Siphonic Roof Drainage Explained: Risks, Advantages and Design Considerations

Siphonic roof drainage is the most effective rainwater discharge technology for flat and low-gradient roofs. Compared to gravity drainage systems, siphonic drainage boasts at least twice higher flow rate, offering better protection from leaks and overflows. However, to function correctly, siphonic drainage solutions must be impeccably designed. We talked with our expert, Maciej Teterka, and our recent customer, Tadmet Company, about the importance of professional design and other key considerations in ensuring optimal performance and long service life of siphonic drainage solutions.

_{Introduction to siphonic drainage}

Siphonic roof drainage systems have revolutionized the way rainwater is managed on rooftops. Unlike traditional gravity drainage systems, which rely on the roof gradient to guide water into the downpipes, siphonic systems utilize a vacuum (or siphon) effect, resulting in a higher flow rate and the ability to handle larger volumes of precipitation.

_{What is siphonic roof drainage and how it works}

Conventional drainage systems require pipes to be installed at a downward slope and are designed to operate with partial filling. As rainwater flows into the drains, a vortex forms, drawing air into pipes and pushing the water toward the pipe walls. This means that a lower amount of water can be removed at a time, and larger-diameter pipes must be used to compensate for the presence of air.

At lower precipitation intensities, siphonic systems operate in the same way as their gravity counterparts. However, during periods of heavy rain, they rely on the siphon effect, which creates a full-bore water flow.

The anti-vortex design of air baffles prevents air from entering the drains altogether. Once full-bore flow begins, negative pressure forms within the system, creating a pulling effect that accelerates water flow. At full system capacity, the collector pipes run completely filled with water; thus, considerably higher volumes can be drained swiftly, minimizing the risks of water accumulation on the roof. 

Main benefits of siphonic drainage systems

• Exceptional drainage capacity: Compared to gravity drainage, siphonic systems operate at much higher flow rates and require fewer outlets. This is particularly important for large-footprint buildings with flat or low-gradient roofs, such as warehouses, malls, residential complexes, factories, distribution and logistics centers.

• Enhanced safety and reliability: Thanks to high-velocity flow, siphonic roof drainage systems less prone to blockages and require minimal maintenance.

• Space-saving design: The connecting pipes are laid parallel to the ceiling with no need for slope. Smaller pipe diameters can be used without sacrificing drainage efficiency, and multiple outlet drains can be connected to a single downpipe. This way, siphonic systems free up more interior space and offer greater design flexibility.

• Reduced piping costs: As the water is carried away through fewer and smaller pipes, siphonic systems allow for saving up to 55% of material costs compared to gravity alternatives.

Potential risks of siphonic systems

Like any advanced building services technology, siphonic roof drainage can pose certain risks that must be assessed and mitigated.

• Need for meticulous design and installation: Errors in pipe routing, outlet placement, or system sizing can prevent the system from operating in siphonic mode, leading to insufficient drainage, excess water accumulation, and increased static loads on the roof — potentially risking structural damage. Thus, it is crucial to engage experienced engineers and qualified installers familiar with siphonic principles.

• Importance of system integrity: Siphonic systems operate below atmospheric pressure and must withstand harsh weather events. If the components used are not suitable for negative water pressures, UV exposure or local environmental conditions, it can lead to leakages, drain collapse or even system failure.

• Necessity for correct maintenance: Neglecting the recommended inspection and cleaning schedule of baffles and drains may reduce system efficiency. By opting for low-maintenance siphonic drainage solutions, property owners can save time and minimize the risk of system malfunction.

_{Pipelife siphonic roof drainage: Market-leading solution}

Pipelife siphonic systems have been developed in accordance with the highest national and international standards (VDI 3806, DIN 1986-100 and UNI EN 1519) and are recommended for roofs with a minimum surface area of 150 m2 and a maximum inclination of 3%.

• Pipelife's DN75 siphonic drainage outlets have been tested to perform at a flow rate of up to 20 l/s, surpassing standard siphonic drainage systems by 60%.

• The air baffles are manufactured from automotive-industry-standard ASA plastic and boast a self-cleaning design.

• The pre-welded tailpipes speed up the installation process while minimizing error risks.

Due to these characteristics, our siphonic drainage systems go above and beyond current industry standards, offering exceptional durability and performance.

_{Planning of siphonic drainage systems}

Every siphonic roof drainage model must be tailored for a specific precipitation amount. If the actual rainfall intensity happens to be lower, the drains will simply function like a traditional gravitational system. However, in case the rainfall intensity considerably exceeds the anticipated amount, the system may struggle to cope with the large influx of rain and even fail.

Therefore, typical precipitation amounts for the area are accounted for already in the design stage. The size of the system is also determined by considering how often the typical rainfall intensity is likely to be exceeded.  

“It is not optimal to install a setup for precipitation amounts that may occur only once every few decades,” says Maciej Teterka, Technical Support Specialist at Pipelife Poland. “For such cases, designing emergency overflow systems is a better and more cost-effective solution.”

_{Tailored siphonic drainage system design}

To ensure customers can always count on fast, professional design services, Pipelife works with top-tier designers specializing in building services and boasting in-depth expertise in siphonic drainage solutions.

• Each system is meticulously designed and individually fitted to ensure optimal water drainage and prevent potential issues like flooding or roof collapse.

• In a few days of receiving information on roof plan drawings and cross-sections, Pipelife’s experts create an optimized design and share it with the customer.

• The design services are available for Pipelife’s customers in all markets.