16.1 Storm Drainage Scope, Disposal, and Prohibited Connections
Key Takeaways
- IPC 1101.3 prohibits storm water from being drained into sewers intended for sewage only.
- IPC 1104.1 prohibits conductor pipes from being used as soil, waste or vent pipes, and prohibits soil, waste or vent pipes from being used as conductors.
- IPC 1104.2 prohibits floor drains from being connected to a storm drain.
- IPC 1101.8 requires cleanouts in the storm drainage system complying with the sanitary drainage cleanout provisions, excepting subsurface drainage systems.
- IPC 1101.9 requires storm drainage systems to be provided with backwater valves as required for sanitary drainage systems in accordance with Section 714.
Ten items hiding in plain sight
The PSI content outline gives Storm Drainage Systems 10 scored items on the Journeyworker exam — tied for third largest behind Sanitary Drainage (20), Vents (18) and Water Supply (15), and five times the weight of Residential Fire Sprinklers. It is also the chapter that candidates most often skip, because most day-to-day residential work never touches it.
IPC 1101.1: the provisions of Chapter 11 govern the materials, design, construction and installation of storm drainage.
Disposal — IPC 1101.2
Rainwater from roofs and storm water from paved areas, yards, courts and courtyards shall drain to an approved place of disposal. For one- and two-family dwellings, and where approved, storm water is permitted to discharge onto flat areas, such as streets or lawns, provided that the storm water flows away from the building.
Two regimes: commercial and multifamily storm water goes to an approved place of disposal; one- and two-family dwellings may discharge to grade where approved, provided the water moves away from the structure.
Prohibited drainage — IPC 1101.3
Storm water shall not be drained into sewers intended for sewage only.
This is the foundational separation rule, and it is mirrored on the sanitary side by IPC 703.3 (sanitary and storm sewers shall be separate where the public system is separate) and completed by IPC 703.6 and 1109.1: where the public sewer is a combined system, the storm sewer shall be connected independently to the public sewer.
Colorado field note (not exam-scored): Board Rule 1.2(E)(1)(xx) adds a sentence to 1101.3: storm water from roof drains shall not discharge over public walkways except where an approved grated trough or trench drain sized to accept the calculated discharge is installed in the walkway, with the discharge diverted vertically from a height not greater than 12 inches to the grate. Secondary drains are excepted. This is a real winter-safety rule in a state where discharge over a sidewalk becomes ice.
Conductors, leaders and floor drains — IPC 1104
| Rule | Requirement |
|---|---|
| 1104.1 Prohibited use | Conductor pipes shall not be used as soil, waste or vent pipes, and soil, waste or vent pipes shall not be used as conductors |
| 1104.2 Floor drains | Floor drains shall not be connected to a storm drain |
1104.2 is absolute in the model code. A floor drain carries whatever hits the floor — mop water, spills, oils — and belongs on the sanitary system. The only related allowances elsewhere in the code are IPC 1002.1 Exception 3, which permits floor drains in multilevel parking structures discharging to a building storm sewer to go untrapped (or to connect to a main trap where they discharge to a combined sewer), and IPC 301.6, which permits indirectly connected floor drains, sumps and pumps at the base of an elevator shaft.
The shared rules with sanitary drainage
Chapter 11 borrows heavily from Chapter 7, and the cross-references are exam material.
