5 Common Misconceptions About Siphonic Roof Drainage Systems

Quick Summary

Siphonic systems are still relatively new in many Indonesian construction markets, so it's natural that a number of inaccurate perceptions have taken hold — some overly skeptical, others overselling. This article addresses the five misconceptions we encounter most often from consultants, contractors, and building owners.

Misconception #1: "Siphonic Is Only for Prestigious, High-End Buildings"

The reality: siphonic is most relevant precisely for functional, wide-span buildings — factories, warehouses, distribution centers — not just iconic architecture. The reasoning is simple: the larger the roof area and the greater the water flow that needs managing, the greater the pipe and outlet efficiency advantage siphonic offers. Ordinary warehouses and factories are actually the most technically logical application, not merely a prestige upgrade.

Misconception #2: "Siphonic Is More Prone to Blockages Than Gravity"

The reality: blockage risk fundamentally comes down to outlet strainer design and routine maintenance — not the full-bore flow principle itself. Blockage issues reported in the field for siphonic systems generally trace back to incorrect design or installation (such as flawed hydraulic calculations), not the anti-vortex baffle mechanism itself. A siphonic system designed and installed according to EN 12056-3 principles is not inherently more blockage-prone than gravity.

Misconception #3: "Siphonic Systems Require No Maintenance at All"

The reality: this is actually the opposite misconception, and equally important to correct — so client expectations stay realistic. Siphonic outlets still need periodic cleaning of debris and leaves that could clog the strainer, just like any roof drain. What sets siphonic apart isn't "zero maintenance" — it's that there are far fewer points to maintain (due to fewer downpipes). That's a genuine advantage, but it doesn't mean no maintenance at all.

Misconception #4: "Siphonic Is Always More Expensive Overall"

The reality: per-unit outlet and design costs for siphonic are indeed higher than conventional roof drains. But that's only part of the total project cost. For wide-span roofs, savings on pipe quantity, structural penetration count, and installation complexity are often significant enough to make total project cost more competitive — sometimes even lower than gravity, depending on roof scale and configuration. We cover this in more detail with illustrative figures in our Case Study article.

Misconception #5: "A Regular Plumbing Contractor Can Design Siphonic the Same Way as Gravity"

The reality: this is one of the riskiest misconceptions. Conventional gravity design is relatively standardized and widely mastered by plumbing contractors. Siphonic is fundamentally different — its hydraulic calculations are sensitive to details like available drop head, manifold layout, and flow balance between outlets. An incorrect design can cause the system to fail to "prime" (never reaching the intended full-bore vacuum condition), produce noise issues, or simply underperform. This is exactly why EN 12056-3 explicitly calls for design by experienced specialists, rather than simply applying familiar gravity design conventions.

Frequently Asked Questions

Are all siphonic system failures caused by bad design?
Most cases we've encountered do trace back to calculation or installation errors, rather than the full-bore flow principle itself — which is exactly why choosing an experienced design team matters so much.

How often do siphonic outlets need cleaning?
The frequency is similar to conventional roof drains generally, depending on the building's surrounding environment (areas with more trees/dust need more frequent checks) — but the number of points requiring inspection is far fewer than gravity for the same roof area.

If I already have a regular plumbing contractor, do I still need a separate siphonic specialist?
For physical installation, an experienced plumbing contractor can typically still carry out the work — but the hydraulic calculation and design of the siphonic system itself should involve a team with specific experience in this system.

Have another question or misconception you'd like clarified? Contact RAFA's engineering team — or start by testing your project's requirements with our interactive calculator.