9 February, 2022 (Updated 23 February, 2024)

How to calculate fall gradient when installing drainage pipes?

If you’re new to installing underground drainage systems, you might be surprised to learn that the pipes should be installed perfectly level. The pipes have...

What is a fall in underground drainage pipes?

If you’re new to installing underground drainage systems, you might be surprised to learn that the pipes should be installed perfectly level. The pipes have to be laid with a slight slope. A fall is the height by which a drainage pipe drops over a given distance to create a slope. There is a good reason why pipes need to be installed with a fall.

Why do you need a gradient on a drainage pipe?

If a drainage pipe is installed perfectly level, the wastewater will slow down and the solids will settle in the pipe causing blockages. However, you can’t just guess how steeply to slope the pipe you are installing. If you install a drainage pipe with too much fall - i.e. sloping too steeply - the water in the pipe will flow too fast and leave the solids behind again causing blockages. So what angle should you install your drainage pipe at and how can you check it is correct?

What is the recommended fall for a soil pipe?

The maximum fall for a waste pipe is 1 in 40. The minimum fall is 1 in 110 (apart from vertical pipes, but that’s a different story). So pipes with a gradient between 1 in 40 and 1 in 110 should have adequate flow to prevent blockages from occurring.

How to calculate fall and gradient.

When digging a trench for drainage, you can measure the fall with the use of a level and a tape measure or using a pipe laser level. For example, this Imex Laser IPLTG300 features a screen to show you whether you are achieving the perfect fall.

If you are just doing a one-off or DIY project, you might want to consider a more old-school trick like the one shown in this video:

What if the pipe is bent, will it cause blockages?

Drainage pipes can warp when they heat up in the sun if they aren’t stored correctly. If you have a badly warped pipe, you should avoid installing it because it is likely to lead to blockages. Consider cutting it into smaller lengths to create rocker pipes for instance, to avoid wasting it completely. The best option if you are a groundworks contractor that installs a lot of underground drainage pipes is to use The SiteStak System to prevent your pipes from warping in the first place.