5.4.6 Design 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
- step and level changes.
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#
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 recognise waterproofing may extend or continue across different substrates and/or other materials, products or building systems, or that materials and components may be interchangeable between systems. To ensure performance will be maintained checks should be undertaken on chemical and adhesive compatibility before installation. Manufacturers often have material compatibility data that can be referred to, however undertaking on-site tests prior to full application are recommended to verify predicted suitability. Substrates should be suitably primed, prepared, dried, cured, cleaned of dirt, dust or other debris and/or protected in accordance with manufacturer’s instructions throughout the site works.
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.
Steps and level changes #
Situations where steps and level changes occur within buildings are not uncommon and it is often necessary to consider where normal damp proofing arrangements are likely to become ineffective, resulting in a need for waterproofing to be applied.
Issues to be taken into account include:
- nature of both the substructure and superstructure constructions and their resistance/susceptibility to liquid water and/or water vapour. For example, use of timber or light gauge steel framed superstructures, requirements for drained and/or vented cavities, ability for timber sole plates to breathe, inclusion of insulation(s)
- height difference between finished floor levels
- height of soil retention and/or presence of voids
- robustness of damp proofing or waterproofing solution(s) proposed
- accessibility for repair
- ground conditions and hydrology
- inclusion of subsurface drainage
- provision and continuity of damp proofing to walls and/or floors, interface with the above ground structure (particularly relevant should hazardous gases be present)
- relationship between internal and external ground levels
- continuation at returns, possibly of a different construction type (for example a change from solid to cavity walls or from concrete to masonry construction). Such situations should be considered in three dimensions.
Last updated: 2nd January 2024