Using basements to maximise space without expanding a building’s footprint is increasingly popular, but keeping groundwater out can be difficult, and remediation costly. James Berry of the Property Care Association looks at the options
Although hidden away, structural waterproofing can be particularly sensitive to failures, largely due to poor workmanship, the inappropriate use of materials, or simple bad design. As a result, a range of factors need to be carefully considered when undertaking such works, to ensure a positive outcome.
Architects often approach our organisation – the Property Care Association (PCA), which represents the UK’s structural waterproofing sector – for advice. There are some key considerations when designing a water-tight basement, as follows.
It’s important to get help right from the start. Ideally, a design team will include a geotechnical specialist and a waterproofing specialist, and I would advise that both are engaged at the earliest possible stage. These specialists can give key advice on the ground conditions and in assessing the site.
It’s important that the geology, hydrogeology, and topography as well as the external drainage options and groundwater conditions of a site, are carefully considered. The type of soil and the water table also need to be evaluated, as well as any ground gas concerns.
There are a number of resources available to architects. BS 8102:2009 Code of Practice for Below Ground Structures Against Water from the Ground is considered the ‘waterproofer’s bible.’
Other useful documents include the Construction Industry Research and Information Association 139 and 140 Water Resisting Basements, as well as the PCA Code of Practice for Waterproofing of Existing Below Ground Structures.
Understanding the target grade of the environment is key to choosing a suitable waterproofing system. BS 8102: 2009 details three grades, each providing a different level of dryness. Examples of a grade 1 structure include a car park and plant room without electrical equipment, where it’s considered tolerable to have some seepage and damp areas. No water penetration is acceptable for grade 2 structures, and for damp areas that are tolerable, ventilation might be required. Similarly, grade 3 accepts no water penetration, but this performance level also specifies ventilation, dehumidification or air conditioning as necessary and appropriate to the intended use.
The target grade and site conditions inform which of the resulting three waterproofing systems, (Type A, B or C) is most suitable. In some cases, a combination of systems will be appropriate.
Type A, usually referred to as ‘barrier protection,’ is a structure with no integral protection against water penetration.
It relies totally on a waterproofing membrane to keep water out. Type A forms of waterproofing may be applied internally or to the outside of the structure or, in some cases, sandwiched between two skins of masonry or concrete.
Type B are structures built with a water-resistant shell, usually constructed out of reinforced concrete to an appropriate design code (which gives guidance in the grade of concrete to be used and spacing of the reinforcing steel). Special additives may also be used.
Type C or ‘drained cavity’ systems rely on a drained cavity within the basement structure. There is a permanent reliance on the cavity to collect groundwater that enters through the fabric of the structure. The drainage system directs the water to a drain or sump, where it can be removed from the building by gravity or pumping.
Contingencies & contractors
First and foremost, the construction teams applying, installing and building the underground space must be fully aware of the critical nature of what they are doing, the importance of accuracy, and the implications of any defects and errors.
Contingency planning for any localised defects must also be included as part of the overall water-resisting design for the structure. If feasible repair is not possible, then it may be required to look at what remedial measures may be available in respect to the risk posed by groundwater.
It is, therefore, essential to engage contractors with the necessary expertise in this complex, highly skilled aspect of construction. The PCA can signpost architects to members with the qualifications and experience to ensure their project is a success. The association oversees the industry-recognised qualification of Certificated Surveyor in Structural Waterproofing (CSSW), attained by professionals in the sector.
The PCA also has a register of waterproofing design specialists who have signed-up to annual CPDs, and are able to provide standalone design services. Along with a geotechnical specialist, they make up the core components of a waterproofing design team.
James Berry is the technical manager of national trade body the Property Care Association (PCA)