Case Study: Reducing Pipe Material on Wide-Span Roofs (Factories & Warehouses)

Quick Summary

This article uses an illustrative scenario — not a specific named client project, but representative figures for a 5,000 m² warehouse/factory roof in a typical Greater Jakarta industrial area — to concretely show the difference in outlet and pipe requirements between conventional gravity and siphonic systems under realistic conditions. All figures are calculated using the same standard hydraulic principles used in our interactive calculator, not rough estimates.

Project Scenario

• Effective roof area: 5,000 m²
• Design rainfall intensity: 350 mm/h (representative of a Cikarang/Greater Jakarta industrial area at a return period commonly used for industrial buildings)
• Runoff coefficient (C): 1.0 (flat metal roof, typical for factories/warehouses)
• Total design flow (Q): ≈ 486 L/s

Scenario A — Conventional Gravity

Using conventional outlets without an anti-vortex baffle (field-referenced capacity: DN200 ≈ 57.8 L/s):

• Outlets required: ≈ 9 units (486 ÷ 57.8, rounded up)
• Since standard gravity field practice is one outlet per downpipe (no manifolding), this means 9 separate downpipes, each penetrating the roof structure independently
• The horizontal collector pipe, running at a 1–2% slope, must carry the combined flow before discharging to the final drainage point — under these conditions, the required horizontal pipe size can end up considerably larger than the vertical stack itself (we cover this phenomenon in more detail in our Standards & Regulations article), because horizontal gravity flow capacity is constrained by the available slope rather than benefiting from full gravitational acceleration as in a vertical pipe

Scenario B — Siphonic (RAFA DN110, 60 L/s)

Using siphonic anti-vortex outlets (RAFA DN110, 60 L/s capacity):

• Outlets required: ≈ 9 units as well (486 ÷ 60) — the roof-surface outlet count is fairly similar in this case, since the per-outlet capacities happen to be close
• However, these nine outlets can be manifolded into a far smaller number of pipe stacks — not nine separate downpipes — since full-bore flow allows multiple outlets to share a single pipe run without losing capacity
• Horizontal pipework can be installed with no slope at all, avoiding the "oversized horizontal pipe" issue seen in the gravity scenario

What Actually Differs: Not Outlet Count, But the Pipe Network

Interestingly, in the scenario above, the number of roof-surface outlet points between gravity and siphonic turns out to be roughly comparable — because we deliberately used a moderate siphonic outlet capacity (60 L/s) for a fair comparison. The real difference emerges elsewhere:

1. Number of downpipes/stacks — gravity needs one downpipe per outlet (no shared routing), while siphonic can combine many outlets into far fewer stacks
2. Horizontal pipe size and slope — this is the most significant difference; siphonic requires no slope, resulting in much smaller pipe sizes and far more routing flexibility beneath the roof structure
3. Number of roof structural penetrations — fewer downpipes means fewer long-term potential leak points

The implication: total horizontal pipe length and material volume, structural penetration count, and installation complexity in the gravity scenario tend to be considerably higher — which ultimately affects installation cost and long-term leak risk. We've deliberately omitted specific cost figures here, since actual costs depend heavily on material pricing, labor rates, and project-specific specifications — we recommend calculating based on your project's actual Bill of Quantities.

Important Note

This scenario is simplified for educational purposes and uses standard hydraulic formulas (not rough assumptions) — but every real project has additional variables (structural column layout, building height, roof configuration, maintenance access) that can meaningfully change the final result. We recommend always running project-specific calculations for your own conditions — either using our calculator as an initial estimate, or through a full technical consultation with our team for final design.

Frequently Asked Questions

Does this result apply to all roof sizes?
No. On smaller roofs, the difference between gravity and siphonic is usually far less dramatic — siphonic's advantage becomes clearer specifically on wide-span roofs with large design flows.

Why is the roof-surface outlet count nearly identical between the two scenarios?
Because we chose a moderate siphonic outlet capacity for a fair comparison, rather than an exaggerated figure. Using a higher-capacity siphonic outlet would reduce the outlet count further still — but the most important comparison is actually in the pipe network, not just outlet count.

Can I get this kind of calculation for my own project?
Yes. Enter your roof data into our interactive calculator for an initial estimate, then contact our team for a more detailed design calculation.

Want to see this comparison for your own project? Try our interactive calculator, or contact RAFA's engineering team for a technical consultation.