10.1 Storm Drainage Basics: Disposal, Materials, Rainfall Rates & Roof Area

Storm drainage terms (IPC Chapter 2)

• Conductor: a pipe inside the building that carries storm water from the roof to a storm or combined building drain.
• Leader: an exterior drainage pipe that carries storm water from roof or gutter drains to an approved place of disposal.
• Building storm drain: the storm piping inside the building that extends 30 inches beyond the exterior walls. Building storm sewer: the piping from the end of the building storm drain to the public storm sewer or other point of disposal.
• Roof drain: a drain installed to receive water collecting on a roof and discharge it into a leader or conductor.
• Subsoil drain: a drain that collects subsurface or seepage water and carries it to a place of disposal.

General rules (IPC 1101 to 1105)

• 1101.2 disposal: rainwater from roofs and storm water from paved areas, yards and courts must drain to an approved place of disposal. For one- and two-family dwellings, and where approved, storm water may discharge onto flat areas such as streets or lawns if it flows away from the building. Virginia exception: rainwater nonpotable water systems are allowed under Chapter 13.
• 1101.3: storm water shall not be drained into sewers intended for sewage only.
• 1101.4: conductors and the building storm drain are tested per 312. Under 312.8, storm drains inside a building get the DWV water or air test.
• 1101.5: a storm pipe may not be reduced in size in the direction of flow.
• 1101.6: changes in direction use the drainage fittings of Table 706.3.
• 1101.7 roof design: roofs must be designed for the maximum possible depth of ponding water, assuming all primary roof drains are blocked. Virginia deleted the model text's last sentence.
• 1101.8: storm drain cleanouts follow the sanitary cleanout rules of 708, except for subsurface drainage.
• 1101.9: backwater valves follow the sanitary rules of 714.
• 1103.1: leaders and storm drains connected to a combined sewer must be trapped, either with an individual trap on each conductor's branch or with one trap in the main storm drain just before it connects to the combined sewer. Under 1103.2 to 1103.4, traps are the same material as the piping and the same size as the horizontal drain they serve, with an accessible cleanout on the building side. Storm drains connected to a building storm sewer need no trap.
• 1104.1: 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 shall not be connected to a storm drain.
• 1105.1: roof drains are installed per the manufacturer's instructions, and the roofing membrane must not obstruct the drain inlet. Virginia deleted 1105.2, the model rule that sized storm piping from the roof drain's published flow rate.

Materials (IPC 1102)

Rainfall rate (IPC 1106.1 and Appendix B)

Storm piping is sized on the 100-year hourly rainfall rate shown in Figures 1106.1(1) to 1106.1(5), which are National Weather Service maps, or on other rates from approved local weather data. A "100-year" storm has a 1% chance of being equaled or exceeded in any year.

Appendix B is informative, not part of the code, but it lists rates for selected cities:

For a locality not listed, use the figure or the rate the building official approves.

Projected roof area and vertical walls (IPC 1106.4)

Storm piping is sized on the roof's horizontally projected area, its plan-view footprint, not the sloped surface area. A 40-by-100-foot building has 4,000 square feet of projected roof area, whether the roof is flat or steep.

1106.4: In sizing roof drains and storm drainage piping, one-half of the area of any vertical wall that diverts rainwater to the roof shall be added to the projected roof area for inclusion in calculating the required size of vertical conductors, leaders and horizontal storm drainage piping.

The IPC rule is simply half of each wall that diverts water to the roof. It has no separate percentages for corners or for opposing walls, which some other codes use.

Worked example: a 50-by-80-foot low roof (4,000 sq ft) adjoins a two-story wall 50 feet long that rises 24 feet above it.

• Wall area: 50 × 24 = 1,200 sq ft. Half: 600 sq ft.
• Design area: 4,000 + 600 = 4,600 sq ft.
• With two roof drains sharing the area equally, each serves 2,300 sq ft.

Converting rainfall to flow

One inch of rain per hour on 1 square foot is 1/12 cubic foot per hour. At 7.48 gallons per cubic foot, that is 0.623 gallon per hour, or about 0.0104 gpm:

$$Q_{\text{gpm}} = 0.0104 \times A_{\text{sq ft}} \times R_{\text{in/hr}} \approx \frac{A \times R}{96}$$

The model 2021 IPC states this as Equation 11-1 in Section 1106.2.1. Virginia deleted 1106.2.1 along with the model's gpm tables, but the relationship still underlies Virginia's area tables, and Virginia 1114.1 uses the same constant: each gpm of continuous discharge equals 96 square feet of roof area at a rainfall rate of 1 inch per hour.

Example: a 36,000 sq ft roof in Norfolk (3.4 inches per hour): 0.0104 × 36,000 × 3.4 ≈ 1,273 gpm at the design storm.

Storm and sanitary are separate

• Inside a building, conductors and storm drains are separate from soil, waste and vent piping (1104.1).
• 1109.1: where the public sewer is a combined sanitary and storm system, the storm sewer connects to it independently. 703.6 says the same of the sanitary sewer. The two are not joined inside the building, and the 2021 IPC has no rule for converting storm flow into drainage fixture units to size a shared pipe.