5.3.17 Drainage under buildings
Drainage system under buildings shall be suitably designed and supported to ensure an effective and satisfactory performance for the life of the building. Issues to be taken into account include:
- ground supported pipework
- suspended drainage
- examples of site installations
- drainage beneath specialist foundations.
Pipework support should take account of the ground conditions and ensure that the drainage is not adversely affected by ground movement.
Pipework under suspended floors should not be supported on ground or fill that is susceptible to movement without adequate provision being made to:
- maintain minimum design gradients
- protect against backfall
- protect against leakage.
See Clause 5.3.14 for Pipework passing through substructure walls.
5.3.17.1 Ground supported pipework
Where the ground is not at risk of settlement or heave, drainage can be installed in a trench with suitable bedding and backfill.
Where fill is used to support drainage on made-up ground at risk of settlement, it should be a maximum depth of 600mm, well graded, inert and without hazardous materials.
The fill should be placed and mechanically compacted in layers not exceeding 225mm in depth, to form a stable mass. Any fill in excess of 600mm which supports drainage should be designed by an engineer to avoid settlement. Drainage pipes should be bedded into the compacted fill.
In ground at risk of settlement, drainage can be laid in the normal manner, but at a steeper gradient than recommended minimums, to allow for any settlement and avoid backfalls. Easy or rest bend connections to above ground drainage can be made with proprietary settlement pipe sockets, which provide for more movement than a standard pipe socket connection. Alternatively, drainage can be suspended.
5.3.17.2 Suspended drainage
In ground at risk of significant heave or settlement, a suspended drainage installation should be used.
Pipe supports should be adjustable to achieve a consistent gradient and rigid enough for rodding without causing dislodgement. Support brackets should be suitable for an underfloor environment. Lightly pre-galvanised thin strapping can easily rust and collapse, so it should not be used. Purpose-made stainless steel or suitably galvanised brackets should be used.
Where suspended horizontal drainage is used, it should be supported as per the manufacturer’s recommendations, typically 900mm to 1,000mm centres for 110mm nominal diameter pipes and at every socket or joint. Long pipe runs (typically 5 or greater) should be designed to accommodate thermal movement.
5.3.17.3 Examples of site installations
Ground supported drainage are often seen bedded, backfilled and strapped to suspended floors as shown in photos 1 and 2. In many cases, the strappings are flexible lightly galvanised type, fixed to floor beams on one side of the drain. These are
unacceptable as ground settlement here can cause the straps to pull the drain out of alignment. Equally, such strappings are less able to resist movement of the pipework during rodding, thereby increasing the risk of disconnection and leakage.
Where a fully suspended design is adopted, rigid proprietary brackets which clamp around the drainage pipe and have adjustable threaded support rods should be used eg as shown in photos 3-5. Some brackets clamp to concrete beams, and
others are built into the floor structure. In each case, it is necessary to ensure the pipes are fully supported at the sockets and at horizontal centres in accordance with the pipe manufacturer’s instructions.
5.3.17.4 Drainage beneath specialist foundations
Where drains are located beneath raft foundations or where ground movement is likely, the design of the pipework and support system should be carried out by a suitably qualified engineer in accordance with Technical Requirement R5.
Last updated: 2nd January 2025