Building standards technical handbook 2019: non-domestic

The building standards technical handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004. This handbook applies to a building warrant submitted on or after 1 October 2019 and to building work which does not require a warrant commenced from that date.

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3.5 Existing drains

Mandatory Standard

Standard 3.5

Every building must not be constructed over an existing drain (including a field drain) that is to remain active.


This standard does not apply where it is not reasonably practicable to re-route an existing drain.

3.5.0 Introduction

The purpose of this standard is to ensure that existing drains continue to function properly without causing harm to the building, the drain itself or to the health of the occupants. The standard is not intended to prevent the construction of small domestic extensions over existing drain lines, but each installation should be looked at carefully to ensure the most effective and workable solution is chosen.

Generally, public sewers are nor permitted beneath buildings. Where it is proposed to construct a building over the line of an existing sewer, the sewer should be re-routed around the building. Permission will be required from the Water Authority for any work that is to be carried out to a public sewer.

In exceptional circumstances, if it not possible to re-route an existing sewer away from the building, for instance if a gap building site in a terrace is being developed, the Water Authority may permit a building to be constructed over it. Where it is necessary to build over a public sewer, approval of the Water Authority will be required.

Disused drains and sewers offer ideal harbourage to rats and frequently offer a route for them to move between drains and the surface. They can also collapse causing subsidence.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirement of this standard (regulation 12, schedule 6).

3.5.1 Existing drains

A survey should be carried out to establish the geography and topography of the building site and ascertain whether there are any existing field drains. Where a building site requires that an existing drain (including a field drain) must remain active and be re-routed or retained, particular methods of construction and protection should be carefully considered. The guidance contained in clauses 3.5.2, 3.5.3 and 3.5.4 should be taken into account and any new drain should be constructed in accordance with the guidance to Standards 3.6 and/or 3.7.

3.5.2 Re-routing of drains

Where a building is erected over a private drain, including a field drain that is to remain active, the drain should be re-routed if reasonably practicable or re-constructed in a manner appropriate to the conditions of the site.

It would be unreasonable for drains to be re-routed around a limited life building, but care should be taken that no undue loading is transmitted to the drain that might cause damage.

3.5.3 Re-construction of drains

The condition of any drain or sewer that is to be built over should be determined. If in poor condition and/or relatively shallow then consideration should be given to re-construction if re-routing is not reasonably practicable.

The strength of a pipeline should be determined, decided or specified before construction work is undertaken. Drains should be designed and constructed in accordance with the recommendations in BS EN 752-3: 1997 and BS EN 1295-1: 1998. During construction, it should be ensured that the assumptions made in the design are safeguarded or adapted to changed conditions.

Protection of drains - every drain or sewer should be protected from damage by construction traffic and heavy machinery. Providing barriers to keep such traffic away from the line of the drain or sewer may be appropriate. Heavy materials should not be stored over drains or sewers.

3.5.4 Drains passing through structures

Where a drain passes through a structure, including a manhole or inspection chamber, a detail should be devised to allow sufficient flexibility to avoid damage of the pipe due to movement. A rigid connection however may be appropriate if the drain and the structure are an integral construction on a rigid foundation. Where drains pass under or close to structures, similar precautions should be considered. Drains should be constructed and laid in accordance with the recommendations of BS EN 1610: 1998.

3.5.5 Sealing disused drains

Disused sewers or drains provide ideal nesting places for rats. In order to prevent this, they should be disconnected from the drainage system as near as possible to the point of connection. This should be done in a manner that does not damage any pipe that is still in use and ensures that the drainage system is watertight. This may be carried out, for example, by removing the pipe from a junction and placing a stopper in the branch of the junction fitting. Where the connection is to a public sewer, the Water Authority should be consulted.

Disused sewers and drains less than 1.5m from the surface and in open ground should be, as far as reasonably practicable, removed. Other pipes should be capped at both ends and at any point of connection, to ensure rats cannot gain entry.

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