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Roof Scupper Sizing

Calculate the minimum required width for roof scuppers using the standard weir flow formula. Factors in catchment area, primary drainage flow, and secondary emergency overflow requirements per IPC code.

Roof Catchment Area

sq ft

Rainfall Rate

in/hr

Max Allowable Head

inches

Secondary/Emergency Scuppers

Standard plumbing code (IPC 1107) requires secondary drainage systems to be sized independently of the primary drains. Emergency scuppers must have an invert (bottom) at least 2" above the roof surface and 2" below the top of the parapet wall.

Weir Flow Dynamics

A scupper acts as a "Broad-Crested Weir." The flow through it depends entirely on the width and the "Head"—the height of the water above the bottom of the opening.

Head Depth: Doubling the head depth significantly more than doubles the flow. If your parapet is low, you need a much wider scupper to keep head depth down.
Free Discharge: This calculator assumes "Free Discharge," meaning the water falls away from the wall without being restricted by a downspout or conductor head.
The Multiplier: In the US, the standard conversion for drainage is 0.0104 GPM per 1 square foot per 1 inch of rain per hour.

Required Width

10.1"

Minimum Scupper Opening

Total Peak Flow

156 GPM

0.348 CFS (Cubic Feet / Sec)

ASCE 7 Standard

Calculated using the standard weir coefficient (3.32) for rectangular scupper openings.

For estimation purposes only. Always consult a licensed professional before beginning work. Full Trade Safety Notice →

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What is The Physics of Roof Scuppers (Rectangular Weirs)?

A roof scupper is simply a rectangular hole cut through a parapet wall to allow water to drain off a flat roof. In fluid dynamics, this is known as a 'Broad-Crested Weir.' Unlike a vertical pipe drain which sucks water down using gravity (creating high-velocity flow), a scupper relies entirely on water statically overflowing its edge. Because of this, the flow capacity of a scupper is intimately bound to the 'Head Depth'—the physical height of the water allowed to pool on the roof before it spills out.

Mathematical Foundation

Standard Weir Flow Formula

Q = 3.32 \times L \times H^{1.5}

Q

= Flow Rate in Cubic Feet per Second (CFS).

3.32

= The empirical discharge coefficient (C_d) for a rectangular, broad-crested weir discharging freely into the air.

L

= The physical width of the scupper opening in feet.

H^{1.5}

= The Head Depth (the height of the water above the bottom of the scupper) measured in feet, raised to the 1.5 power.

Rainfall Conversion Constant

\text{GPM} = \text{Area} \times \text{Rain Rate} \times 0.0104

0.0104

= The US Engineering constant required to instantly convert square feet of roof area, multiplied by inches per hour of rain, straight into Gallons Per Minute (GPM).

Laws & Principles

The Head Depth Danger: The Weir formula dictates that flow increases exponentially with Head Depth (H^1.5). However, you cannot simply allow water to pool 6 inches deep on a roof to force it through a small scupper. Water weighs 62.4 pounds per cubic foot. A 6-inch deep ponding area places catastrophic, unintended structural loads on the roof trusses, often leading to collapse. Most architectural specifications limit Head Depth to 2 or 3 inches maximum.
The Parapet Location Rule: IPC Code 1107 dictates that secondary emergency scuppers must be installed with their invert (bottom edge) exactly 2 inches above the lowest point of the roof membrane. This ensures they only activate if the primary internal drains are fully clogged.
The Downspout Bottleneck: The weir formula operates on the strict assumption of 'Free Discharge'—meaning the water shoots out of the wall and falls freely through the air. If you pipe the scupper directly into a tight metal 'conductor head' and a 3-inch downspout, the downspout becomes the new bottleneck, and the weir calculation is voided.

Step-by-Step Example Walkthrough

" An architect is designing secondary emergency scuppers for a 5,000 square foot commercial roof in a coastal city with a 3.0-inch/hour design storm. The structural engineer strictly forbids the water 'Head Depth' from exceeding 3 inches during an emergency. "

1. Calculate Peak Runoff: 5,000 sq ft × 3.0-in/hr × 0.0104 = 156 Gallons Per Minute (GPM).
2. Convert into CFS: 156 GPM / 448.8 = 0.347 Cubic Feet per Second (CFS).
3. Convert Max Head to Feet: 3 inches max depth / 12 = 0.25 feet of Head (H).
4. Apply Weir Formula for Width (L): L = Q / (3.32 × H^1.5).
5. Solve for Width: 0.347 / (3.32 × 0.125) = 0.836 feet wide.
6. Convert to Inches: 0.836 feet × 12 = 10.03 inches.

Final Result: To safely eject 156 GPM of stormwater without letting the roof pool deeper than 3 inches, the sheet metal contractor must fabricate a scupper opening that is at least 10 inches wide.

Quick Answer: How do you size a roof scupper?

