2021 IPC Chapters 11, 12 and 13
Rain is the largest flow a commercial building ever has to shed, and it arrives all at once. Chapter 11 of the 2021 IPC turns a rainfall rate and a roof area into gallons per minute, then sizes the drains, leaders, and piping that carry it away. It also makes a second, independent path for the day the first one clogs. This lesson teaches that method with a full worked example, then gives short pointers to Chapter 12 (medical gas and oxygen) and Chapter 13 (nonpotable water). Remember the local note from Lesson 1: Houston enforces the Uniform Plumbing Code with amendments, so check which code your city has adopted.
Roof and paved-area water must go to an approved place of disposal (1101.2). Only one- and two-family dwellings, or a site where the official approves it, may discharge onto a lawn or street, and only if the water flows away from the building. Storm water may never go into a sewer meant for sewage only (1101.3), and a floor drain may never be connected to a storm drain (1104.2). Conductors are not soil, waste, or vent pipes, and the reverse (1104.1). Storm piping is never reduced in size in the direction of flow (1101.5), needs cleanouts like sanitary piping (1101.8), and gets backwater valves where Section 714 would require them (1101.9). A trap is required on a leader only when it connects to a combined sewer, and a building storm sewer needs none (1103.1).
Materials follow Chapter 7: Table 702.1 for above-ground conductors, Table 702.2 for the underground building storm drain, and Table 1102.4 for the building storm sewer. Roof drains, other than siphonic ones, must be tested and rated (1102.6). Install them to the manufacturer's instructions, and do not let the roofing membrane block the drain's inside opening (1105.1).
Size everything from the 100-year, one-hour rainfall rate, read from the maps in Figures 1106.1(1) through 1106.1(5), or from other rates in approved local weather data (1106.1). This course does not quote a rate as a code number. Treat it as a given design value for your site, and always look it up. The steps are the same every time:
The constant 96.2 is only a unit conversion: one inch of rain on one square foot is 144 cubic inches, a gallon is 231, so that is 0.623 gallon per hour, or 0.0104 gpm, and 1 / 0.0104 is about 96.
Table 1106.3 gives the capacity of a round vertical leader. Table 1106.2 gives the capacity of storm drain piping, vertical and at four slopes. The rows below are the ones this lesson uses, rebuilt in our own layout.
| Pipe size | Leader, Table 1106.3 (gpm) | Vertical drain (gpm) | Horizontal 1/16 in per ft | Horizontal 1/8 in per ft | Horizontal 1/4 in per ft |
|---|---|---|---|---|---|
| 3 in | 92 | 87 | 39 | 55 | 79 |
| 4 in | 192 | 180 | 81 | 115 | 163 |
| 5 in | 360 | 311 | 117 | 165 | 234 |
| 6 in | 563 | 538 | 243 | 344 | 487 |
| 8 in | 1,208 | 1,117 | 505 | 714 | 1,010 |
Horizontal pipe carries much less than vertical pipe, and a flatter slope carries less again. Roof gutters are sized from Table 1106.6, which lists semicircular and rectangular gutters by slope. For example, a 6 inch semicircular gutter at 1/8 inch per foot carries 110 gpm and an 8 inch one carries 247 gpm.
If a roof edge or parapet would trap water when the primary drains clog, a secondary drain or scupper is required (1108.1). Think of it as a spare that does not depend on the first. It needs its own end point of discharge, above grade, in a place occupants or maintenance staff will notice (1108.2). That visible discharge is the warning that a primary drain is blocked. Where primary and secondary drains come as one assembly, their inlets and outlets must stay independent.
The secondary system is sized under Section 1106 for the same rainfall rate as the primary, and the primary flow is ignored (1108.3). The structure must also carry the water that would pond. Section 1101.7 has the roof designed for the deepest ponding, with every primary drain blocked, plus the head needed above the secondary inlet to pass the flow. The 2021 IBC (Section 1611.1) converts that depth to a load. Each inch of water on a roof weighs about 5.2 pounds per square foot, so 4 inches is about 21 psf, before the roof deflects and holds more.
Worked example. A one-story Group B clinic has a flat roof of 60 by 100 feet (6,000 sq ft) with a stair tower wall, 40 feet long and 10 feet tall, that sheds onto it. Area: half of the wall area is 0.5 x 40 x 10 = 200 sq ft, so A = 6,200 sq ft. Rate: the local design value is given as 5 in/hr. Flow: 5 x 6,200 / 96.2 = 322 gpm. With two primary drains, each takes 3,100 sq ft, or 161 gpm, which must be within each drain's published rating at the design ponding. Leaders: 161 gpm is over the 3 inch limit (92, or 87 vertical) and under the 4 inch limits (192 and 180), so each leader is 4 inches. Horizontal storm drain: both leaders join, so 322 gpm. At 1/8 inch per foot a 6 inch pipe carries 344, which works; at 1/16 inch per foot it carries only 243, so that slope needs 8 inch. Secondary: sized again at 5 in/hr for the same areas, so 161 gpm per scupper. Each opening is at least 4 inches high and at least 12.6 inches wide (pi x 4 inch, taking a 4 inch drain as the one that serves that area), and larger if needed to pass the flow without exceeding the design ponding depth (1106.5).
