The maximum permissible water level on the roof is determined, on the one hand, by the structural capacity of the roof construction and, on the other, by the waterproofing heights at rooftop structures and penetrations.

The required water level in mm can be read from the discharge curve of the roof drainage system in the LX system data sheet at the planned discharge in l/s.

The aim of roof drainage is to keep the water level on the roof as low as possible, even during a heavy rainfall event, while still achieving a high discharge from the roof.

During planning, both must therefore be coordinated: which maximum permissible water level does the roof construction allow, and which required water level do the main drainage and the emergency drainage need according to the discharge curve in the LX data sheet?

LORO-X emergency drains with the patented “weir under the hood” allow a higher discharge with a significantly lower head of water above the weir than is necessary with conventional drains or scuppers.

This keeps the water level on the roof minimal with a LORO-X emergency drainage system – despite a slightly higher weir height. Example with a LORO-X emergency drain:

• Weir height of the emergency drain = 60 mm
• Head of water for the discharge of the emergency drainage = 15 mm
• Water level on the roof = 60 + 15 = 75 mm
• Safe: For emergency drains, LORO specifies the effective nominal water level on the roof and not just the head of water, so manual conversion during planning is not necessary.

The required water level corresponds to the water level at the planned discharge of the emergency drainage.

It is made up of the weir height of the emergency drain plus the head of water above the weir for the discharge of the emergency drainage and, for LORO-X emergency drains, is measured from the surface to be drained. With variably installed scupper pipes, the weir height corresponds to the bottom edge of the scupper pipe.

Example with a conventional roof drain according to the standard:

• Weir height of the roof drain = 55 mm
• Head of water for the discharge of the emergency drainage = 55 mm
• Water level on the roof = 55 + 55 = 110 mm
• Caution: Some roof drain manufacturers only state the “head of water” in the technical information for their emergency drains, so the weir height has to be added manually when planning the roof load. This often results in higher water levels on the roof!

The maximum permissible water level on the roof is specified by the planning and results, for example, from the following aspects:

• Structurally permissible roof load (1 mm water level = 1 kg/m² roof load)
• Door threshold heights and height of roof access points on roof areas in use
• Waterproofing height at cable and pipe penetrations
• Waterproofing height at parapets and upstanding building components
• Waterproofing height at skylights or PV systems
• Height of electrical installations on the roof
• Possible flotation of the roof build-up
• Possible overtopping of high points