If you are getting ready to install a septic tank or a new drainage run you will need to be able to get your levels right here is how you do it:

First find out what fall you will need on the drains:

  • Minimum gradient for surface water drainage = 1:100
  • Minimum gradient for foul water drainage = 1:40 (1:80 if wc connected)


Fall = Distance ÷ Ratio

If a drain length is 20m and the ratio is 1:40


20,000mm ÷ 40 = 500mm