Getting compliance in basement projects

The UK continues to experience a boom in residential and commercial basement construction, with land in limited supply. Nik Ullfors of Visqueen looks at how to waterproof correctly to ensure compliance with the latest regulations

Any discussion of living in our cities invariably comes around to high house prices. This, coupled with the limited amount of land available for new builds and conventional extensions, has led to an explosion of new basement building with a 183 per cent increase in relevant planning applications since 2012 according to mortgage lender Halifax.

So, while many are turning to basement developments as a solution to these pressures, it is important to recognise the challenges that expanding underground bring for both the architects designing these structures and the contractors building them.

Getting it right

Unlike the regular walls of a house, which are in contact with water only sporadically, the walls of a basement can be in constant contact with groundwater, which leads to the danger of water ingress – causing damp, flooding and potential structural damage. Ensuring that a basement is therefore protected from groundwater is paramount.

There can be severe financial consequences for designers or builders who get this wrong with incorrectly installed waterproofing potentially leading to professionals being held liable. Between 2005 and 2013 the cost of these claims is estimated at £21m and has affected 890 homes. Added to this is the potential cost
of expensive remedial works to rectify botched waterproofing damage and families having to leave their homes, while work takes place.

NHBC refers to this issue in its guidance for basement waterproofing: “With this current boom in basements, and the frequency of below-ground waterproofing claims to registrations being circa 1600 times greater than foundation-related claims, below ground construction remains a cause of concern.”

The regulations – BS8102:2009

Basements must be designed and constructed to a minimum standard as defined in BS8102:2009 – the code of practice for the protection of below ground structures against water from the ground. The standard outlines the different approaches to waterproofing including the installation of external tanking with a waterproof membrane.

Traditionally, it has been the role of architects to draw up and recommend the waterproofing design for below ground projects. However waterproofing design is a highly specialised area and the vast majority of architects will readily acknowledge that they do not have the detailed training or expertise to undertake this function. Often the only reason they have been doing it is because there are so few people in the industry who are willing to undertake this role!

A lack of design expertise, combined with the need to provide effective overview of the installation on site can lead to incorrectly installed waterproofing measures and thus leaves professionals, contractors and developers vulnerable to claims.

Specialist advice

Waterproofing Design Specialists (WDS) are therefore essential to successful basement developments and are one of the recommendations given in BS8102:2009 which states: “A waterproofing specialist should be included as part of the design team so that an integrated waterproofing solution is created”. Often the use of accredited WDS professionals is also a requirement from structural warranty providers to ensure the required level of protection is delivered.

Identifying suitable specialist companies that have the appropriate design capability and resources can be challenging. Looking out for the latest qualifications is one way, such as CSSW (Certificated Structural Surveyor in Waterproofing). Ensuring the company has the necessary professional indemnity insurance helps architects identify a trustworthy provider and deliver an end-to-end design service and accredited product solutions.

CSSW professionals have an in-depth knowledge of the many considerations required for below ground construction. By understanding the sources of water, how it flows through the soil, and how it interacts with the structure they have the structural and geotechnical knowledge to be able to talk meaningfully to engineers.

Be it designing waterproofing barriers or structural, integral and drained cavity constructions, CSSW professionals will work with stakeholders, and provide full ownership of design briefs, CAD detailing and on-site support.

Building underground has been a response to pressures facing homeowners and businesses as space becomes increasingly scarce and expensive in our urban areas. Effective waterproofing is fundamental to the success of these projects and with the guidance of a qualified WDS professional, architects and designers can ensure that their building is protected from groundwater and meets all necessary standards before the first shovel breaks the ground.

Nik Ullfors is national technical manager of Visqueen