• As flagged up in “Building” last month, (Claims over problem basements cost millions – 2 August 2013), basements are a growing cause of concern for the UK housing industry. In 2012, NHBC reported significant costs with 14 claims together costing in the region of £3m. “Building” highlighted that research by the National House Building Council (NHBC), showed claims on basements built since 2005 have cost the industry a total of nearly £21 million and that 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 which needs to be effectively managed.

    The NHBC is not alone in its concern. UK registration figures highlight that 72% of basement constructions are within the London Boroughs. Earlier this year Kensington and Chelsea council published new draft rules to limit basements to a single storey and impose much tighter limits on how far they can extend under the garden area, following a huge surge in applications as owners sought to by-pass planning restrictions on changes to their homes above ground by massively extending their living space underneath.

    One of the most famous applications approved by the council came from Chelsea FC owner, Roman Abramovich for his home in Kensington. “Building” reported that research carried out by the NHBC has shown that over a quarter of sites surveyed revealed high or unknown water tables, yet proposed an unsuitable Type a (tanked) or Type B (water resisting concrete.)

    Enough is enough, however, and the NHBC has announced plans to revise its guidance on basements over the coming months and to work more closely with the industry to improve basement design and construction In its “Technical Extra (June 2013: Issue 10) “, NHBC advised completing a desk study and ground investigation report as the water table is critical to the structural design of a basement, assessment of buoyancy risks and the selection of the waterproofing system.

    Waterproofing measures should be designed on the basis of ground water to the full height of the retained ground, or on advice from a geotechnical expert regarding geology and hydrogeology. If tanking is being considered then professionals must check that the waterproofing of the basement is continuous right to the outside of the building and that there is a cavity tray to prevent bridging and is this sealed to the basement waterproofing. Basement waterproofing should also be checked to ensure it can withstand large loads form the walls above. Whilst tanking defects account for nearly two-third of claims, podium slabs share some of the blame.

    Recent large losses have involved properties adjacent to the deck surface suffering from rising damp and failed tanking where the membrane was cut to allow for drainage. Podium decks generally have a greater exposure to the environment, often with decorative features, including paving and planters. The below-basement performance is critical to the surface water drainage and waterproofing system to prevent leaking. Drainage maintenance risks and potential risk of perched water must be taken into account at the design stage.

    The NHBC has shown it is taking the issue very seriously. After its research also revealed that in over three-quarters of claims, the Site Notification and Initial Notice had not been completed correctly when declaring basement construction it is now requiring builders to declare basement construction on the SNIN form as part of its risk management process. One of its project managers will be involved in any basement development to assess the proposals and assist in achieving satisfactory construction.

    As a helpful hint, it is advised that sites with basement or semi-basements (for example, on sloping sites where only part of the lower storey forms a basement) should be clearly identified on the SNIN when the application for a Buildmark warranty is made. Going forward, it you are considering a basement excavation, you need to:

    • ensure proposals to use basements or semi-basements are identified when making the application;
    • ensure proposals comply with the requirements of BS 8102:2009 “Code of Practice for protection of below ground structures against water from the Ground”
    • complete a desk study and ground investigation report, clearly indicating ground water table risks or design for the worst-case scenario;
    • ensure waterproofing junctions are adequately detailed;
    • ensure podium decks have a suitable drainage systems with allowance for maintenance, especially when using decorative planters or within the vicinity of trees; and
    • ensure adjacent surface water catchment areas and surrounding ground water flows are limited.

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