How to deal with drainage issues

Drainage problems can be caused by a number of reasons and often the immediate symptom of the problem is rectified, but not the underlying issue. The most common claim for both foul and storm water drainage is inadequate falls in underground pipework.

Understand how to identify and avoid this issue.

What are the requirements?

Approved Document H indicates the minimum permissible fall for 100mm pipework is 1:80, provided there are adequate flow rates over 6.3 litres/second. These requirements are reflected in Chapter 9 of our Technical Manual

What are the common problems?

Achieving a consistent fall can be very difficult. Bent or warped piping can occur particularly when 6m long plastic pipes are used, as these longer pipes can bend during storage.

Inconsistent falls can also be an issue. Failing to divide the total available fall between the junctions and chambers often leads to some sections with greater than required falls, followed by some sections towards the end of the run with less than adequate falls.

How to avoid drainage issues:

  • Ensuring an engineer has calculated adequate falls to accommodate 100mm pipes
  • Using 150mm diameter pipes if falls are suspect (1:150 achievable with 150mm pipes)
  • Checking falls that appear suspect with a 1200mm spirit level
  • Ensuring a suitably qualified person sets the invert levels of the inspection chambers between runs of pipework on the basis of the falls available
  • Ensuring pipes used are straight and not bowed excessively (particularly 6m long plastic pipes)
  • Using a taught line or laser between inspection chambers to ensure a constant gradient
  • Using adequate granular material to bed pipes on so that levels can be adjusted easily
  • Using suitably trained operatives to lay the pipes.

For full details of our technical requirements please see Chapter 9 of our Technical Manual.

Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.

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