Substructure, Ground Floors, Drainage and Basements

5.4Waterproofing of basements and other below ground structures

This chapter gives guidance on meeting the Technical Requirements for the waterproofing of basements and other structures below, or near to, ground level.


This chapter includes guidance for walls, floors and foundations below, or near to, ground level that are intended to prevent the passage of water from the ground (including from sources such as run-off, burst pipes etc.) entering the building near to or below ground level.

Guidance for the following types of waterproofing systems is included in this chapter:

  • Type A waterproofing barriers
  • Type B structurally integral construction
  • Type C drained cavity construction.

Constructions that are at risk of coming into contact with water and generally require waterproofing include:

  • basements
  • semi-basements
  • below ground parking areas
  • lift pits
  • cellars
  • storage or plant rooms
  • service ducts, or similar, that are connected to the below ground structure
  • stepped floor slabs where the retained ground is greater than 150mm.

Types of construction that, depending on the findings of a risk assessment, may require waterproofing include:

  • external walls where the lowest finished floor level is less than 150mm higher than the external ground level
  • voids caused by split levels.

Typical examples of construction types:

Definitions for this chapter

For the purposes of this chapter the following definitions apply:

Cavity drain membrane Semi-flexible sheet designed to form a cavity that intercepts water penetrating the structure and directs it to a suitable drainage point.
See Type C drained cavity construction.
Combined systemFor the purposes of this chapter, a combined system includes:
Type A and Type B
Type A and Type C
Type B and Type C
It does not include Type A and Type A of different performance characteristics as recognised in BS 8102.
Ground barrier A barrier used to resist the ingress of moisture and/or ground gases into the building.
Lowest finished floor level The top surface of the lowest finished floor, including lift pit floors, car park surfaces and other similar surfaces. Excluding coverings such as carpet and tiles.
Type A waterproofing
A waterproofing barrier applied to the structural element being waterproofed, also known as tanking.
Type A fully bonded barrier Type A barrier that forms part of a composite structural wall, including liquid applied and cementitious systems. Post-applied sheet membranes are not considered to be fully bonded barriers for the purposes of this chapter. Typically, when membranes, they will prevent water from tracking from a defect in the membrane to a crack/joint in the structure.
Type A post applied membraneA sheet membrane applied to the completed structure typically with hot or cold adhesive.
Type B structurally integral construction Where the water-resistant properties of the retaining structure provide waterproofing to the building.
This chapter provides specific guidance for the use of Type B concrete systems cast in-situ, with or without water resisting admixtures. The principles are applicable to other Type B systems listed in BS 8102.
Type C drained cavity construction Construction that incorporates a cavity, generally formed with a cavity drain membrane.
Water is removed from the cavity via a managed drainage system.
Waterproofing design specialist A suitably qualified person co-ordinating the team involved in the design of waterproofing to basements and other below ground structures.
Waterproofing system A fully assessed and certified system of compatible materials and components used to provide waterproofing. These are normally considered to be Type A, B or C as defined above.
Retained groundIn this chapter retained ground levels are taken from the top of the retained ground to the lowest finished floor level.