Digging Below Existing Buildings with Safeguard

Digging Below Existing Buildings with Safeguard

In the first of two articles, Hudson Lambert, Director at Safeguard Europe, looks at dig outs below existing buildings.

New basements remain a highly popular way of adding extra space, yet are not to be undertaken lightly – as recent high profile house collapses show. Apart from the structurals, it’s also crucial to get the waterproofing right.

The construction of basements underneath existing houses has been growing in popularity since the late 1990s driven by increasing property prices and demand for increased living space in city centres. The trend was originally limited to the most desirable parts of London, however, rising property prices mean that ‘dig out’ basements can now be cost effective in other areas of the UK where property prices are at a premium.

However, basements are an area of risk for the builder, as figures released by the NHBC demonstrate. Back in 2013, it revealed that claims on basements between 2005 and 2013 had amounted to nearly £21 million. Consequently, it is important that a specialist – with a Certificated Surveyor in Structural Waterproofing (CSSW) qualification – be consulted at the design stage of all projects.

Habitable space underneath an existing property is created using underpinning. Familiar to many through its use as a solution to subsidence, this involves increasing the depth of the existing foundations to allow earth to be safely removed from underneath the building to create the new basement space.

Underpinning relies on the ability of a wall to span unsupported for a short length. This allows a section of wall to be undermined and a new structure to be built directly underneath.

In this way, new sections of lower level basement foundation and wall can be built in a sequence until the existing wall is supported for its full length. There are several types of underpinning used in basement construction.

For obvious reasons, it is not practically possible to apply a waterproofing system to the external face of a basement wall constructed in this manner. For this reason, the choice of waterproofing systems is limited to those that can be applied to the internal face of the basement wall.

If a basement is to be a habitable room – a living room or bedroom – no water penetration is acceptable, according to the British Standard concerning waterproofing, BS 8102: 2009 Protection of below ground structures against water from the ground.

There two main types of system for internal waterproofing of such basements. These are typically used in combination as shown above.

Primary water-resistance is typically achieved by applying a tanking slurry Vandex to the internal face of the wall.

This then should be combined with ‘drained protection’, where water is directed down the walls into a channel running around the perimeter of the room using a profiled cavity drain membrane – such as the Oldroyd system (Oldroyd Xv).

This allows compliance with NHBC Chapter 5.4 which states that where Grade 3 protection (BS 8102:2009) is required, and below ground wall retains more than 600 mm, measured from the lowest finished floor level, the waterproofing design should include a combination of two types of waterproofing systems.

Specialists must specify

It is worth taking the time up front to choose the right system and getting the detailing right. Dig out basements can increase the profitability of renovation projects in any of the more expensive areas of the country. However, this kind of work is firmly in the domain of the specialist contractor and time needs to be taken choosing the right waterproofing system and getting the detailing correct.

Specialist waterproofing manufacturers such as Safeguard will be able to provide details of specialist contractors capable of carrying out this type of work.

For more technical information and diagrams click here.

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