Use the Roof Scupper (Weir Flow) Calculator to determine the exact sheet-metal opening required to meet the International Plumbing Code. Enter your Roof Area, the local IPC Rainfall Rate, and the maximum structural Head Depth (usually 2 to 4 inches). The calculator utilizes standard hydraulic weir equations to instantly output the exact minimum Scupper Width needed to safely eject the storm surge.

Architectural Drainage Scenarios

The Long, Narrow Trough

An engineer is designing a warehouse with a very weak wooden roof deck. They must clear 300 GPM of water but cannot allow the water 'Head' to exceed a microscopic 1.5 inches deep, or the roof will collapse. Because the Head Depth is so shallow, the water has almost no kinetic push. The calculator proves that to eject 300 GPM at only 1.5 inches deep, they must abandon standard 8-inch scuppers and instead build a massive, continuous 48-inch wide sheet metal trough across the parapet.

The Downspout Choke Point

A roofer perfectly calculates that their 12-inch wide scupper will effortlessly clear a 4-inch Florida rainstorm. However, to make the building look neat, they pipe the massive 12-inch scupper down into an aesthetic 3-inch residential aluminum downspout. During the first hurricane, the scupper ejects perfectly, but the tiny downspout backs up instantly. The water fills the conductor head, submerges the scupper, and voids the 'free discharge' weir physics, flooding the building.

The Width Equation

Scupper Width (L) Formula

Width (L) = Total Flow (CFS) / (3.32 × Head Depth^1.5)

Notice the exponent 1.5 on the Head Depth. This proves that water height is exponentially more powerful than scupper width. If you double the width of a scupper, you only double its flow. If you double the allowable water height, the flow increases by nearly three times.

Code Compliance & Mistakes

Do This

✓Enforce the IPC 2-inch rule. If you are installing secondary (emergency) scuppers, the IPC completely forbids installing them at the exact same height as the primary drains. You must set the bottom of the emergency scupper exactly 2 inches higher than the roof membrane. They act as an alarm system; if water comes out of them, the primary system has critically failed.
✓Oversize Conductor Heads. If you are required to pipe the scupper into a downspout rather than letting it splash onto the ground, the metal catcher box (conductor head) must be massively oversized—typically twice the width of the scupper opening—to prevent splash-back and aerodynamic choking.

Avoid This

✗Don't place scuppers strictly on the corners. Roofers commonly put scuppers in the exact corners of a parapet wall because it looks clean. However, high-wind storms push water into corners, causing erratic, swirling cross-currents that slash the weir flow efficiency. Keep scuppers at least 24 inches away from 90-degree parapet corners.
✗Don't assume a 4x4 scupper is 16 inches of flow. If code requires a 8-inch wide scupper to clear the water at a 2-inch head, you cannot substitute 'two 4-inch scuppers.' Because water sticks to the sidewalls (end-contraction friction), two small rectangles process significantly less water than one wide rectangle.

Head Depth vs. Capacity (12-inch Wide Scupper)

Water Depth (Head)	Flow Capacity (12" Width)	Roof Load Factor (Ponding Weight)
1.0 Inch Deep	42 GPM	5.2 lbs per sq ft
2.0 Inches Deep	119 GPM	10.4 lbs per sq ft
3.0 Inches Deep	220 GPM	15.6 lbs per sq ft
4.0 Inches Deep	339 GPM	20.8 lbs per sq ft
6.0 Inches Deep	623 GPM	31.2 lbs per sq ft (Extreme Collapse Risk)

Frequently Asked Questions

What does "Head Depth" actually mean?

Head Depth is strictly the height of the water sitting above the bottom lip (the invert) of the scupper opening. It represents how deep the water is puddling on the roof. The deeper the puddle, the heavier the physical water pressure, and the faster the water is violently squeezed out of the scupper opening.

Are scuppers better than vertical pipe drains?

They serve different purposes. Vertical pipe drains drop straight down through the heated interior of the building, meaning they never freeze, making them ideal for massive commercial roofs in northern climates. Scuppers punch straight through the exterior walls. They are cheaper and infinitely safer as emergency backups because an external PVC pipe cannot shatter inside your drywall.

Why do emergency scuppers whistle during high winds?

Because emergency scuppers are installed 2 inches above the membrane, they are completely dry 99% of the time. They are essentially open tunnels leading straight into the parapet wall. High-velocity lateral winds blow into these empty tunnels, turning them into giant sheet-metal flutes. Architectural baffles are typically installed to block the wind while allowing water to burst out.

Is there a minimum size for any scupper?

Yes. Even if the math states you only need a 1-inch width to clear a tiny overhang, the International Plumbing Code mandates that the minimum physical dimension of any scupper opening be at least 4 inches (e.g., 4" wide by 4" tall). Anything smaller runs an extreme risk of becoming instantly clogged by a single wet leaf or a plastic bag.