Siphonic Drainage
High-Capacity Rainwater Discharge for Large Roofs.
Siphonic drainage is a modern rainwater system that discharges rainwater from large roofs at high velocity using a vacuum effect, allowing fewer downpipes and a more compact horizontal pipe layout.
Request a Quote for Your Project →Siphonic drainage achieves high-flow discharge by running the pipework completely full.
In conventional systems, air and water flow through the pipe together. In a siphonic system, special roof outlets limit the entry of air, so the pipework approaches full-bore flow.
A siphonic rainwater system consists of special siphonic roof outlets, an HDPE pipe network, horizontal collector pipes, vertical downpipes and safe discharge points. The vacuum effect draws water through the system at high velocity.
This solution is used particularly on factories, warehouses, logistics centres, shopping centres, airports, sports halls and large industrial roofs. As fewer downpipes are needed, it offers a major advantage in coordinating the architecture and building services.
Which details matter in a siphonic drainage system?
In siphonic drainage, the choice of outlets, pipe diameters, length of horizontal runs, quality of HDPE welding and the calculated rainfall flow directly affect system performance.
Siphonic Roof Outlets
One of the most important components of a siphonic system is the special roof outlet. These outlets are designed to limit the entry of air while collecting rainwater, helping to create full-bore flow and a vacuum effect in the pipework.
When selecting outlets, the roof area, rainfall intensity, roof pitch, parapet details, waterproofing layers and discharge capacity must all be assessed together.
Technical installation details
| Function | To admit rainwater into the system with minimal air |
|---|---|
| Suitable applications | Flat roofs, industrial roofs, warehouse and factory roofs, shopping centres and large commercial buildings |
| Points to note | Waterproofing details, outlet level, overflow line, leaf guard and maintenance access |
HDPE Pipe System
HDPE is the most widely used pipe material in siphonic drainage systems. Because HDPE pipes can be joined by welding, they form a watertight, durable and long-lasting system.
As the pipework operates with full-bore flow, pipe hangers, weld quality, expansion allowances and fixing points are extremely important. Incorrect support or poor welds can lead to vibration, sagging and leaks.
Technical installation details
| Pipe used | HDPE rainwater pipes and welded fittings |
|---|---|
| Jointing method | Electrofusion welding, butt welding or suitable HDPE jointing systems |
| Advantage | Welded watertight joints, high strength, suitability for industrial applications |
| Points to note | Bracket spacing, fixed points, weld quality, expansion and vibration control |
Hydraulic Calculation and Flow Planning
A siphonic drainage system is not installed with pipe sizes chosen by eye. The roof area, rainfall intensity, number of outlets, pipe routing, vertical drop and total pressure loss are assessed by a dedicated hydraulic calculation.
For the system to work correctly, the pipes must reach full-bore flow at a given rainfall level. A system that is incorrectly calculated may suffer from noise, inadequate discharge, ponding on the roof or unbalanced flow in the pipework.
Values taken into account in the calculation
| Calculation inputs | Roof area, rainfall intensity, outlet capacity, pipe length, vertical height |
|---|---|
| System behaviour | Full-bore flow, vacuum effect, high flow velocity |
| Risks | Incorrect pipe sizing, unbalanced flow, noise, inadequate discharge, ponding on the roof |
Large Roof and Industrial Applications
Siphonic drainage offers the greatest advantage on large roofs. In buildings such as factories, warehouses, logistics centres and shopping centres, the large roof area would require many downpipes with a conventional system.
A siphonic system can achieve high-flow discharge with fewer downpipes. This provides flexibility in interior planning, coordination around columns, façade design and external infrastructure.
Advantages
| Suitable buildings | Factories, warehouses, logistics centres, shopping centres, airports, sports halls, large commercial buildings |
|---|---|
| Advantages | High flow, fewer downpipes, compact horizontal runs, architectural flexibility |
| Disadvantages | Requires specialist calculation and installation; poor installation can seriously impair system performance |
| Maintenance | Outlet cleaning, leaf guard checks, pipe hangers and overflow lines must be inspected periodically |
What should you consider when choosing a siphonic drainage system?
1. A hydraulic calculation is essential.
Siphonic drainage is not sized using conventional pipe sizing rules. The roof area, rainfall intensity and pipe routing must be determined by a dedicated calculation.
2. The choice of outlets is critical.
Special roof outlets must be used to create the siphonic effect, and the number and capacity of outlets must be calculated correctly.
3. HDPE weld quality is important.
As the pipework operates with full-bore flow, watertight welds and a correct support system are essential for a long service life.
4. The emergency overflow must not be forgotten.
Overflow protection must be planned to prevent ponding on the roof in the event of extreme rainfall or a blocked outlet.
Let’s plan the right siphonic drainage system for your large roofs.
We will prepare a safe siphonic rainwater system based on the roof area, rainfall data, number of outlets, HDPE pipe routing and discharge capacity.