Basement design process (simplified)
- Establish basement use; current and future flexibility
- Site survey and exploratory works
- Design proposals to define type of construction, water tight class and thermal performance
- Detailed structural design integrated with design of waterproofing
The appropriate design of basements is well established and achievable, provided design and construction guidance is implemented. The general principle is to assess the risk of water reaching the below ground structure and to select an appropriate form of construction, structure and system of waterproofing to achieve the required internal environment.
To do this the designer needs to understand the expectations of the client, the proposed and likely future use of the basement space and its associated performance requirements in terms of building regulations. It is essential that an appropriate site investigation is carried out to establish the soil and ground water conditions. Evaluation of these factors provides the basis for selection of an appropriate construction method, structural solution and system of waterproofing.
It is strongly advised that a three dimensional (3D) review of structure and waterproofing is undertaken to identify and avoid any complex geometries, which will not be readily identified from normal twodimensional details.
See a simple checklist for minimising risk in basement design.
Roles and responsibilities
Aspects of the design process are inter-related and there are likely to be a number of options available; particularly for straightforward residential properties.
Of particular importance for new-build basements is a unified approach to establishing an appropriate design solution and defining the roles and responsibilities of the design team from the outset. It was common for the design of the waterproofing system to be the responsibility of the architect however, in BS 8102:2009 there is emphasis on including a specialist waterproofing advisor as part of the design team so that an integrated waterproofing system is created. This can be an architect or another consultant, manufacturer or supplier, provided they have the relevant expertise. An exception to this is when the construction method is classified as ‘structurally integral protection’; when it may form part of the structural engineer’s brief, a specialist waterproofing advisor may still be required.
The client should be advised of any implications related to choice of construction and waterproofing with regards to the expected building use, future flexibility and associated maintenance requirements.
Basement use - current and future
It is essential that the current and proposed use of a basement space is established early in design development, in order to provide the relevant performance criteria for the subsequent choice of waterproofing system, construction method and structural design.
BS 8102 designates building uses against three grades of water tightness. These range from car parking areas, where some seepage and damp patches are tolerated, to ventilated residential and commercial areas where no water penetration is acceptable. Standards and forms of construction and waterproofing suitable for each grade of usage are provided.
The previous edition of the British Standard referred to Grade 4 environments. This was omitted in the later version since the only difference from Grade 3 is the performance level related to ventilation, dehumidification or air conditions. BS 5454 provides specific guidance related to the storage of exhibition or archival documents.
A Grade 2 environment may be acceptable for permanent workshops or garages. However, since usage may change, it is better to construct a basement to a Grade 3 environment than to upgrade it later. In a high risk situation, the client and designer may wish to opt for additional waterproofing or vapour control.
See Grades of basements