Parapet scuppers are one of the most failure-prone elements on commercial flat roofs — yet they are among the most lightly specified. A scupper that is undersized, improperly flashed, or missing its secondary overflow counterpart can result in catastrophic ponding loads that exceed the structural capacity of a roof deck. IBC Section 1503.4 requires secondary drainage on all roofs with parapets, but enforcement during construction frequently falls through the cracks between trades. The architect's specification is the line of defense.
CSI MasterFormat Coverage for Scuppers
Scuppers and their associated through-wall flashings span multiple specification sections depending on their function:
| Section | Scope |
|---|---|
| 07 62 00 | Custom-fabricated scupper boxes, through-wall sleeves, and associated counter flashings |
| 07 71 00 | Proprietary prefabricated scupper assemblies with integral flashing systems |
| 22 14 29 | Overflow (secondary) scuppers when coordinated with the plumbing drainage design |
| 05 50 00 | Structural openings in metal parapets for scupper installation |
Most architects specify scuppers in Division 07 62 00, with a coordination note in Division 22 identifying the overflow scupper location and invert elevation. The structural opening must be coordinated with Division 05 or the structural drawings.
IBC and Drainage Code Requirements
International Building Code Section 1503.4 establishes the primary and secondary drainage requirements:
- Primary drainage: Roof must drain to drains or scuppers sized to handle a 100-year, 1-hour rainfall event for the building location
- Secondary (overflow) drainage: Required for all roofs with parapets; must be capable of handling 100% of the design storm independently of the primary system
- Overflow scupper invert: Set 2 inches above the primary scupper sill elevation to ensure it only activates when primary is overwhelmed
- Structural design: ASCE 7 Section 8 requires the structural system to be designed for the ponding loads that would accumulate if primary drainage fails to the point of overflow scupper activation
Structural implication: The 2-inch offset between primary and overflow scupper inverts means that if the primary scupper fails completely, 2 inches of water can accumulate before the overflow scupper activates. At 5.2 psf per inch of water depth over the full tributary area, the structural engineer must confirm the roof deck can handle this load. Verify structural design accounts for this condition — it is often missed on projects that use prescriptive drainage without a drainage engineer.
Scupper Sizing: The Drainage Formula
Scupper sizing is governed by the drainage area each scupper serves and the design rainfall intensity. The basic approach follows IBC Appendix D and SMACNA Chapter 1:
Step 1: Determine Design Rainfall Rate
Use ASCE 7-22 Chapter 8 or local jurisdiction tables to find the 100-year, 1-hour rainfall rate (in inches per hour) for the building location. Values range from approximately 2 in/hr in dry western climates to 8+ in/hr in humid southern climates.
Step 2: Calculate Required Flow Area
SMACNA provides the following formula for rectangular scupper sizing:
Where:
Q = Required flow (gallons per minute)
C = Runoff coefficient (use 1.0 for roofs)
I = Rainfall intensity (inches per hour)
A = Tributary area (acres; 1 acre = 43,560 sq ft)
Convert to scupper area using: Q = 0.0104 × W × D^1.5 (for rectangular scupper)
Where W = scupper width (inches), D = scupper depth (inches)
Practical Sizing Table
The following table provides approximate scupper capacity for common sizes at a 4 in/hr design rate, as a preliminary sizing reference (verify with drainage engineer for final design):
| Scupper Opening (W × D) | Approx. Flow (GPM) | Approx. Roof Area Served |
|---|---|---|
| 4" × 3" | ~28 GPM | ~700 sq ft |
| 6" × 4" | ~64 GPM | ~1,600 sq ft |
| 8" × 4" | ~86 GPM | ~2,150 sq ft |
| 12" × 6" | ~228 GPM | ~5,700 sq ft |
| 18" × 6" | ~342 GPM | ~8,550 sq ft |
Through-Wall Scupper Flashing Design
The scupper assembly is only as watertight as its through-wall flashing. A scupper consists of three components that must be separately specified:
- Scupper box: The metal sleeve that lines the through-wall opening, sized to the scupper opening dimensions plus clearance for the flashing laps
- Head flashing: Extends from the scupper box up the parapet interior face, minimum 8 inches, to intercept water above the scupper
- Sill flashing: Turns down the exterior face of the parapet, minimum 3 inches, to carry discharge water away from the wall
Through-Wall Flashing Installation Sequence
The installation sequence matters — flashing installed out of order cannot be properly lapped:
- Install through-wall sleeve (scupper box) in the structural opening
- Install sill flashing against the interior parapet face, lapped under the scupper box
- Integrate sill flashing with the roof membrane — the membrane should lap onto the sill flashing, not under it
- Install head flashing over the scupper box, lapping the interior parapet face membrane or coping cap base flashing
- Seal all laps with compatible sealant; avoid relying solely on sealant at any joint that experiences water head pressure
Scupper Material Requirements
A. Steel: 24 gauge (0.0239 inch) minimum, galvanized steel ASTM A653, G90 coating; Galvalume (ASTM A792) where exposed.
B. Aluminum: .040 inch (1.0 mm) minimum, ASTM B209, Alloy 3003-H14.
C. Stainless Steel: Type 304, 26 gauge minimum, where coastal or aggressive environment is noted.
D. Prohibition: Do not use galvanized steel in direct contact with copper or lead flashing — galvanic corrosion will occur. Use stainless or aluminum in these conditions.
