Under IPC 1108.2, where must a secondary (emergency overflow) roof drain system discharge?

Apart from the primary system, above grade where it will be seen. Section 1108.2 requires secondary roof drain systems to have an end point of discharge separate from the primary system, above grade and where occupants or maintenance staff would normally see it. Overflow then signals that the primary drains are blocked.

A secondary scupper replaces a roof drain that would need to be 4 inches for the same roof area. Under IPC 1108.3, what minimum opening is required?

At least 4 inches high and about 12.6 inches wide. Section 1108.3 requires scupper openings at least 4 inches high, with a width equal to the circumference of the roof drain required for the area served. For a 4-inch drain, that width is π × 4 ≈ 12.6 inches. Flow through the primary system is ignored when sizing the secondary system.

What does IPC 1111.1 require of subsoil drains?

Perforated or open-jointed pipe at least 4 inches, to an approved outlet. Section 1111.1 requires subsoil drains to be open-jointed, horizontally split or perforated pipe from Table 1102.5, at least 4 inches in diameter. They discharge to a trapped area drain, sump, dry well or approved location above ground, with an accessible backwater valve if the building is subject to backwater. The subsoil sump needs no gastight cover or vent.

Secondary Roof Drainage, Scuppers, Controlle | Free Guide 2026

Key Takeaways

Secondary (emergency overflow) roof drains or scuppers are required where the roof perimeter would trap water if the primary drains backed up (IPC 1108.1).
Secondary roof drain systems must have a discharge point separate from the primary system, above grade where building occupants or maintenance staff would normally see it (IPC 1108.2).
Secondary drains are sized under 1106 without counting any flow through the primary system, and scuppers must be at least 4 inches high and as wide as the circumference of the roof drain required for the same area (IPC 1108.3, 1106.5).
Controlled-flow roof drain systems must be engineered, with at least two roof drains on roofs of 10,000 square feet or less and at least four on larger roofs (IPC 1110.1, 1110.4).
Subsoil drains must be at least 4 inches, discharge to a trapped area drain, sump, dry well or approved location above ground, and have an accessible backwater valve where backwater is possible (IPC 1111.1).

Why this section matters

A blocked roof drain on a flat roof with parapets can collapse the roof under ponded water. The IPC therefore requires a separate secondary (emergency overflow) system wherever water could be trapped. This section also covers the groundwater side of storm drainage: subsoil drains, subdrains and storm sumps. All rules are from IPC Chapter 11, 2021 edition.

Roof ponding design (1101.7)

Roofs must be designed for the maximum possible depth of ponded water, assuming all primary drains are blocked. That depth includes the head of water over the secondary inlets needed to pass the design rainfall. The roof designer sets this under IBC Section 1611. The plumber's job is to set the secondary inlets at the elevations on the drawings and to keep the two systems separate.

Secondary (emergency) roof drains (1108)

Rule	Requirement
1108.1, when required	Where roof drains are required and the roof perimeter construction (parapets, walls) extends above the roof so that water would be trapped if the primary drains back up
1108.1, combined assemblies	Where primary and secondary drains are made as one assembly, each drain's inlet and outlet must be independent
1108.2, separate systems	The secondary system's end point of discharge must be separate from the primary system. It discharges above grade where occupants or maintenance staff would normally see it, so an overflow signals that the primary drains are blocked
1108.3, sizing	Sized per Section 1106. The flow through the primary system is not counted when sizing the secondary system

Typical arrangements:

Overflow roof drains: set with a dam or raised inlet about 2 inches above the primary drain inlet, as the design requires. They pipe separately to a visible outlet, such as a downspout nozzle on the wall.
Scuppers: openings through the parapet.

Scuppers (1106.5 and 1108.3)

Scuppers may serve as primary drainage, secondary drainage or both. Their number, size, location and inlet elevation must keep ponding below the depth the roof was designed for (IBC 1611.1 and IPC 1101.7).
A scupper opening must be at least 4 inches high, with a width at least equal to the circumference of the roof drain that would be required for the same roof area.
Do not count primary flow when locating and sizing secondary scuppers.

Example. If a 4-inch roof drain would be required for an area, a secondary scupper serving it must be at least 4 inches high and about 12.6 inches wide, since $3.1416 \times 4 \approx 12.6$ .

Siphonic and controlled-flow roofs (1107 and 1110)

Siphonic roof drain systems are engineered to ASME A112.6.9 and ASPE 45 (1107.1).
Controlled-flow roofs (1110) are designed to store water and release it slowly through flow-control devices. They must be engineered under 316.1 and designed for the 1106.1 rainfall rate.
- Control devices are protected by strainers (1110.3).
- Discharge may not exceed the design values (1110.2).
- At least two roof drains are required for roof areas of 10,000 square feet or less, and at least four on larger roofs (1110.4).

Subsoil drains (1111)

A subsoil drain collects groundwater around foundations. It is open-jointed, horizontally split or perforated pipe from Table 1102.5, at least 4 inches in diameter.
Where the building is subject to backwater, protect the subsoil drain with an accessible backwater valve.
Subsoil drains discharge to a trapped area drain, a sump, a dry well, or an approved location above ground.
A subsoil sump does not need a gastight cover or a vent. Its pump and pit follow 1113.1.

Building subdrains (1112)

Building subdrains below the public sewer level discharge into a sump or receiving tank. The contents are lifted automatically into the drainage system, following 1113.1 for storm-type sumps or 712 for sanitary sumps.

Storm sumps and pumps (1113.1)

Rule	Requirement
1113.1.1	Pump capacity and head suited to the expected use
1113.1.2	Pit at least 18 inches in diameter and 24 inches deep unless approved otherwise; accessible; all drainage enters by gravity; tile, steel, plastic, cast iron, concrete or other approved material; removable cover strong enough for expected loads; solid bottom supporting the pump
1113.1.3	Electrical outlets meet NFPA 70
1113.1.4	Discharge piping meets 1102.2–1102.4, with a gate valve and a full-flow check valve, sized no smaller than the pump discharge. In one- and two-family dwellings only a check valve is required

Compare this with sanitary sumps (712): those need a gastight cover and a vent, and pumps receiving water closets must pass 2-inch solids.

How storm and sanitary systems relate

Storm water never goes into a sanitary-only sewer (1101.3), and floor drains never go into a storm drain (1104.2).
On a combined public sewer, the sanitary and storm building sewers connect independently (703.6, 1109.1), and the storm leaders are trapped (1103.1).
Air-conditioning condensate is regulated by IPC 314. It may discharge to the receptors listed there, or outdoors where it does not cause a nuisance.

Exam traps

Secondary discharge must be separate and visible. Never tie it into the primary storm piping.
Scupper size: at least 4 inches high, and as wide as the circumference of the equivalent roof drain.
Ignore primary flow when sizing the secondary system.
Subsoil drains: at least 4 inches. Their sumps need no gastight cover or vent.

Alabama Journeyman Plumber

8.2 Secondary (Emergency) Roof Drainage, Scuppers, Controlled Flow, Subsoil Drains & Storm Sumps