| Storm rule | Points to | Content |
|---|---|---|
| 1101.4 Tests | Section 312 | Conductors and the building storm drain are tested per 312 — storm drains within a building are tested by water or air per 312.2 or 312.3 (312.8) |
| 1101.5 Change in size | — | The size of a drainage pipe shall not be reduced in the direction of flow (same as 704.2) |
| 1101.6 Fittings and connections | Table 706.3 | Connections and changes in direction use approved drainage-type fittings per Table 706.3; fittings shall not obstruct or retard flow |
| 1101.8 Cleanouts required | Section 708 | Cleanouts in the storm drainage system comply with the sanitary drainage cleanout provisions. Exception: subsurface drainage system |
| 1101.9 Backwater valves | Section 714 | Storm drainage systems shall be provided with backwater valves as required for sanitary drainage systems |
| 1102.1 Materials | Chapter 7 | Materials and methods comply with Section 1102 and the applicable provisions of Chapter 7 |
1101.7 Roof design [BS]: roofs shall be designed for the maximum possible depth of water that will pond, determined by the relative levels of roof deck and overflow weirs, scuppers, edges or serviceable drains combined with deflected structural elements. In determining the maximum possible depth of water, all primary roof drainage means shall be assumed to be blocked. The maximum depth includes the height of water required above the inlet of the secondary drainage means to achieve the required flow rate for the design rainfall rate of Section 1106.
That assumption — primary drains blocked — is why secondary drainage exists and why it is sized independently.
Traps on storm drainage — IPC 1103
A rule that surprises people: storm drains are usually untrapped, but not always.
1103.1 Main trap: Leaders and storm drains connected to a combined sewer shall be trapped. Individual storm water traps shall be installed on the storm water drain branch serving each conductor, or a single trap shall be installed in the main storm drain just before its connection with the combined building sewer or the public sewer. Leaders and storm drains connected to a building storm sewer shall not be required to be trapped.
| Connected to | Trap required? |
|---|---|
| A combined sewer (sanitary and storm together) | Yes — individually per conductor, or one main trap just before the combined sewer connection |
| A building storm sewer (separate storm system) | No |
1103.2 Material: storm water traps shall be of the same material as the piping system to which they are attached.
1103.3 Size: traps for individual conductors shall be the same size as the horizontal drain to which they are connected.
1103.4 Cleanout: a cleanout shall be installed on the building side of the trap and shall be provided with access.
The logic: a combined sewer carries sewage, so a trap is needed to block gas from entering the building through an open roof drain. A dedicated storm sewer carries only rainwater, and the open conductors actually help ventilate it.
Storm drainage and sanitary drainage side by side
| Topic | Sanitary (Chapter 7) | Storm (Chapter 11) |
|---|---|---|
| Sizing basis | Drainage fixture units | Flow rate in gpm from rainfall x area |
| Sizing tables | 710.1(1), 710.1(2) | 1106.2, 1106.3, 1106.6 |
| Slope | Table 704.1 | Table 1106.2 tabulates capacity at four slopes |
| Traps | Every fixture (1002.1) | Only where connected to a combined sewer (1103.1) |
| Cleanouts | Section 708 | Same provisions (1101.8), except subsurface systems |
| Backwater valves | Section 714 | Same requirement (1101.9) |
| Reduction in direction of flow | Prohibited (704.2) | Prohibited (1101.5) |
| Fittings | Table 706.3 | Table 706.3 (1101.6) |
| Tests | Section 312 | Section 312 (1101.4, 312.8) |
| Sumps | 712 — gastight cover, vented | 1113 — cover need not be gastight, subsoil sump needs no vent (1111.1) |
The single biggest conceptual difference: storm drainage is sized on flow, not on fixture units. There is no such thing as a storm drainage fixture unit in the 2021 IPC.
What is covered where in this guide
| Section | Topic |
|---|---|
| 16.1 | Scope, disposal, prohibited connections, traps |
| 16.2 | Materials, conductors, roof drains |
| 16.3 | Secondary (emergency) roof drainage and controlled flow |
| 16.4 | Rainfall rates and the flow-rate conversion |
| 16.5 | Sizing conductors, leaders and storm drains |
| 16.6 | Gutters, scuppers, subsoil drains, subdrains and sumps |
May a floor drain be connected to a storm drain under IPC 1104.2?
A building's leaders and storm drains connect to a combined public sewer. What does IPC 1103.1 require?
Under IPC 1101.7, what assumption governs when determining the maximum possible depth of ponded water on a roof?
Which requirement does IPC Chapter 11 import unchanged from the sanitary drainage chapter?