Calculate rain gutter - how to do it properly


The calculation of the necessary diameter of rain gutters and the entire roof drainage including the downpipes is a rather uncomplicated thing. In this article you will find the used values ​​in the DIN and the calculation process explained.

The calculation is done according to standardized sizes

According to DIN EN 12056-3, some - quite confusing - values ​​are needed to calculate the correct dimensioning of the roof drainage. Some of the values ​​can be looked up in tables, others you have to find out yourself.

Relevant values ​​in DIN EN 12056-3

  • rainfall
  • Rainwater runoff, also referred to as Q, and
  • Rainwater discharge dispenser, also referred to as qr
  • runoff
  • Nominal size (of pipes)
  • Roof floor area

The rainfall is the amount of precipitation that hits one hectare per second - that is, the intensity of precipitation in l / s per hectare.

The rainwater runoff is defined as the amount of rainfall in l / s that is fed to the runoff, the rainwater runoff is the rainwater runoff in relation to a certain area size, also in l / s per hectare.

The ratio of the two values ​​gives the so-called discharge coefficient. For non-water-storing roofs it is 1.0, because nothing is seeped or stored here, with gravel roofs 0.5 and green roofs usually 0.3.

Additional tables, which are listed in the DIN EN, can then be used to determine the exact ratio of the roof base area to the rain pipe diameter based on the precipitation values. That's a pretty nice math problem, though.

This is how the calculation works

  • Paper and pen
  • calculator

1. Determine rainfall

You need to determine the amount of rainfall and rainwater for your area

2. Share rainfall by number of basic connections

Where there are several basic connections, of course, share the amount accumulating.

3. Determine the dimensioning of the downpipe

You can do that from DIN EN 12056-3.

4. Determine the allocation of the gutter.

The DIN EN 12056-3 can also be used for this.

Tips & Tricks

If the whole job of calculating your hair makes you gray, just let them do it to an architect - there that calculation is almost a daily bread, usually in minutes. This saves you a lot of headaches and a lot of effort.


Video Board: How do I calculate the volume of stormwater runoff for my drainage system?