Overflow (Secondary) Scupper Specification
Overflow scuppers are required by IBC 1503.4 and must be specified explicitly — they are frequently omitted or assumed to be provided by another trade:
A. Provide overflow scuppers at each primary scupper location, or as otherwise indicated on the drawings.
B. Overflow scupper sill: Set 2 inches above the primary scupper sill elevation — no exceptions.
C. Overflow scupper opening: Size to match the primary scupper flow capacity per Section 2.1.
D. Overflow scuppers shall discharge to a location visible from grade or to an area where discharge is perceptible to building occupants.
E. Through-wall flashing for overflow scuppers: Same requirements as primary scuppers per Section 2.2.
Scupper-to-Conductor Head Connection
When scuppers discharge into exterior conductor heads (leader heads) and downspouts rather than open air, the connection must be specified. Common failure modes:
- Scupper outlet projects too far past the wall face, preventing correct alignment with conductor head opening
- Conductor head is undersized for the scupper discharge volume
- No sealant or crimped connection between scupper outlet and conductor head, allowing water to run down the wall face
- Dissimilar metals at the scupper-to-conductor head joint causing galvanic corrosion over time
Specify: "Scupper outlet shall project [dimension] inches beyond the wall face to align with the conductor head opening. Provide crimped connection or sealant joint between scupper outlet and conductor head inlet. Where scupper and conductor head are dissimilar metals, install an isolating gasket or use matching materials."
Submittal Requirements for Parapet Scuppers
Division 07 62 00 scupper submittals should include:
- Shop drawings showing scupper box profile, through-wall sleeve dimensions, head and sill flashing configurations, and lap joint locations
- Drainage engineer's calculation confirming scupper sizing meets IBC 1503.4 for the design rainfall rate and tributary area
- Overflow scupper invert elevations relative to primary scupper sills
- Material certifications for scupper box and flashing materials
- Sealant product data and compatibility documentation for all sealant joints
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Frequently Asked Questions
What CSI MasterFormat section covers parapet scuppers?
Parapet scuppers and their associated through-wall flashings are typically specified under Division 07 62 00 (Sheet Metal Flashing and Trim) when custom-fabricated, or Division 07 71 00 (Roof Specialties) when part of a proprietary drainage system. The scupper opening may also be referenced in Division 22 (Plumbing) when emergency overflow design is coordinated with the plumbing engineer. For most commercial projects, scuppers are in Division 07 with coordination notes to Division 22.
How do you size a parapet scupper for drainage?
Scupper sizing is governed by IBC Section 1503.4 and ASCE 7 rainfall intensity data. The basic formula: Required flow area (sq in) = Tributary roof area (sq ft) × Design rainfall rate (in/hr) / Flow coefficient. SMACNA provides sizing tables for common conditions. A rule of thumb for preliminary sizing: one 4-inch-wide × 2-inch-tall scupper can handle approximately 1,000 square feet of roof area in a 4-inch-per-hour rainfall event, but this must be confirmed by the drainage engineer using local intensity data.
What is an emergency overflow scupper and is it required?
An emergency overflow scupper is required by IBC Section 1503.4 for roofs with parapets. It provides a backup drainage path if the primary drain or scupper becomes blocked. The overflow scupper must be set 2 inches above the primary scupper sill, at a location visible from grade or designed to discharge in a location that alerts building occupants. Failure to provide secondary drainage is one of the most common IBC violations found during plan review on commercial low-slope roofs.
What material should be used for parapet scupper fabrication?
SMACNA recommends minimum 24 gauge (0.0239 inch) galvanized steel or .040 inch (1.0 mm) aluminum for parapet scuppers. Stainless steel (Type 304, 26 gauge minimum) is preferred in coastal or chemical environments. Where scuppers discharge into gutters or conductor heads with lead or copper components, stainless or aluminum is required to prevent galvanic corrosion at the interface.
How far should a scupper project beyond the face of the parapet?
SMACNA recommends that scupper outlets project a minimum of 1/2 inch beyond the face of the parapet or wall to ensure water clears the wall face during discharge. Where scuppers discharge to open air (not into a conductor head or gutter), a 1-inch to 2-inch projection is preferred to prevent staining of the wall face. If the scupper connects to an exterior conductor head or downspout, the projection must align with the conductor head opening, and the connection must be weather-sealed.