Waterproofing of basements and other below ground structures

Also see:

5.4.4Ground conditions

The waterproofing system shall take account of ground conditions.

The ground conditions should be fully considered by the engineer and waterproofing design specialist in the design of the waterproofing system.

NHBC may request investigation and a report of the ground conditions where the below ground waterproofed structure:

  • retains more than 600mm of ground, measured from the top of the retained ground to the lowest finished floor level
  • comprises more than 15% of the perimeter of an individual building (e.g. terraced homes, apartment blocks and detached garages), measured on plan.

The ground conditions report should take into account appropriate investigations, as described in Table 1.

Table 1: Investigation of ground conditions

Further investigationGuidance and information
Desk study, including review of:
■ ground water and flooding issues
■ flood potential of the site
■ available ground water data
■ SuDS impact assessment
■ flood risk assessment
■ topography of the site
■ effects of adjacent surface finishes.
Contaminated or aggressive ground and/or
ground water conditions.
Testing required where there is the potential for chemically aggressive ground and/or ground water.
Water level change, including potential for flash flooding and waterlogging.Identifying likely fluctuations and short-term flooding events.
Impact assessment of ground water flow where the construction is likely to have a ‘damming’ effect. Interpretative report by a qualified engineer, hydrologist or hydrogeologist to include:
■ assessment of the direction of ground water flow
■ damming effects on the ground water regime
■ damming effect of adjacent structures.

Where it is necessary to establish the water table, a detailed hydrogeological assessment should be undertaken by a suitably qualified engineer, and include:

  • long-term water level monitoring over at least one year to capture seasonal fluctuations
  • short-term flooding events that typically occur during autumn and spring
  • information based on a suitable number of boreholes monitored at intervals of three months or less.