Large flat roofs demand drainage that’s fast, predictable, and space-efficient. The Zurn Z131S Siphonic Overflow Roof Drain is designed to achieve full-bore (siphonic) flow during major rain events while protecting the roof system with a secure membrane clamp and deck plate assembly. Below is a practical guide to how it works, where it’s used, and how to get reliable results on site.

Zurn Z131S – Profile View

What Is the Z131S?

The Z131S is a siphonic overflow roof drain that transitions to negative-pressure (siphonic) flow at design water depths to rapidly evacuate stormwater. Its Dura-coated cast iron body delivers longevity, while the air baffle helps sustain siphonic action. A membrane flashing clamp and deck plate secure the roofing system for a watertight, durable installation.

Key Features & Benefits

• Siphonic Overflow Operation: Enables high flow rates and efficient pipe routing.
• Dura-Coated Cast Iron: Corrosion-resistant body designed for rooftop environments.
• Air Baffle: Helps maintain full-bore (air-excluded) flow during peak events.
• Membrane Flashing Clamp & Deck Plate: Positive, watertight integration with roof assemblies.
• Space Efficiency: Supports flatter pipe runs and fewer downspouts versus conventional gravity only systems.

Zurn Z131S – Siphonic Overflow Drain

Ideal Applications

• Large commercial and institutional roofs
• Sites with limited ceiling space for sloped piping
• Projects seeking reduced downspout counts and pipe sizes
• Retrofits targeting faster roof dewatering and overflow protection

How the Z131S Works (Plain-English)

1. During rainfall, water enters the drain similar to a standard gravity system.
2. As the water depth rises, air is restricted at the inlet via the baffle, transitioning the piping to full-bore flow.
3. With air excluded, negative pressure forms in the piping, accelerating water removal across long, near-level runs.
4. In extreme events, the unit’s overflow capabilities help protect the roof from ponding.

Installation Best Practices

• Follow deck opening dimensions and fastener patterns per the Zurn submittal.
• Set the drain flush and evenly torque the flashing clamp to the membrane manufacturer’s guidance.
• Confirm the baffle orientation and clearances so siphonic action won’t be disturbed.
• Coordinate pipe sizing/headers with the siphonic design to avoid air re-entry.
• Water-test and inspect strainers/baffles after major storms to maintain peak performance.

Zurn Z131S – Top View

Case Study: Campus Roof Modernization

Challenge: A university needed to rehab a sprawling flat roof with limited fall for sloped piping and frequent ponding.

Solution: Engineers specified Zurn Z131S drains and a siphonic header to move water rapidly with flatter mains and fewer vertical drops.

Outcome: Peak-storm performance improved, ponding disappeared, and the ceiling plenum regained usable space thanks to smaller, near-level piping.

Conclusion

The Zurn Z131S Siphonic Overflow Roof Drain delivers fast stormwater removal, robust waterproofing, and installation flexibility. For large roofs or tight routing conditions, its siphonic design, durable body, and secure membrane interface make it a dependable choice for long-term protection.