as an aid to the code user. See Chapter 1 for state agency authority and building applications.)

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2019 CALIFORNIA PLUMBING CODE 385

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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






2019 CALIFORNIA PLUMBING CODE 387

APPENDIX D any other purpose or distributed to any other persons or parti es.

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




























12
 12
 12






388 2019 CALIFORNIA PLUMBING CODE

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



















2019 CALIFORNIA PLUMBING CODE 389

APPENDIX D any other purpose or distributed to any other persons or parti es.

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






























390 2019 CALIFORNIA PLUMBING CODE

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
























2019 CALIFORNIA PLUMBING CODE 391

APPENDIX D any other purpose or distributed to any other persons or parti es.

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





















  • 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).

392 2019 CALIFORNIA PLUMBING CODE

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
12
12
12
12






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.

2019 CALIFORNIA PLUMBING CODE 393

any other purpose or distributed to any other persons or parti es.

394 2019 CALIFORNIA PLUMBING CODE

CALIFORNIA PLUMBING CODE – MATRIX ADOPTION TABLE

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▸Contents — California Plumbing Code (Title 24, Part 5)
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 STANDARD FOR PEX TUBING SYSTEMS FOR …
  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

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