11.3 Secondary (Emergency) Roof Drainage & Subsoil Systems

Secondary (Emergency) Roof Drainage & Subsoil Systems

Secondary (emergency overflow) roof drainage and subsoil systems address two structural hazards: a roof collapsing from ponded water, and a foundation failing from groundwater pressure. IPC Sections 1101.7 and 1108 govern overflow relief, and IPC Sections 1111 through 1113 govern subsoil drains, building subdrains and sumps. IBC Section 1502 sends roof drainage design to the IPC, and IBC Section 1805.4 covers foundation drainage. The plans examiner checks the overflow sizing, the separate observable discharge, and the sump and valve details.

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Why Secondary Drainage Is Required (IPC 1101.7 and 1108.1)

Primary roof drains clog. Leaves, plastic bags, construction debris, gravel ballast and bird nests can block strainers during the same storm that delivers the design rainfall.

Ponding Instability

Water is heavy: 1 cubic foot weighs 62.4 pounds, which is 5.2 pounds per square foot (psf) per inch of depth. Consider a $25{,}000\text{ sq ft}$ flat roof inside a 12-inch parapet. If the primary drains clog and 6 inches of water collects:

$W = 25{,}000\text{ sq ft} \times 6\text{ in} \times 5.2\text{ psf/in} = 780{,}000\text{ pounds} \approx 390\text{ tons}$

The load makes the framing deflect. The sag forms a deeper depression that collects more water, which causes more deflection. Engineers call this cycle ponding instability, and without overflow relief it can end in collapse.

Roof Design Depth (IPC 1101.7)

Roofs must be designed for the maximum possible depth of water that can pond on them, set by the relative levels of the roof deck and the overflow weirs, scuppers, edges or serviceable drains, combined with the deflected structure. In this check, all primary roof drainage is assumed to be blocked, and the depth includes the head of water needed above the secondary inlet to pass the design flow.

The Code Trigger (IPC 1108.1)

Where roof drains are required, secondary roof drains or scuppers must be provided where the roof perimeter construction extends above the roof so that water will be trapped if the primary drains allow buildup. Parapets are the usual trigger. Where primary and secondary drains are made as a single assembly, the inlet and outlet of each drain must be independent.

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Separate Discharge and Full Sizing (IPC 1108.2 and 1108.3)

Two principles govern secondary drainage design.

1. Separate, Observable Discharge (IPC 1108.2)

Secondary roof drain systems must have an end point of discharge separate from the primary system. Discharge must be above grade, in a location that would normally be observed by the building occupants or maintenance personnel. Tying secondary conductors into the primary storm drain defeats both requirements: the shared outlet is not separate, and a blockage downstream would disable both systems at once.

┌────────────────────────────────────────────────────────────────────────┐
│      PRIMARY & SECONDARY ROOF DRAINAGE (IPC SECTIONS 1108.2–1108.3)     │
├────────────────────────────────────┬───────────────────────────────────┤
│        PRIMARY DRAINAGE SYSTEM     │    SECONDARY (EMERGENCY) SYSTEM   │
├────────────────────────────────────┼───────────────────────────────────┤
│ • Drain inlet at the roof low point│ • Inlet set above the primary     │
│ • Operates in every rain event     │ • Operates ONLY when water rises  │
│                                    │   above the primary drains        │
│ • Sized per Section 1106           │ • Sized per Section 1106 at the   │
│                                    │   same rainfall rate, with no     │
│                                    │   credit for the primary system   │
│ • Discharges to the storm sewer or │ • Discharges ABOVE GRADE where    │
│   other approved disposal          │   occupants or staff would see it │
├────────────────────────────────────┴───────────────────────────────────┤
│ The secondary system's end point of discharge must be separate from the│
│ primary system (IPC 1108.2).                                           │
└────────────────────────────────────────────────────────────────────────┘

2. Full Sizing Without Primary Credit (IPC 1108.3)

Secondary roof drain systems are sized under Section 1106 using the same rainfall rate as the primary system (the 100-year hourly rate of 1106.1). The flow through the primary system is not considered, because the primary drains are assumed to be blocked.

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Secondary Roof Drains vs. Overflow Scuppers (IPC 1108.3)

Designers can meet the requirement with secondary roof drains or with scuppers through the parapet.

                    SECONDARY OVERFLOW CONFIGURATIONS

    OPTION A: SECONDARY ROOF DRAIN          OPTION B: PARAPET SCUPPER

    Inlet raised by a water dam             Opening at least 4" high;
    above the primary drain inlet           width = circumference of the
          ┌───┐                             roof drain required for the area
      ┌───┤   ├───┐                         ┌─────────────────┐
      │   └───┘   │                         │  OPEN SCUPPER   │
  ════╪═══════════╪══════ Roof Deck         └────────┬────────┘
      │           │                                  │
      ▼           ▼                                  ▼
  [Separate secondary piping,              ═══════════════════ Roof Deck
   discharging above grade]

Comparison: Secondary Drains vs. Scuppers

Worked check: A roof area needs a 4-inch roof drain. A scupper serving the same area must be at least 4 inches high and $\pi \times 4 \approx 12.6$ inches wide.

