8.1 Roof Drains, Rainfall Rates & Storm Drain Sizing (IPC 1101–1106)

Why this section matters

Roof Drain Systems carries 5 questions. The 2021 IPC changed storm sizing from roof-area tables to flow-rate (gpm) tables. Many older study aids still use the area tables, so learn the gpm method from Chapter 11 of your 2021 book.

Where storm water goes (1101, 1104, 1109)

• 1101.2: Rainwater from roofs and storm water from paved areas, yards and courts must drain to an approved place of disposal. One- and two-family dwellings may discharge onto flat areas such as lawns or streets where approved, as long as the water flows away from the building.
• 1101.3: Storm water may not drain into sewers intended for sewage only.
• 1101.4: Conductors and building storm drains are tested under 312, which for inside storm drains means 312.8.
• 1101.5: Storm drain size may not be reduced in the direction of flow.
• 1101.6: Use drainage-pattern fittings as in Table 706.3.
• 1101.8: Provide cleanouts as for sanitary drains, except on subsurface drains.
• 1101.9: Provide backwater valves as for sanitary drains (714).
• 1104.1: Conductors may not be used as soil, waste or vent pipes, and soil, waste and vent pipes may not be used as conductors.
• 1104.2: Floor drains may not connect to a storm drain.
• 1109.1: Where the public sewer is combined, the storm sewer connects to it independently of the sanitary sewer.

Materials (1102)

Traps on storm drains (1103)

• 1103.1: Leaders and storm drains that connect to a combined sewer must be trapped. Use either an individual trap on the storm branch serving each conductor, or a single trap in the main storm drain just before it joins the combined sewer. Storm drains connected to a separate storm sewer need no trap.
• Traps are the same material as the piping (1103.2), and an individual conductor's trap is the size of the horizontal drain it connects to (1103.3).
• Provide a cleanout on the building side of the trap, with access (1103.4).

Roof drains (1105)

• Install per the manufacturer. The roofing membrane may not obstruct the drain's inside opening (1105.1).
• 1105.2: Size the storm drainage using the drain's published flow rate, based on the head of water above it. The design flow is based on the maximum anticipated ponding at the drain.

Sizing step 1: the design flow (1106.1–1106.4)

• 1106.1: Base sizes on the 100-year, 1-hour rainfall rate from Figures 1106.1(1)–(5) or approved local weather data. Appendix B (informative) lists city values. For Alabama: Huntsville 3.6, Birmingham 3.8, Montgomery 4.2 and Mobile 4.6 inches per hour.
• 1106.2.1, Equation 11-1:

$$Q\ (\text{gpm}) = 0.0104 \times R\ (\text{in/h}) \times A\ (\text{sq ft})$$

The factor comes from converting 1 inch per hour falling on 1 square foot into gallons per minute.

• 1106.4: Add one-half the area of any vertical wall that diverts rain onto the roof to the projected roof area.
• 1106.2: Check the calculated flow against the roof drain manufacturer's published rating for the model and size.

Sizing step 2: pipe, leaders and gutters

Table 1106.2, storm drain piping (capacity in gpm):

Table 1106.3, vertical leaders (sheet-metal downspouts): 2-inch (or 2 × 2) = 30 gpm; 2-1/2-inch = 54; 3-inch (or 2 × 4) = 92; 4-inch (or 3 × 4-1/4) = 192; 5-inch = 360; 6-inch = 563; 8-inch = 1,208.

Table 1106.6, horizontal gutters (selected): a 5-inch semicircular gutter at 1/8 in/ft = 74 gpm; a 6-inch semicircular gutter at 1/8 in/ft = 110 gpm; a 3 × 5-inch rectangular gutter at 1/4 in/ft = 157 gpm.

Worked example: Montgomery commercial roof

A flat roof of 12,000 square feet drains to three roof drains of equal area. A 20-foot-high, 60-foot-long wall of an adjoining taller building sheds rain onto the roof.

1. Wall area to add: $\tfrac{1}{2} \times 20 \times 60 = 600$ square feet. Total = 12,600 square feet.
2. Rate: Montgomery is 4.2 inches per hour.
3. Total flow: $0.0104 \times 4.2 \times 12{,}600 \approx 550$ gpm. Each drain serves about $550 \div 3 \approx 183$ gpm.
4. Vertical conductor for one drain: a 4-inch vertical drain carries 180 gpm, just short of 183, so use a 5-inch conductor (311 gpm).
5. Horizontal storm drain at 1/8 in/ft carrying all three drains (550 gpm): 6-inch = 344, too small. 8-inch = 714, OK.
6. Building storm sewer at 1/4 in/ft (550 gpm): 6-inch = 487, too small. 8-inch = 1,010, OK.
7. Confirm each roof drain's published flow rating covers 183 gpm at the design ponding depth.

Siphonic and controlled-flow systems

• 1107.1: Siphonic roof drain systems are engineered under ASME A112.6.9 and ASPE 45.
• 1110: Controlled-flow systems store water on the roof on purpose and must be engineered (see section 8.2).

Exam traps

• gpm, not square feet: the 2021 tables are flow-based. Convert the roof area with Equation 11-1 first.
• 100-year, 1-hour rate: for example, 4.6 in/h for Mobile.
• Add half the vertical wall area.
• Combined sewer: storm leaders need traps, and storm and sanitary sewers connect to it independently.