Parapet Drainage with Siphonic Flow

FLAT ROOF >> The use of parapet drainage systems with suction power over the parapet is new. Pressure-resistant and backflow-proof drainage pipes are located in front of the facade, we describe the advantages.

Just like with internal siphonic drainage, the reliable performance verification is of high importance here as well. Planners and contractors must be able to rely on complete system solutions with manufacturer-guaranteed system design and performance.

It is therefore logical that the entire system from the drain through the fittings and pipes to the outlet should come from a single source.

Advantages of Parapet Drainage

• No penetrations into the building and therefore no fire protection issues
• Free usability of the interior space, as there are no pipe systems in the building
• No noise problems in the building
• Fewer downpipes and drains thanks to powerful siphonic flow
• Low installation costs thanks to quick installation with push-fit sleeves
• Planning reliability through data sheets with measured flow curve and CAD drawings

External Drainage for Flat Roofs

Parapet means roof edge upstand as a wall-like structure at the roof edge of a building together with the roof edge finish instead of a gutter. As already known from round gutter drainage, parapet siphonic flow systems are also an external drainage. The roof structure is planned with a slope towards the parapet or subsequently renovated with slope insulation towards the parapet, so that sufficient water can be supplied to the parapet drains.

Fewer Drainage Points Due to High Performance

Particularly advantageous in this context is the small number of necessary drains or low points due to the high drainage performance of the individual roof drainage systems. For example, the parapet siphonic flow system from LORO offers up to 32 liters per second drainage performance with one drain. It can replace up to ten conventional 500 millimeter wide parapet openings or up to ten standard parapet single drains with gravity flow as a main or emergency drainage system.

Main Drainage and Emergency Drainage

• Main drainage has the task of safely draining the water from the roof through the roof drainage system into the sewer during normal rain. A rainwater standpipe with cleaning opening at the end of the downpipe forms the interface to the underground pipe connection.
• Emergency drainage has the task of draining the water during heavy rain events not into the sewer, but into the open. This refers to freely floodable areas around the building or specially designed rainwater retention devices. An emergency drainage system with drain and downpipe for controlled drainage of rainwater is highly recommended. Conventional solutions such as spouts without downpipe or simple openings in the parapet as overflow outlets do not meet the requirements of modern buildings.

To prevent rainwater from constantly flowing into the open during normal rain events, the emergency drainage may only begin draining from a defined water level on the roof. To avoid having to install the emergency drain at a higher level, for example, LORO's emergency drainage systems feature a weir under the suction hood. This allows the emergency drain to be installed and sealed at the same level as the main drain. Particularly advantageous here is the control of the suction power of the water-air mixture, for which the weir as a retention element under the suction hood is of central importance. Under normal conditions, the water is retained at the weir of the emergency drain system. In the event of a heavy rain event, strong suction power is activated in a controlled manner in this rapid drain system, so that in an emergency the water is quickly sucked from the roof and discharged into the open.

The common term pressure flow can easily be misunderstood, as it is understood to mean a pressing water column. In fact, it is a hanging water-air mixture column that must be imagined as suspended from the roof drain, with the connected pipe system pulling or sucking through vacuum formation. The control of suction power is achieved through the shape of the entire system, formed by drains, pipes and fittings.

This means that depending on the shape of the entire system, the suction power develops with varying intensity. Essential for suction power is that the water-air mixture in the downpipe pulls the water-air mixture in the drain after it through vacuum. This pulling effect between the lower water-air mixture and the upper water-air mixture can be low (gravity flow) or strong (siphonic flow) depending on the system design.

As with a vacuum cleaner, the suction power is essentially controlled by the ventilation of the system. The ventilation opening of a vacuum cleaner, which can be pushed open and closed to change the suction power, is matched in this roof drainage system by ventilation openings at the drain and on the downpipe. The shape and size of all ventilation openings must be optimally designed to ensure safe gravity flow or safe siphonic flow.

Generally, however, especially for siphonic flow, a pressure-resistant and backflow-proof pipe system is always used. Steel drainage pipes have proven themselves for all types of roof drainage. Through the weir under the hood, these roof drainage systems achieve their full performance already at an absolute water level of 75 millimeters.

The safety of a parapet roof drainage system consists primarily of reliably maintaining defined limit values. To ensure this, the design and the resulting performance are decisive. The system design is defined as a CAD drawing and installed accordingly on site. Each of these systems includes an LX data sheet with measured flow curve, in which the water level on the roof (in millimeters) is compared to the drainage performance of the system (in liters per second). This allows planning which defined drainage performance the system delivers at which water level. The essential limit values are the maximum water level of 35 millimeters for main drainage with gravity flow, 55 millimeters for main drainage with siphonic flow, and - according to the manufacturer's standard - a maximum of 75 millimeters absolute water level for emergency drainage. Through the weir under the hood, these roof drainage systems achieve their full performance already at an absolute water level of 75 millimeters.

Conclusion: Take Advantage of Benefits

Parapet roof drainage systems - as gravity flow or siphonic flow drainage - maintain limit values. With just a few steps, the roofing contractor can create the drainage planning themselves. <<