Expert: Cailler - 12/21/2006

Question
What is the standard pitch for civil storm drain piping (outside the building)?  I'm looking at civil drawings and the slopes seem to be all over the map.  Does it depend on pipe size?

Answer
Hello and thank you for your question.

Several standards, depending on your country, are fixing minimum slopes for storm water piping, depending on pipe diameter and expected water heights inside the pipe. Pipe elbow angles or "T" section angles should generally be kept less than 45 degrees. The pipe section should also not be decreased along the direction of flow.

The DIN Standard, for example, gives minimum pitch for civil storm drain piping as follows: (DIN 1986 T 1.6)

Minimum slopes outside of buildings for storm water piping : 1/DN slope with h/d=1 (pipe full of water) for pipe diameter DN>150mm
(If ID<150, then h/d<0.7)

Example: If you have a straight pipe with normal diameter DN 300mm (generally minimum for storm water pipes), the pipe slope should be greater than 1/300 (0.3%) with a minimum water velocity of 0.5m/s. If you have elbows and a long straight pipe of course, the slope will have to be increased.

It is in practice generally not a problem to respect these minimum slopes related to pipe diameter, as you generally follow the natural uneven topography of a site and your piping is kept relatively short due to the placement of manholes. The topography is probably the main reason why you are seeing so many different slopes on your drawing.

Hope this could help clear your query.

Best regards