Drainage below ground

5.3.10Septic tanks

Septic tanks shall have suitable drainage connections. Issues to be taken into account include:

  1. outfall
  2. flow velocity
  3. soakaways for septic tanks
  4. field drains
  5. underdrains.


The designer should ensure at an early stage that consent for discharge will be given, or select an alternative method of drainage. Certain locations and ground conditions may preclude the use of septic tanks. Septic tank sewage systems should have:

  • satisfactory outfall disposal
  • placement that accounts for topography and ensures that water is drained away from the building.

Where a septic tank drainage system is to be installed, NHBC requires:

  • evidence of a satisfactory percolation test
  • copies of relevant consents and approvals before work commences.

Flow velocity

A dip pipe should be provided with:

  • the top limb rising above scum level , and
  • the bottom limb extending about 450mm below top water level.

Soakaways for septic tanks

Soakaways in porous subsoils

A soakaway may be used where the outfall from a septic tank is to discharge to a porous subsoil at a level above that of the winter water table. Soakaway constructions generally consist of an excavation filled with brick bats or other large pieces of inert material, or unfilled but lined, e.g. with dry laid brickwork or precast concrete (porous or perforated) rings, from which the effluent may percolate into the surrounding ground. Proprietary septic tanks should be assessed in accordance with Technical Requirement R3.

Soakaways which are not filled should be covered by a slab incorporating an inspection cover.

The size of the soakaway should be determined as described in this chapter and the area of the bottom of the soakaway should equal the area of trench bottom in Chart 1 below.

Where the porous strata is overlaid by less permeable subsoil, a borehole may be permitted by the appropriate authority.

Soakaways in less porous subsoils

In less porous subsoils, a sub-surface irrigation system may be used, which should be designed:

  • using approved means to determine the percolation rate
  • according to the area of sub-surface drainage from which the length of land drain can be found, determined by the following procedure.

Percolation test procedure for septic tanks:

The results of the percolation test should be used in accordance with Table 6 to determine a suitable method of drainage.

Table 6: Suitable methods of drainage

Percolation value (s)Suitability for less porous subsoils
Up to 100Chart 1 to determine the field drain trench area.
Chart 2 to determine the pipe length to provide this area.
100 to 140As above, but underdrains are also necessary.
Over 140The soil is unsuitable for field drains.

Table 7: Capacity based on potential occupancy

Number of persons/bed spaces Minimum capacity (litres)
< 4 2700

Field drains

Field drains should be:

  • sited according to topography, ensuring that water is drained away from the building
  • formed with perforated pipe, laid at least 500mm below the surface
  • laid in trenches with a uniform gradient less than 1:200 with undisturbed ground 2m wide between trenches and at least 8m from any building and 10m from any water course
  • laid on a 150mm bed of clinker, clean gravel or broken stone (20mm–50mm grade) and trenches filled to a level 50mm above the pipe and covered with strips of plastic material to prevent entry of silt
  • backfilled with as dug material.

Where the level of the water table is expected to rise in the winter months to within 1m of the field drain invert, it is not acceptable to use subsurface irrigation.


Where underdrains are necessary, drainage trenches should be constructed a minimum of 600mm deeper than the pipe level specified in the design.

The lower part of the drainage trenches should be filled with pea gravel. A second system of drainage pipes should be laid on the bottom of the trenches to convey surplus drainage to an outfall in a surface ditch or watercourse.