Waterproofing of basements and other below ground structures

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5.4.6Design considerations

The waterproofing of all elements, including walls, floors and foundations, forming below ground structures shall be suitable for intended use. Issues to be taken into account include:

  • grade of waterproofing protection
  • waterproofing systems, materials and components
  • interface with the above ground structure
  • joints, abutments and service penetrations.

Grade of waterproofing protection

Waterproofing systems should be designed to resist the passage of water and moisture to internal surfaces.

The waterproofing grade should be appropriate for the proposed use of the internal space and the equipment located within.

Table 2: Waterproofing grades

GradeDescription Generally required for:
Grade 3No water penetration acceptable and a dry environment provided where maintained by adequate ventilation. Habitable accommodation.
Grade 2 No water penetration is acceptable although damp areas are tolerated.Non-habitable areas, such as car parks, storage or plant rooms where the internal finishes are not readily damaged by moisture.
(Some water ingress may occur where openings are provided in car parks,
e.g. for ventilation. To minimise potential for standing water, refer to Chapter 9.1 ‘A consistent approach to finishes’. Car parks should be provided with drainage to a suitable outfall).
Grade 1 Some seepage and damp areas are tolerable, dependent on intended use. Retaining walls typically used to form external lightwells.
(Drainage may be required to deal with seepage).

Where there is doubt about potential use, minimum Grade 3 protection should be considered in the waterproofing design.

Waterproofing systems, materials and components

Components forming the waterproofing system should be predefined and assessed to demonstrate suitable performance.

The assessment should specifically consider compatibility where materials and components are intended to be interchangeable between systems.

The design information and documentation should detail waterproofing systems, materials and components in accordance with the manufacturer’s recommendations.

Proprietary waterproofing systems, materials and components should be assessed in accordance with Technical Requirement R3.

Interface with the above ground structure

Waterproofing should extend at least 150mm above the external ground level and connect with the superstructure damp-proofing. This can generally be achieved by linking the below ground waterproofing system to a continuous cavity tray.

The connection between the below and above ground waterproofing should be bonded and formed with appropriate materials.

Where the waterproofing is linked to the above ground structure via a cavity tray, the materials should:

  • compress to form a watertight seal
  • be capable of taking the load.

Bitumen-based materials in accordance with BS 6398 or suitable materials assessed in accordance with Technical Requirement R3 should be used.

Joints, abutments and service penetrations

The design of waterproofing systems should include the correct method and detailing to form joints, abutments and service penetrations, including those between:

  • the waterproofing system and superstructure damp proofing
  • horizontal and vertical waterproofing
  • system components.

The manufacturer should confirm compatibility between different materials where they are used to form joints.

Details of how junctions and abutments are formed should be provided to site personnel. Proprietary components that are part of, or compatible with, the waterproofing system should be used for complex joints, abutments and service penetrations.

Penetrations through the waterproofing should be avoided where possible. Where penetrations cannot be avoided, the design should detail the method of waterproofing to ensure that it is watertight and durable.

Penetrations, including those for wall ties, services and drainage systems, should:

  • be suitably separated to allow for proprietary seals to be correctly installed
  • account for differential settlement and movement between the structure/finishes and services.