The IPC does not set a fixed height for the secondary inlet. The structural engineer coordinates that elevation, plus the head of water over the inlet, with the ponding load of 1101.7.

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Reviewing the Secondary Discharge Location

The secondary discharge point is one of the most common plan review failures.

The "Alarm" Principle

The secondary system works as a visual alarm. When maintenance staff see water pouring from a wall nozzle or scupper, they know the primary drains are blocked and can clear them before the roof is overloaded.

Terminations That Fail IPC 1108.2

• Discharging into an underground storm sewer, manhole or detention vault, which is not above grade and not observable.
• Connecting to the primary conductors or building storm drain, which removes the separate end point.
• Discharging into a crawlspace, basement or interior sump.

Examiners should also question an outlet that would spill onto a doorway, exit path or electrical equipment, even though the IPC does not list those locations.

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Subsoil Drains (IPC Section 1111.1)

Subsoil drains collect groundwater around foundations and below slabs, relieving pressure against walls and floors.

            SUBSOIL FOUNDATION DRAIN DETAIL (IPC 1111.1 + IBC 1805.4.2)

                     Finished Grade
             ───────────────────────────────
             Backfill
             ───────────────────────────────
             Approved filter membrane over the drain (IBC 1805.4.2)
             ┌─────────────────────────────────────────────────┐
             │ Gravel / crushed stone, at least 6" over the pipe│
             │   ┌─────────────────┐                           │
             │   │ 4" MIN PERFORATED│  (IPC 1111.1)             │
             │   │ OR OPEN-JOINTED  │                           │
             │   └─────────────────┘                           │
             │ Gravel / crushed stone, at least 2" under pipe  │
             └─────────────────────────────────────────────────┘
             Foundation footing

IPC 1111.1 requires:

1. Pipe type and size: Open-jointed, horizontally split or perforated pipe, not less than 4 inches in diameter.
2. Materials (Table 1102.5): Cast iron (ASTM A74, A888, CISPI 301), polyethylene (ASTM F667 and CSA B182 series), PVC sewer pipe (ASTM D2729, D3034, F891), Type 316L stainless steel (ASME A112.3.1) or vitrified clay (ASTM C4, C700).
3. Backwater protection: Where the building is subject to backwater, the subsoil drain is protected by an accessibly located backwater valve.
4. Discharge: To a trapped area drain, sump, dry well or approved location above ground.
5. Sump relief: A subsoil sump is not required to have a gastight cover or a vent, but the sump and pumping system must comply with Section 1113.1.

Foundation drain bedding (IBC 1805.4.2): Where perforated pipe or drain tile is used in a foundation drain, the pipe rests on at least 2 inches of gravel or crushed stone and is covered with at least 6 inches of the same material. The top of the joints or perforations and the top of the drain are protected with an approved filter membrane. These bedding dimensions come from the building code, not from IPC 1111.

Disposal limit (IPC 1101.3): Storm water must not drain into sewers intended for sewage only.

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Building Subdrains and Sumps (IPC 1112 and 1113)

Building subdrains (1112.1): A building subdrain below the level of the public sewer discharges into a sump or receiving tank. Its contents are automatically lifted into the drainage system, using sump and pumping equipment that complies with 1113.1.

Sump Pit (IPC 1113.1.2)

• Size: At least 18 inches in diameter and 24 inches deep, unless otherwise approved.
• Gravity inflow: The pit has access and is located so that all drainage flows into it by gravity.
• Construction: Tile, steel, plastic, cast iron, concrete or other approved material, with a removable cover strong enough for the loads in that area. The pit floor is solid and permanently supports the pump.

Pump and Discharge (IPC 1113.1.1, 1113.1.3 and 1113.1.4)

• Capacity: The pump has a capacity and head appropriate to the anticipated use. The IPC sets no fixed gpm minimum and does not require a duplex pump.
• Electrical: Outlets, where required, meet NFPA 70.
• Discharge piping: Materials per 1102.2, 1102.3 or 1102.4, with a gate valve and a full-flow check valve. Pipe and fittings are at least the size of the pump discharge tapping.
• Dwelling exception: In one- and two-family dwellings, only a check valve is required on the pump discharge.

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Plan Reviewer Verification Checklist: Secondary & Subsoil Drainage

• Secondary Drainage Provided: Every roof where perimeter construction could trap water has secondary drains or scuppers (1108.1).
• Single Assemblies: Combined primary and secondary drain assemblies have independent inlets and outlets (1108.1).
• Full Capacity: Secondary drains are sized per 1106 at the primary rainfall rate, with no credit for primary flow (1108.3).
• Separate, Observable Discharge: The secondary end point is separate from the primary system and above grade where it would normally be seen (1108.2).
• Scupper Geometry: Scuppers are at least 4 inches high and as wide as the circumference of the required roof drain (1108.3).
• Structural Coordination: The roof is designed for the maximum ponding depth with the primary drains blocked (1101.7).
• Subsoil Drains: Pipe is at least 4 inches and from Table 1102.5, with a backwater valve where the building is subject to backwater (1111.1).
• Sumps: The pit is at least 18 by 24 inches, and the discharge has a gate valve and full-flow check valve, or only a check valve for one- and two-family dwellings (1113.1).