Information about building over sewers and surface water soakaways
What do I do if I want to build over or within three metres of a sewer?
This requirement applies to work carried out:
- over a drain, sewer or disposal main which is shown on any map of sewers, or
- on any site or in such a manner as may result in interference with the use of, or obstruction of the access of any person to, any drain, sewer or disposal main which is shown on any map of sewers.
Undue risk in the event of failure of the drain or sewer
Buildings should not be constructed over or within three metres of drains or sewers in fine sands, fine silty sands, saturated silts, and peat unless special measures are taken in the design and construction of the foundations to prevent undue risk to the building in the event of failure of the drain or sewer.
Special measures do not need to be taken if the drain or sewer is:
- above the level of the foundations, and
- above the groundwater level, and
- no more than one metre deep
Special measures do need to be taken where a building is constructed over or within three metres of:
- any rising main
- any drain or sewer constructed from brick or masonry
- any drain or sewer in poor condition,
In these circumstances, the building would be exposed to a high level of risk in the event of failure of the drain or sewer.
Protection of the drain or sewer during construction
Any drain or sewer should be protected from damage by construction traffic and heavy machinery.
Where piling works are being carried out care should be taken to avoid damage to any drain or sewer.
Buildings or extensions should not be constructed over manholes or inspection chambers or other access fittings on any sewer serving more than one property.
Satisfactory diversionary routes should be available so that the drain or sewer could be reconstructed without affecting the building.
Protection from settlement
- Where a drain or sewer runs under a building, at least 100mm of granular or other suitable flexible filling should be provided around the pipe.
- On sites where excessive subsidence is possible, additional flexible joints may be advisable or other solutions adopted, such as suspended drainage.
- Where the crown of the pipe is within 300mm of the underside of the slab, special protection should be provided.
- Where a drain or sewer running below a building is less than two metres deep, the foundation should be extended locally so that the drain or sewer passes through the wall.
- Where a drain or sewer runs through a wall or foundation, suitable measures should be taken to prevent damage or misalignment.
- Where the drain or sewer is more than two metres deep, to invert and pass beneath the foundations, the foundations should be designed as a lintel spanning over the line of the drain or sewer. The span of the lintel should extend at least 1.5m either side of the pipe and should be designed so that no load is transmitted on to the drain or sewer.
- A drain trench should not be excavated lower than the foundations of any building nearby.
If a sewer is to be built over, we will be expecting one of the following measures to be taken:
- If there is sufficient space to divert the sewer around the building at a future date should something go wrong, the protective measures mentioned above will be sufficient,
- Where there is not sufficient space to divert the sewer at a later date an inspection chamber will be required at each end of the sewer prior to its entry under and re-emergence from under the building and the pipe relaid under the new building.
How do you design surface water soakaways?
Soakaways must store the immediate stormwater run-off for its efficient infiltration into the adjacent soil. They must discharge their stored water sufficiently quickly to provide the necessary capacity to receive run-off from a subsequent storm. The time taken for discharge depends upon the soakaway shape and size and the surrounding soil's infiltration characteristics. They can be constructed in many different forms and from a range of materials. The following information should only be used as a guide.
If the stormwater run-off is to be taken to a soakaway, there are certain things that you need to do:
- Let your Building Control Officer check the soil around the building
- Calculate the roof area to be drained into each soakaway
- Ensure soakaways will be sited at least five metres from buildings and the boundaries of the site
- Let your Building Control Officer inspect the drainage to the soakaway
If the soil is not considered to have an adequate degree of permeability or the roof area to be drained into the soakaway exceeds 100 square metres you will need to:
- Carry out an on-site infiltration test
- Decide on the construction type for the soakaway
- Calculate the required storage volume and
- Consider space requirements, site layout and time for emptying
The above should be carried out in accordance with BRE Digest 365. The calculation process is complicated and you may wish to seek help.
In most cases where the soil drains well, and the roof area is less than 100 square metres, you will be able to construct a traditional type soakaway.