A controlled flow system holds water on the roof and meters it out. The roof is designed for the stored water, the system is an engineered design submitted under Section 316.1, the control devices are protected by strainers, and a roof of 10,000 square feet or less needs at least two drains, more than that at least four (1110). Siphonic systems run full and follow ASME A112.6.9 and ASPE 45 (1107). Subsoil drains around foundations are at least 4 inches in diameter, open-jointed, split, or perforated, and get a backwater valve where the building is subject to backwater (1111). A subdrain below the public sewer level discharges to a sump (1112). The sump pit is at least 18 inches across and 24 inches deep unless approved, and the discharge line carries a gate valve and a full-flow check valve, with only a check valve allowed for one- and two-family dwellings (1113). Storm systems inside a building are tested with water or air (312.8; see Lesson 14).
The design is a short, intense downpour, not annual rainfall, so Gulf Coast rates are often higher than in drier parts of the country. A leader that looks fine on a drawing from another climate may be too small here. Flat roofs with parapets are common on small commercial and community buildings, so secondary drainage matters, and low, flat ground raises backwater risk (1101.9 and 1111). Confirm the rate on the plans against the approved local source, and confirm which plumbing code the city has adopted.
In the field
A rehab crew reroofs a flat community hall that has parapets on four sides. The new tapered insulation raises the roof surface, and the membrane laps over the old scupper. Now the scupper opening is only 2 inches tall and its invert sits at the same height as the primary drain rim. Ask where the water goes if the drain clogs. Check that the scupper opening is at least 4 inches high, that its invert is above the primary inlet, that the membrane does not cover a drain opening, and that the secondary discharge lands where someone will see it.
Chapter 12 is short. It covers nonflammable medical gas piping and nonmedical oxygen piping, with maintenance and operation left to the fire code (1201.1). Medical gas, inhalation anesthetic, and vacuum piping are installed, tested, and labeled to NFPA 99, not to portable systems or cylinder storage (1202.1). Nonmedical oxygen follows NFPA 51 and NFPA 55 (1203.1). Read the adopted edition of NFPA 99 before inspecting a clinic system.
Chapter 13 covers collection, storage, treatment, and distribution of nonpotable water, including rainwater harvesting (Section 1303) and reclaimed water from a treatment facility (Section 1304). The system needs a permit with construction documents and calculations (1301.4). Water for toilet and urinal flushing is filtered to 100 microns or finer (1301.2.2), chlorine is limited to 4 ppm (1301.2.1), and every outlet carries a sign reading that the water is nonpotable and not for drinking, with letters at least 0.5 inch high and a pictograph (1301.3). Any connection to the potable supply is protected against backflow under Section 608 (1301.5). Harvested rainwater comes from above-ground impervious roofs (1303.2), passes a debris excluder (1303.3) and an automatic first-flush diverter (1303.4), and its gutters slope at least 1/8 inch per foot (1303.5.1).
1. A Group B office building has a flat roof enclosed by parapets on all four sides, with two roof drains. Under Section 1108.1, what else must the roof have?
2. A Group B roof of 4,800 square feet drains to one roof drain. The local design rainfall rate is given as 5 inches per hour. Using Q = R x A / 96.2, the design flow is about:
3. The single roof drain in question 2 passes 250 gpm to a round vertical leader. Using Table 1106.3 (3 inch, 92 gpm; 4 inch, 192 gpm; 5 inch, 360 gpm; 6 inch, 563 gpm), the smallest leader is:
4. A Group M store's horizontal building storm drain must carry 300 gpm and is set at 1/4 inch per foot. Using Table 1106.2 (4 inch, 163 gpm; 5 inch, 234 gpm; 6 inch, 487 gpm; 8 inch, 1,010 gpm at that slope), the smallest pipe is:
5. During a reroof inspection of a Group A-3 community hall, an inspector finds a new parapet scupper with a 3 inch tall opening, wide enough for the flow. Under Sections 1106.5 and 1108.3, this scupper is:
6. A parapet scupper serves a roof area for which a 6 inch roof drain would be required. Under Section 1106.5, its minimum width is closest to:
7. A 2,000 square foot flat roof on a Group B building is bounded on one side by a taller wall, 30 feet long and 20 feet high, that sheds rain onto it. Under Section 1106.4, the area used for sizing is:
8. A contractor renovating a Group B building proposes to connect a new roof leader to an existing building sewer that carries sanitary sewage only. Under Section 1101.3, this connection is:
9. A designer of a Group S-1 warehouse wants to save pipe by connecting a mechanical room floor drain to the roof leader's storm drain. Under Section 1104.2, this is:
10. An engineered controlled flow roof drain system serves a 14,000 square foot roof on a Group F-1 building. Under Section 1110.4, the minimum number of roof drains is:
11. A Group B building has a building subdrain below the public sewer level that discharges to a sump with a pump. Unless otherwise approved, the sump pit under Section 1113.1.2 is at least:
12. A school (Group E) plans a nonpotable rainwater system to flush water closets. Under Section 1301.2.2, the flushing water must be filtered by:
13. An inspector finds a hose bibb on a rainwater system at a Group B building tagged with a small label in 1/4 inch letters and no pictograph. Under Section 1301.3, this is:
14. Section 1202.1 says nonflammable medical gas, inhalation anesthetic, and vacuum piping in a clinic must be installed, tested, and labeled in accordance with which document?