10.4 Subsoil Drains, Building Subdrains, Storm Sumps, Controlled-Flow Roofs & Continuous Flow

Subsoil drains (IPC 1111.1)

Subsoil (foundation) drains relieve groundwater pressure against basement walls and slabs.

• Pipe: open-jointed, horizontally split or perforated pipe from Table 1102.5: cast iron, PE (for example ASTM F405), PVC (ASTM D2729, D3034, F891), stainless steel or vitrified clay.
• Size: at least 4 inches in diameter.
• Backwater: where the building is subject to backwater, the subsoil drain must be protected by an accessibly located backwater valve.
• Discharge: to a trapped area drain, a sump, a dry well, or an approved location above ground.
• The subsoil sump does not need a gastight cover or a vent. Clear groundwater does not produce sewer gas the way sewage does.
• The sump and pump follow 1113.1.

Gravel envelopes, filter fabric and pipe socks keep fine soil out of the perforations. The manufacturer and the designer specify them; the IPC does not set their dimensions.

Building subdrains (IPC 1112.1)

Building subdrains below the public sewer level discharge into a sump or receiving tank. Its contents are lifted automatically into the drainage system, in the same way as building sumps, and the equipment follows 1113.1. (For sewage, see Section 6.4 and IPC 712.)

Storm and subsoil sumps (IPC 1113.1)

Disposal: storm water and groundwater go to the storm system or another approved point of disposal. Under 1101.3, storm water may not drain into a sewer intended for sewage only.

Design practice: commercial storm sumps often use two pumps with an alternator and a high-water alarm, so one pump can fail without flooding. These are design choices, not IPC requirements.

Controlled-flow roof drain systems (IPC 1110)

A controlled-flow system stores rainwater on the roof temporarily and releases it at a limited rate, which reduces the peak flow into the storm piping and the public system.

• 1110.1: the roof must be designed to store water where the storm drainage is engineered for controlled flow. The system is an engineered system under Section 316.1 (design, submittal, approval, inspection and testing), based on the rainfall rate required by 1106.1.
• 1110.2: control devices must be installed so the rate of discharge does not exceed the design values.
• 1110.3: runoff control is by control devices, which must be protected by strainers.
• 1110.4: at least two roof drains are required for roof areas of 10,000 square feet or less, and at least four for roofs over 10,000 square feet.

Because water is stored on purpose, the structural design (IBC 1611) and the emergency overflow (1108) are especially important on these roofs.

Continuous flow into storm drains (Virginia 1114.1)

Where there is a continuous or semicontinuous discharge into the building storm drain or building storm sewer, such as from a pump, ejector, air conditioning plant, or similar device, each gallon per minute of such discharge shall be computed as being equivalent to 96 square feet of roof area, based on a rainfall rate of 1 inch per hour.

Because the Virginia tables give areas for each rainfall rate, convert the pump flow to area at the same rate you are using:

• At the 1-inch-per-hour column: add 96 square feet per gpm.
• At the 3-inch-per-hour column: add 96 ÷ 3 = 32 square feet per gpm.

Worked example: a 6,000 sq ft roof in a 3-inch-per-hour area drains to a horizontal storm drain at 1/8 inch per foot. A sump pump also discharges 25 gpm into that drain.

• Pump equivalent at 3 inches per hour: 25 × 32 = 800 sq ft.
• Total design area: 6,000 + 800 = 6,800 sq ft in the 3-inch-per-hour column.
• Virginia Table 1106.3 at 1/8 inch per foot and 3 inches per hour: 6-inch allows 7,133 sq ft, which works; 5-inch allows 4,453, which is too small. Use 6-inch.

The sanitary system has a different conversion: 1 gpm = 2 dfu (709.3). Do not mix the two.

Rainwater and graywater reuse (Virginia Chapter 13)

Virginia rewrote IPC Chapter 13 (VCC 2901.1.1, items 35 to 43) and added definitions for nonpotable water systems, nonpotable fixtures and outlets, and "rainwater" (natural precipitation, including snowmelt, from roof surfaces only). Related rules:

• 1101.2 exception: rainwater nonpotable water systems are permitted under Chapter 13.
• 602.2.1: nonpotable water may serve nonpotable fixtures and outlets, such as water closets, urinals, irrigation and hose connections for vehicle washing, under Chapter 13.
• 608.9: nonpotable piping and outlets are identified in purple and labeled "CAUTION: NONPOTABLE WATER — DO NOT DRINK."
• A potable water makeup to a rainwater cistern must be protected against backflow, for example by an air gap.