10.3 Secondary (Emergency) Roof Drains, Scuppers & Roof Ponding
Key Takeaways
Secondary (emergency overflow) roof drains or scuppers are required where the roof perimeter construction extends above the roof so that water would be trapped if the primary drains allow buildup (IPC 1108.1).
A secondary roof drain system must have an end point of discharge separate from the primary system, above grade, where building occupants or maintenance staff would normally see it (1108.2).
Virginia's 1108.3 sizes secondary drains under 1106, using the same rainfall rate as the primary system, without counting any flow through the primary system.
Scuppers must be at least 4 inches high, with a width at least equal to the circumference of the roof drain required for the area served (1106.5 and 1108.3).
Each inch of ponded water weighs about 5.2 pounds per square foot, and roofs must be designed for the maximum possible ponding depth with all primary drains blocked (1101.7).
Why secondary drainage matters
Water weighs 62.4 pounds per cubic foot, so each inch of depth adds 5.2 pounds per square foot (62.4 ÷ 12).
| Depth of ponding | Load | Load on a 10,000 sq ft roof |
|---|---|---|
| 1 inch | 5.2 psf | 52,000 lb |
| 2 inches | 10.4 psf | 104,000 lb |
| 4 inches | 20.8 psf | 208,000 lb |
| 6 inches | 31.2 psf | 312,000 lb |
A roof that deflects under water forms a bowl, which collects more water and deflects further. This progressive failure is called ponding instability. The IBC and IPC handle it with two linked requirements: design the roof for the worst ponding, and provide a second way for water to leave.
- 1101.7 roof design: roofs must be designed for the maximum possible depth of water that will pond on them, set by the relative levels of the roof deck and the overflow weirs, scuppers, edges or drains, together with structural deflection. All primary roof drainage is assumed to be blocked. Virginia deleted the model's last sentence. The structural design itself follows IBC Section 1611.
When secondary drainage is required (IPC 1108.1)
Where roof drains are required, secondary (emergency overflow) roof drains or scuppers must be provided where the roof perimeter construction extends above the roof such that water would be entrapped if the primary drains allowed water to build up. Parapets are the most common case. Where primary and secondary drains are made as a single assembly, the inlet and outlet for each drain must be independent.
Separate systems and visible discharge (IPC 1108.2)
Secondary roof drain systems shall have the end point of discharge separate from the primary system. Discharge shall be above grade, in a location that would normally be observed by the building occupants or maintenance personnel.
- The secondary system is a separate system. Its piping does not join the primary conductors or the building storm drain, because a backup in the primary system would then disable the overflow too.
- Water coming out of an overflow nozzle or scupper is a warning that the primary drains are blocked. A visible location means someone sees it and clears the drains.
- Typical outlets are an overflow nozzle or downspout boot on an exterior wall, or a scupper through the parapet.
Sizing (IPC 1108.3, as amended, and 1106.5)
- Virginia 1108.3: secondary roof drain systems are sized in accordance with Section 1106, based on the rainfall rate for which the primary system is sized. Use the same Virginia area tables (1106.2(1), 1106.2(2) and 1106.3).
- The flow through the primary system shall not be considered when sizing the secondary system. Each system must handle the design storm alone.
- Scuppers are sized so the ponding depth does not exceed the depth the roof was designed for (1101.7). They need an opening at least 4 inches high, with a width equal to the circumference of the roof drain required for the area served.
- 1106.5 parapet wall scuppers: where scuppers are used for primary drainage, secondary drainage or both, their quantity, size, location and inlet elevation must keep ponding from exceeding the roof's design depth under IBC 1611.1. The same minimum dimensions apply: at least 4 inches high, and a width at least equal to the circumference of a roof drain sized for the same roof area. Primary flow is not counted when sizing secondary scuppers.
The IPC does not set a fixed inlet height for secondary drains or scuppers. The inlet elevation comes from the structural design so that the water depth stays within what the roof can carry.
Scupper sizing example
A roof area requires a 4-inch roof drain.
- Circumference of a 4-inch drain: π × 4 ≈ 12.6 inches.
- The scupper must be at least 12.6 inches wide and at least 4 inches high. A 4-by-4-inch scupper would be too narrow.
For a 6-inch drain: π × 6 ≈ 18.8 inches wide, still at least 4 inches high.
Estimating scupper flow
Engineers often estimate scupper or weir capacity with the Francis weir formula, Q = 3.33 × L × H^1.5, where Q is in cubic feet per second and L (opening width) and H (head of water above the scupper sill) are in feet. Flow rises quickly with head, which is why the inlet elevation and the roof's allowable water depth must be designed together. This formula is engineering background, not a code requirement.
Siphonic roof drainage (IPC 1107)
Siphonic roof drains and drainage systems are designed to ASME A112.6.9 and ASPE 45. They run full-bore with smaller pipe, so they are engineered systems rather than sized from the tables.
Inspection points
- The secondary system is fully separate, from the roof inlet to the outlet.
- The outlet is above grade and readily observable.
- The secondary system is sized by the same rainfall rate as the primary, with no credit for primary flow.
- Scuppers are at least 4 inches high, with a width at least equal to the required drain's circumference.
- Combined primary/secondary drain bodies have independent inlets and outlets.
Standing water 5 inches deep covers a 15,000-square-foot roof. Using 5.2 pounds per square foot per inch of depth, what is the total water load?
195,000 pounds
390,000 pounds
468,000 pounds
780,000 pounds
Under IPC 1108.2, where must a secondary roof drain system discharge?
Into the primary building storm drain, downstream of the last primary conductor
Into the sanitary sewer, so that any overflow water is treated before release
To a separate end point above grade where occupants or staff will see it
To an underground dry well, so that overflow does not stain the building
A scupper serves a roof area that would require a 4-inch roof drain. Under IPC 1106.5 and 1108.3, what are its minimum opening dimensions?
At least 2 inches high and 12 inches wide, matching the parapet cap
At least 4 inches high and 4 inches wide, a minimum 16-square-inch opening
At least 4 inches high and about 12.6 inches wide, the drain's circumference
At least 6 inches high and 6 inches wide, regardless of the drain size
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