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Avoiding problems over basement construction

More basements are being constructed by SME builders, but basement claims can be challenging to investigate and expensive to remedy. So how do you make sure you avoid potential problems?

In recent times, basements have become a relevant and attractive addition to lots of houses, with many constructed by SME builders.

But as new claims figures and experience shows, basement claims by their very nature are difficult to investigate and costly to repair, which can result in significant disruption to homeowners.

Disproportionate claims expenditure

A relatively small number of basements – just 14 – resulted in disproportionately high claims expenditure of £3m in the last year alone, according to our latest figures.

Claims on basements built since 2005 have cost the industry a total of nearly £21m, affecting nearly 890 homes.

Changing weather patterns, a growth in the popularity of basements and a push for more sustainable urban drainage mean that basements present an ongoing risk that needs to be effectively managed.

National House-building Council figures show that large loss claims in relation to basements (those costing more than £250,000) have risen from £400,000 in 2010 to £1.5m in 2011 and £1.7m in 2012.

When combined with other smaller claims, the total costs have risen from around £3m in both 2010 and 2011 to £4.8m in 2012. These figures are expected to rise further, as some claims are still ongoing.

Ensuring you avoid problems

The NHBC has undertaken two recent surveys of around 1,700 sites highlighting the main problems surrounding the construction of basements, resulting in advice for builders to ensure problems are avoided.

“The majority of the large claims from 2012 were as a result of water ingress and incorrect installation of the damp-proof membranes”

The research found that tanking defects were the main cause of claims, accounting for 66 per cent of volume and 62 per cent of basement claims since 2006. These related mainly to the detailing of joints at ground level, floor-level finishes and openings for services, pipes and cables.

The surveys found that more than a quarter (28 per cent) of sites surveyed reported high or unknown water tables, but still proposed a Type A (tanked) or Type B (water-resisting concrete) basement, which may not be suitable for the site.

The majority of the large claims from 2012 were as a result of water ingress and incorrect installation of the damp-proof membranes.

Revised guidance on the pipeline

The NHBC is now looking to revise its standards and guidance on basements, working closely with SME builders and the wider industry to improve basement design and construction.

It has advised the housing industry to carry out suitable desk studies and ground investigation reports before construction, as water levels are critical to the structural design of basements.

The surveys also highlighted the importance of adopting the correct waterproofing system and water stops to provide enhanced resistance at joints.

This new guidance, as a result of surveying hundreds of live sites, will hopefully inform the industry and help reduce the risk for homeowners having to endure problematic basements and inconvenience at a significant financial cost to the sector.

Technical Extra is available online on the NHBC website.

Mark Jones is head of housebuilding standards at the NHBC

Readers' comments (1)

  • Nothing surprises me in construction. For whatever reason many organisations such as manufacturers, suppliers, principal contractors, and even so called specialist trade groups claim to be experts in particular fields of construction but when you dig into them you find very limited technical knowledge or expertise.
    So hardly surprising things go wrong and claims are rife.
    CDM regs have tried to improve things but as ever price dictates. Procurement departments and specifyers should do their homework and look at qualifications, capability and third party references of contractors before they engage their services.

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