Calculating drainage capacity: Here’s how to do it
To ensure proper drainage indoors and outdoors, it is essential to verify that the installed drains and drainage pipes are suitable for the volume and pressure of water encountered at the installation site. In doing so, specific standards and/or the local rainfall volume, as well as the pressure resistance of the pipes used, must be taken into account. How should this be done?
- Determining the volume of wastewater generated
- Find a floor drain with the appropriate drainage capacity
Minimum flow rates in accordance with DIN EN 1253-1
The DIN EN 1253-1 standard specifies the minimum volume of water that drains of various nominal sizes must be capable of discharging at a water level of 20 mm. Many manufacturers only state a single figure (drainage capacity). Please note! This is very likely measured at a water level of 20 mm.
You should therefore check whether the technical product documentation also specifies the drainage capacity at a water level of 10 mm (optionally in a drainage curve).
Minimum flow rates for drains and conventional roof drains (not for pressurised flow)
Nominal size of the drain outlet 1) | Floor drains (gRost) | ||
DN / OD | DN / ID | Minimum flow rates l/s | Backwater height a mm |
| 32 | 0.4 | 20 | |
| 30 | 0.4 | ||
| 40 | 0.6 | ||
| 40 | 0.6 | ||
| 50 | 0.8 | ||
| 50 | 0.8 | ||
| 63 | 0.8 | ||
| 75 | 0.8 | ||
| 70 | 0.8 | ||
| 75 | 0.8 | ||
| 90 | 80 | 0.8 | |
| 100 | 1.4 | ||
| 100 | 1.4 | ||
| 110 | 1.4 | ||
| 125 | 2.8 | ||
| 125 | 2.8 | ||
| 150 | 4.0 | ||
| 160 | 4.0 | ||
1) Any dimensions not listed in this table must be tested using the next higher nominal size.
Practical example
Is the volume of wastewater generated compatible with the drainage capacity of the floor drain?
| Water flow rate | Drainage capacity of flat drainage systems |
| Standard shower heads approx. 12–15 l/min = max. 0.25 l/s | = 0.4 l/s Linearis Infinity 60 with odour trap at 10 mm backflow |
| = 0.5 l/s Linearis Infinity 60 with odour trap at 20 mm backwater | |
| Large rain showers approx. 20–30 l/min = max. 0.50 l/s | = 0.6 l/s Linearis Infinity 90 with odour trap at 10 mm backwater |
| = 0.8 l/s Linearis Infinity 90 with odour trap at 20 mm water column |
r(D, T): Calculated rainfall intensity in l/(s × ha) according to statistics
C: Runoff coefficient
A: Effective catchment area in m²
Minimum flow rates following rainfall runoff
All drains designed to collect rainwater must be dimensioned to accommodate the local rainwater discharge rate Q. This value is calculated using the formula provided here. Please note that during heavy rainfall, larger volumes of rainwater may occur for short periods. This must be taken into account by implementing appropriate measures, such as installing emergency overflows or providing pressure relief for gravity sewers.
Pressure resistance of the pipework
When selecting pipes, fittings, joints and brackets to be installed downstream of drains, care must be taken to ensure that they can withstand the positive and negative pressures expected inside the pipe. Where necessary, components with longitudinal force locking must be used to adequately secure connections such as push-fit sockets. For larger roof drainage systems, the exact internal pressures must be determined precisely by means of overload and flooding calculations.
