Section 3: Subsoil drainage

3.1The provisions which follow assume that the site of the building is not subject to general flooding (see paragraph 0.8) or, if it is, that appropriate steps are being taken.

3.2Where the water table can rise to within 0.25m of the lowest floor of the building, or where surface water could enter or adversely affect the building, either the ground to be covered by the building should be drained by gravity, or other effective means of safeguarding the building should be taken.

3.3If an active subsoil drain is cut during excavation and if it passes under the building it should be:

  • are-laid in pipes with sealed joints and have access points outside the building; or
  • bre-routed around the building; or
  • cre-run to another outfall (see Diagram 3).

3.4Where there is a risk that groundwater beneath or around the building could adversely affect the stability and properties of the ground, consideration should be given to site drainage or other protection (see Section 4: Floors).

3.5For protecting low lying buildings or basements from localised flooding where foul water drainage also receives rainwater, refer to Approved Document H (Drainage and waste disposal). In heavy rainfall these systems surcharge and where preventative measures are not taken this could lead to increased risks of flooding within the property.

3.6Flooding can create blockages in drains and sewers that can lead to backflow of sewage into properties through low level drain gullies, toilets, etc. Guidance on anti-flooding devices is given in a CIRIA publication79.

3.7General excavation work for foundations and services can alter groundwater flows through the site. Where contaminants are present in the ground, consideration should be given to subsoil drainage to prevent the transportation of water- borne contaminants to the foundations or into the building or its services.

  • 1 Distribution of shrinkable clays and principal sulphate:sulphide bearing strata in England and Wales 1 Distribution of shrinkable clays and principal sulphate:sulphide bearing strata in England and Wales
  • 2 Example of a conceptual model for a site showing source–pathway– receptor 2 Example of a conceptual model for a site showing source–pathway– receptor
  • 3 Subsoil drain cut during excavation 3 Subsoil drain cut during excavation
  • 4 Ground supported floor - construction- see par 4.7 4 Ground supported floor - construction- see par 4.7
  • 5 Suspended timber floor - construction - see par 4.14 a (i) Diagram 5 Suspended timber floor - construction - see par 4.14 a (i)
  • 6 Suspended floor - preventing water collection-see par4.14a Diagram 6 Suspended floor - preventing water collection-see par4.14a
  • 7 Typical floors exposed from below Diagram 7 Typical floors exposed from below
  • 8 Damp proof courses- see par 5.5-b Diagram 8 Damp proof courses- see par 5.5 (b)
  • 9 Protecting inner leaf - see par 5.5c Diagram 9 Protecting inner leaf - see par 5.5c
  • 10 Protection of wall head from precipitation - see par 5.9(c) Diagram 10 Protection of wall head from precipitation - see par 5.9(c)
  • 11 Insulated external walls examples -see paragraphs 5.10, 5.13 and 5.17 Diagram 11 Insulated external walls examples -see paragraphs 5.10, 5.13 and 5.17
  • 12 UK zones for exposure to driving rain 12 UK zones for exposure to driving rain
  • 13 Window reveals for use in areas of severe or very severe exposure to driving rain - see paragraph 5.32 Diagram 13 Window reveals for use in areas of severe or very severe exposure to driving rain - see paragraph 5.32
  • 14 Accessible threshold for use in exposed areas- see par 5.33 Diagram 14 Accessible threshold for use in exposed areas- see par 5.33
  • Table 1 Volume change potential for some common clays Table 1 Volume change potential for some common clays
  • Table 2 Examples of sites likely to contain contaminants Table 2 Examples of sites likely to contain contaminants
  • Table 3 Examples of possible contaminants Table 3 Examples of possible contaminants
  • Table 4 Maximum recommended exposure zones for insulated masonry walls Table 4 Maximum recommended exposure zones for insulated masonry walls