How do I build a watertight basement?

Michael Holmes says: There are several ways to construct a basement that will meet Building Regulations. A new version of the ‘Approved Document, Basements for Dwellings’ showing various ways to comply with the latest Building Regulations when constructing a basement is currently being reviewed.

There are two principal issues with basements: construction and waterproofing. The most appropriate construction system will depend on the ground conditions on site. A steel-reinforced raft foundation is commonly used for the floor structure, but in some ground conditions a suspended reinforced concrete floor may be used.

Common solutions for the superstructure include high density hollow core concrete blocks infilled with steel-reinforced concrete, interlocking precast reinforced concrete sheets, or reinforced concrete cast in-situ either in temporary sheeting or boarding that will contain the concrete until it hardens, or within Insulated Concrete Formwork (ICFs), such as Beco, Styro Stone or Logix. Typically, there will be an insulated cavity and an inner wall in masonry, or an insulated timber frame inner wall. With ICFs, there is no need for an inner wall as the expanded polystyrene formwork remains in place as the insulation. Plasterboard can be fixed onto the foam blocks, or onto studwork.

The excavations for the basement may be self-supporting, but in gravel, sand or other ground conditions, or where there are existing buildings nearby, it can be necessary to sheet pile (or to cast interlocking concrete piles) around the basement first to support the ground and then excavate within this. If the basement is constructed below the water table, it will be necessary to drain the excavations and pump away ground water (you may need a licence from the Environment Agency to dispose of the water into the storm drain network).

Making it watertight
Waterproofing can be in the form of a physical membrane, or a chemical damp proof course applied as a render or within the poured concrete. It can be used inside or outside a basement structure, or both. A common solution is a textured ‘studded’ membrane, which prevents water pressure building up against the basement that could lead to leaks through any tiny gaps. Any ground water that gets through the walls is channeled down into a sump from where it’s pumped into a drain. This may be combined on the inside of the basement with a chemical waterproofing system. In the case of ICFs, the use of waterproof concrete, cast in a single pour and carefully linked to the concrete floor forms a watertight structure in itself.

Basement construction is specialist work. Use a specialist contractor (contact The Basement Information Centre at basements.org.uk) who will offer a design and build service with an insurance-backed guarantee.

Featured in the December 2011 issue of Real Homes