Due to the increase in restrictions concerning the piping of rainwater into the ground pipe, the retention of rainwater becomes more and more important. If there is no space for a retention basin or for seepage into the soil, the discharge into the ground pipe via the retention pipe with building-relatedly calculated retention openings will be strongly reduced. Consequently, only a small amount of rainwater flows from the roof into the ground pipe; the rest is held back on the roof in a controlled manner.

The small retention discharge (also called throttle discharge) guarantees that the water that has been retained on the roof can be piped into the ground pipe long after the actual rainfall. Thus, the discharge from the roof is spread over a longer period and the ground pipe is relieved at peak times. 

An urban residential complex with several part roof areas has already been equipped with diverse LORO-X parapet and flat roof drainage systems.

Six months after the completion of the project the municipality introduced restrictions for piping rainwater into the ground pipe in this residential area.

Thus, the existing drainage concept was „too powerful“. That is why the drainage into the ground pipe had to be throttled afterwards.

In the case of normal rain, the discharge into the ground pipe is strongly reduced by the retention pipe. Consequently, only a small amount of rainwater flows from the roof into the ground pipe.

The small retention discharge guarantees that the water that has been retained on the roof can be piped into the ground pipe long after the actual rainfall.

Thus, the discharge from the roof is spread over a longer period and the ground pipe is relieved at peak times.

The basis of the calculation is already determined by the municipality in form of the restrictions of piping water into the ground pipe. The maximum discharge rate of 0.7 l/s per drain (retention drainage) into the ground pipe may not be exceeded.

In order to calculate the amount of rainwater on the roof surface that shall be discharged into the ground pipe, the local rain factors, the size of the roof surface and the presumed duration of precipitation have to be taken into account. By taking the amount of rainwater on the roof and subtracting thereof the amount of rainwater that constitutes the retention discharge, one can determine the amount of rainwater to be retained on the roof (= difference volume).

In the next step, the difference volume in m3 is divided by the roof area in m2. The result of this calculation is the retention water height. In this case 60 mm.

On the basis of the expected retention water height and the given retention drainage, the size of the horizontal retention drain openings in the retention pipe are calculated (green arrow).

For the calculation, the LORO-X calculation programme for retention is used. Thereby a formula that has been confirmed through measuring is applied. The calculation ensures a discharge of exactly the amount of water from the retention water height that is demanded by the restrictions. If a permanent retention of water on the roof is desired, the retention drain openings can be adjusted. When adjusting the openings, the bottom edge of the retention drain openings should match the height of the permanent retention.

The LORO-X VARIOFIT retention unit is a building-related production that is delivered ready for installation. It can be easily installed on already existing parapet and flat roof drains of the series LORO-X RAINSTAR, DRAINLET and DRAINJET. Thus, it is suitable for new buildings as well as for renovation.

If it (a) rains longer than the planned minutes or if (b) the rain is stronger than specified by the DIN standard, the actual water volume will exceed the calculated water volume on the roof. Thus, the retention water height is exceeded and the emergency drainage has to take place into the open.

Due to throttling the main drainage to a small discharge, the emergency drainage has to be designed so powerful that at least the difference between the century rain and the retention discharge can be discharged. Usually, even an additional security measure is taken: the emergency drainage has to be able to drain the complete century rain alone.

The height of the congestion element of the emergency drainage has to be adapted building-relatedly to the retention water height of the main drainage with retention 

The maximum permissible water height indicates how high the water is allowed to congest during the operation of the emergency drainage. In this case 80 mm.

Consequently, for the operation of the emergency drainage only a function range consisting of the difference between the maximum water height and the height of the congestion element is available. In this case, only a small function range of 20 mm is possible although a high discharge rate of 8 l/s is required for the emergency drainage.

The emergency drainage is selected by comparing the flow graphs of different emergency drainage systems that reach the required discharge at the possible function range. Hence, in this case at a function range of 20 mm.

The interplay between main drainage with retention and emergency drainage with variable-height weir is especially important for the retention roof drainage because the weir height of the emergency drainage varies with the retention water height of the main drainage.

LORO-X VARIOFIT flow aids can be easily installed on already existing parapet and flat roof drains of the series LORO-X RAINSTAR, DRAINLET and DRAINJET.

Thus, they are suitable for new buildings as well as for renovation.