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The Regs: Take care when building over drains and sewers

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Geoff Wilkinson looks at problems arising from failure to identify where drains run

One of the most frequent problems we see with small projects arises from the failure to check whether or not there are drains running beneath the footprint of a proposed building or extension.

Building Regulation H4 requires that when building within 3m of a public sewer, or within 1m of a public lateral drain, you’ll need water authority approval before work begins. Even if the drain is not a public sewer or shared drain, failure to identify the location of the drain can still cause major issues if there is a conflict between the line of the drain and the line of the structure above.

Failure to identify the depth of the drain can also cause problems. A shallow drain might be too shallow to enable drainage from the new project to function correctly, or it might clash with the floor slab. Equally, a deep drain might require significantly deeper foundations than were originally assumed or priced for. 

The RIBA Plan of Work for small projects places the production of drainage plans in Stage 4, but this is too late if the drains are found to clash with the building – which can require a full reapplication to be made for planning permission.

Instead I would recommend that a search for the location of drainage should be conducted at Stage 2, when the feasibility of the project is considered, and prior to obtaining a planning consent that may simply not be buildable.

Access points on a sewer may need to be relocated as part of the works and included in the design

It should be remembered that the Building Regulations state that in some soils, such as sands or silt, buildings must not be constructed over or within 3m of a drain unless special measures are in place.

Additional restrictions apply if the drain is a rising main, constructed of brick (a traditional Victorian culvert, for example), or if it is damaged or in poor condition.

Buildings and extensions should not be constructed over a manhole or inspection point on a sewer. Access points on a sewer may need to be relocated as part of the works and included in the design. The extension must also maintain a 3m zone to enable the sewer to be reconstructed in the future if the water authority deem it necessary, and that zone should also be accessible to a mechanical excavator, depending on the depth of the drain.

Building Regulations also restrict the length of drain or sewer that can be built over to a maximum length of 6m.

Lastly, if the drain is greater than 225mm in diameter or greater than 3m in depth, then again special consent will be required, which could affect the viability of the project.

In most cases applying for permission is simple and can be done using a self-certification questionnaire online, although some water authorities are much more helpful than others and a simple national system would help remove regional variations. If you aren’t able to self-certify, you can instead apply for an approved build-over agreement, which should be supplied to the Building Control body prior to the works commencing.

If a build-over agreement is required, then a number of important restrictions are likely to apply. These vary by water authority, but typically include:

  • All new works must be done in like-for-like materials and comply with the requirements of the latest version of ‘Sewers for Adoption’, in conjunction with ‘Protocol on Design and Construction of Adoption of Sewers in England and Wales’. For example, plastic drainage may not be acceptable if the existing drain is manufactured in clay. 
  • The proposed works must not transmit any additional loads to the sewer, and foundations must be taken below the invert (internal pipe base) level.
  • It is your responsibility to check and verify the invert levels and position of the public sewer prior to works on site.
  • Any sewers that are up to 1.1m deep from ground level to invert must maintain a  minimum 150mm gap away from the foundations.
  • Any sewers where the invert level is more than 1.1m below ground level must run at least 500mm away from the foundations.
  • If the invert level is more than 2m below finished ground level, any proposed foundations must be at least 1m from the sewer.
  • Piled foundations must be a minimum of twice the pile diameter or 1.5m (whichever is greater) from the outside of the pile to the outside of the sewer.
  • Driven piles are not permitted within 15m of a public sewer.

So: don’t leave it too late in the project to check where the drains run; ideally, pick it up during the initial site survey.

Geoff Wilkinson is managing director of approved inspector Wilkinson Construction Consultants www.thebuildinginspector.org

This article originally appeared in the May issue of AJ Specification

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