Building over sewers
H4. (1) The erection or extension of a building or work involving the underpinning of a building shall be carried out in a way that is not detrimental to the building or building extension or to the continued maintenance of the drain, sewer or disposal main.
(2) In this paragraph ‘disposal main’ means any pipe, tunnel or conduit used for the conveyance of effluent to or from a sewage disposal works, which is not a public sewer.
(3) In this paragraph and paragraph H5 ‘map of sewers’ means any records kept by a sewerage undertaker under section 199 of the Water Industry Act 1991 (a).
Limits on application
Requirement H4 applies only to work carried out:
(a) over a drain, sewer or disposal main which is shown on any map of sewers;
(b) 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.
In the Secretary of State’s view the requirements of H4 will be met if:
a. the building or extension or work involving underpinning:
i. is constructed or carried out in a manner which will not overload or otherwise cause damage to the drain, sewer or disposal main either during or after the construction;
ii. will not obstruct reasonable access to any manhole or inspection chamber on the drain, sewer or disposal main;
b. in the event of the drain, sewer or disposal main requiring replacement, there is a satisfactory diversionary route or the building or the extension will not unduly obstruct work to replace the drain, sewer or disposal main, on its present alignment;
c. the risk of damage to the building as a result of failure of the drain, sewer or disposal main is not excessive having regard to:
i. the nature of the ground;
ii. the condition, location or construction of the drain, sewer or disposal main;
iii. the nature, volume and pressure of the flow in the drain, sewer or disposal main;
iv. the design and construction of the foundations of the building.
Introduction to provisions
1.1 These provisions apply to the construction, extension or underpinning of a building over or within 3m of the centreline of an existing drain, sewer or disposal main shown on the sewerage undertaker’s sewer records whether that sewer is a public sewer or not.
1.2 Copies of the sewer record maps are held by the sewerage undertaker and by local authorities. These are available for inspection during office hours.
1.3 Where it is proposed to construct a building over or near a drain or sewer shown on any map of sewers, the developer should consult the owner of the drain or sewer, if the owner is not the developer himself. In the case of a public sewer the owner is the sewerage undertaker, who may be able to advise on the condition of the sewer or arrange an inspection.
1.4 If repair or replacement of a public sewer is required it will be carried out by the sewerage undertaker.
1.5 Where it is proposed to construct a building or extension over a sewer which is intended for adoption, it is advisable to consult the sewerage undertaker.
Undue risk in the event of failure of the drain or sewer
1.1 Some soils are easily eroded by groundwater leaking into the drain or sewer. Examples of such soils include fine sands, fine silty sands, saturated silts and peat. Buildings should not be constructed over or within 3m of drains or sewers in such soils unless special measures are taken in the design and construction of foundations to prevent undue risk to the building in the event of failure of the drain or sewer.
Special measures will not be needed if the invert of the drain or sewer is:
a. above the level of the foundations; and
b. above the groundwater level; and
c. no more than 1m deep.
1.2 A building constructed over or within 3m of:
a. any rising main (except those used solely to drain the building);
b. any drain or sewer constructed from brick or masonry;
c. any drain or sewer in poor condition (e.g. the pipes are cracked, fractured, deformed more than 5% or misaligned) would be exposed to a high level of risk in the event of failure of the drain or sewer. Buildings should not be constructed in such a position unless special measures are taken.
1.3 Buildings or extensions should not be constructed over a manhole or inspection chamber or other access fitting on any sewer (serving more than one property). Approved Document H1 Section 2, paragraph 2.53 provides that access points to sewers (serving more than one property) should be in places where they are accessible and apparent for use in an emergency. Buildings and extensions should not be located where they would remove such a provision where this already exists, unless an alternative access point can be provided on the line of the sewer at a location acceptable to the owner (i.e. the sewerage undertaker in the case of a public sewer).
1.4 A satisfactory diversionary route should be available so that the drain or sewer could be reconstructed without affecting the building. This route should not pass within 3m from the building. Where the drain or sewer is more than 1.5m deep and the drain or sewer is accessible to mechanical excavators the alternative route should also have such access.
1.5 The length of drain or sewer under a building should not exceed 6m except with the permission of the owners of the drain or sewer (i.e. the sewerage undertaker in case of a public sewer).
1.6 Buildings or extensions should not be constructed over or within 3m of any drain or sewer more than 3m deep, or greater than 225mm in diameter except with the permission of the owners of the drain or sewer (i.e. the sewerage undertaker in the case of a public sewer).
Protection of the drain or sewer during construction
1.7 Any drain or sewer should be protected from damage by construction traffic and heavy machinery. Protection may be provided by providing barriers to keep such traffic away from the line of the sewer. Heavy materials should not be stored over drains or sewers.
1.8 Where piling works are being carried out care should be taken to avoid damage to any drain or sewer. The position of the drain or sewer should be established by survey. If the drain or sewer is within 1m of the piling, trial holes should be excavated to establish the exact position of the sewer. The location of any connections should also be established. Piling should not be carried out where the distance from the outside of the sewer to the outside of the pile is less than twice the diameter of the pile.
Protection from settlement
1.9 Where a drain or sewer runs under a building at least 100mm of granular or other suitable flexible filling should be provided round 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 (see Approved Document H1, Section 2, paragraph 2.44).
1.10 Where a drain or sewer running below a building is less than 2m deep, the foundation should be extended locally so that the drain or sewer passes through the wall (see paragraph 1.11).
1.11 Where a drain or sewer runs through a wall or foundation suitable measures should be taken to prevent damage or misalignment. For further guidance see Approved Document H1 paragraph 2.24.
1.12 Where the drain or sewer is more than 2m deep to invert and passes 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 onto the drain or sewer.
1.13 A drain trench should not be excavated lower than the foundations of any building nearby. For further guidance see Approved Document H1 paragraph 2.25.