Basements (dig-out)

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 most areas of Greater London and other areas of the UK where property prices are at a premium such as Oxford and Cambridge.

Creating a Basement Under an Existing Property

Habitable space underneath an existing property is created using a technique known as “underpinning”. This involves increasing the depth of the existing foundations to allow earth to be safely removed from underneath the building in order 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. They can be considered as either:

• Mass concrete underpins – Constructed from concrete and not containing significant steel reinforcement
• Reinforced concrete underpins – These contain steel reinforcement that is designed to work structurally

Waterproofing a Dig-out Basement

Figure 1: Basement with dual-layer internal waterproofing; tanking slurry and membrane.

In either case it is not practically possible to apply a waterproofing system to the external face of the basement wall. For this reason the choice of waterproofing systems is limited to those that can be applied to the internal face of the basement wall.

Safeguard offers two main types of system for internal waterproofing of such basements. These are typically used in combination as shown in Figure 1.

Primary water-resistance can be achieved by applying a Drybase Tanking System (Drybase Active Crystalline Tanking Slurry) to internal face of the concrete walls and slab. A secondary drainage layer can be applied on top of the Drybase system using the Oldroyd cavity drainage membrane 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.

Basement Waterproofing Contractors

Basement waterproofing is a job for a skilled underpinning contractor. If you would like a quote from a basement waterproofing company in your area, please contact our technical department on 01403 210204 or submit an enquiry.