Services
Siphonic Rainwater Drainage
Pressure-flow drainage that safely empties large roofs with fewer downpipes.
About the system
A siphonic rainwater drainage system is designed to drain rainwater from roof surfaces safely and economically. The system works on the principle of pressure flow: the pipes run completely full of water, without entrained air.
In conventional gravity drainage the pipes run about one third full and the flow depends on falls. In a siphonic system, the air baffle in the outlet prevents air from entering the pipework; once the pipes are running full, the water column in the vertical stack creates a suction effect that draws water at high velocity. As a result the collector can run horizontally beneath the roof, without falls.
The practical consequence is that the same roof area can be drained with far fewer and smaller-diameter downpipes. That means fewer vertical shafts inside the building, shorter buried drainage runs and greater freedom in the architectural plan. The high flow velocity also keeps the pipework self-cleaning.
The system relies on hydraulic calculation: rainfall intensity, roof area, outlet layout and pipe diameters are solved together. If the calculation is not set up correctly the siphon will not form, so the design is developed alongside the roof system and the structural design.
Applications
- Airport terminals and long-span transport buildings
- Logistics warehouses and distribution centres
- Shopping centres, exhibition and congress buildings
- Stadiums and indoor sports facilities
- Production plants and large-roof industrial buildings
- Projects with limited space for vertical shafts inside the building
Installation process
- 01
Data collection
Local rainfall intensity data, roof area and geometry, parapet and gutter arrangement and the level of the connecting drainage network are established.
- 02
Hydraulic calculation and layout
The number and position of outlets, the collector route and pipe diameters are calculated, ensuring balanced flow to each outlet.
- 03
Roof integration
Outlet bases are detailed together with the roof covering and waterproofing; transitions through standing seam or membrane roofs are made watertight.
- 04
Pipework and bracketing
The collector is installed horizontally beneath the roof; because of the high velocity and pressure, a dedicated bracketing system with guide clamps is used.
- 05
Stack and network connection
Downpipes are dropped and connected to the gravity network after the pressure-breaking transition point.
- 06
Testing and commissioning
A leak test is carried out and the outlets are checked; the system is handed over with periodic cleaning and maintenance instructions.
Available materials
What does a siphonic system gain over a gravity system?
The same roof area needs fewer and smaller downpipes, and because the collector runs level and horizontal, ceiling void heights are preserved. That means fewer shafts inside the building, shorter buried runs outside and usually a lower overall excavation and installation cost.
Does the system work in light rain?
In light rain the system behaves like a gravity drain: the pipes do not fill, so no siphon forms, but the water is still discharged. The siphonic effect starts as the design rainfall is approached and the system reaches full capacity.
Is maintenance difficult?
Maintenance is essentially periodic cleaning of the outlets and leaf guards; the high flow velocity largely keeps the pipework self-cleaning. What matters is that the outlet baffles are never removed and that the roof is inspected before the rainy season.
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Learn moreIs this system right for your project?
Share your architectural design; we will respond within 48 hours with system selection and indicative pricing.