16.3 Secondary Roof Drains, Scuppers, and Controlled Flow Systems

Why secondary drainage exists

A flat roof with a parapet is a basin. If the primary drains block — with ice, leaves, a construction tarp, a roofing bucket — the roof fills. Water weighs about 5.2 pounds per square foot per inch of depth. A few inches of ponding across a large roof is tens of thousands of pounds the structure may not have been designed to carry, and roof collapses from ponding are a recognized failure mode.

IPC 1101.7 encodes the design assumption: in determining the maximum possible depth of ponded water, all primary roof drainage means shall be assumed to be blocked.

When secondary drainage is required — IPC 1108.1

Where roof drains are required, secondary (emergency overflow) roof drains or scuppers shall be provided where the roof perimeter construction extends above the roof in such a manner that water will be entrapped if the primary drains allow buildup for any reason. Where primary and secondary roof drains are manufactured as a single assembly, the inlet and outlet for each drain shall be independent.

Two triggers, both required:

1. Roof drains are required on the roof, and
2. the perimeter construction extends above the roof so water would be entrapped.

A roof that sheds freely over an unrestricted edge does not need secondary drainage, because the edge itself is the overflow.

The single-assembly rule prevents a manufacturer from sharing a throat or an outlet between the primary and secondary path — which would let a single blockage defeat both.

Separate 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.

Three requirements, each with a purpose:

• Separate end point — a shared discharge means a blocked primary line blocks the secondary line too.
• Above grade — a below-grade tie-in can surcharge.
• Normally observed — the secondary system running is the building's alarm that the primary system has failed. Discharging it into a hidden courtyard defeats the warning.

Sizing the secondary system — IPC 1108.3

Secondary (emergency) roof drain systems shall be sized in accordance with Section 1106 based on the rainfall rate for which the primary system is sized. Scuppers shall be sized to prevent the depth of ponding water from exceeding that for which the roof was designed as determined by Section 1101.7. Scuppers shall have an opening dimension of not less than 4 inches in height and have an opening width equal to the circumference of the roof drain required for the area served. The flow through the primary system shall not be considered when sizing the secondary roof drain system.

The last sentence is the one exam items test. You do not credit the primary system with any capacity. The secondary system carries the entire design rainfall on its own, because the design case is that the primary system is blocked.

Scupper dimensions

IPC 1106.5 Parapet wall scuppers states the parallel rule for scuppers used as primary roof drainage:

Where scuppers are used for primary roof drainage or for secondary (emergency overflow) roof drainage or both, the quantity, size, location and inlet elevation of the scuppers shall be chosen to prevent the depth of ponding water on the roof from exceeding the maximum water depth the roof was designed for, as determined by Section 1611.1 of the International Building Code. Scupper openings shall be not less than 4 inches in height and have a width equal to or greater than the circumference of a roof drain sized for the same roof area. The flow through the primary system shall not be considered when locating and sizing secondary scuppers.

Worked scupper width

A roof area requires a 6-inch roof drain. What width must a secondary scupper be?

• Circumference of a 6-inch circle = pi x diameter = 3.1416 x 6 = 18.85 inches.
• Required scupper opening: not less than 4 inches high and not less than approximately 18.9 inches wide.

For an 8-inch drain: 3.1416 x 8 = 25.1 inches wide.

Siphonic roof drainage — IPC 1107

Siphonic roof drains and drainage systems shall be designed in accordance with ASME A112.6.9 and ASPE 45.

A siphonic system runs full bore by design, using an air baffle at the drain to exclude air and create a siphon in the horizontal collector. Because it runs full, the collector can be nearly level and much smaller than a conventional gravity system. It is an engineered design; the IPC delegates it entirely to the two referenced standards.

Note that 1102.6 excludes siphonic roof drains from the ASME A112.6.4 / ASPE-IAPMO Z1034 flow-rating requirement, because a siphonic drain's performance is a property of the whole system, not the drain alone.

Controlled flow roof drain systems — IPC 1110

The opposite approach: deliberately store water on the roof and release it slowly, reducing peak load on the storm sewer.

1110.4 is the most testable line in the section: two drains up to 10,000 square feet, four drains above it. A controlled-flow roof depends on standing water, so a single blocked drain is an unacceptable risk — redundancy is mandatory.

1110.1's cross-reference to Section 316.1 is the alternative engineered design section from Chapter 2 of this guide: a registered design professional, technical data, two sets of signed and sealed construction documents including a riser diagram, and code official approval recorded on the permit.

Putting the primary and secondary systems together

A warehouse has a 12,000 square foot roof surrounded by a 3-foot parapet, in Denver.