17 Dec 2019

The section relates to the design, supply and installation of stormwater soakage systems for residential and non-residential applications.

Stormwater soakage systems are required for properties with no reticulated system provided by council for removing run-off from roofs and sealed hard surfaces.  They can be:

  • soak pits consisting of holes filled with rocks and underground chambers with porous sides and base, lined with filter cloth to provide soakage
  • soak holes comprised of excavated pits with sides and top wrapped in filter cloth and back filled with scoria, circular scoria filled pit constructed from concrete blocks with a precast lid, and pre-cast concrete manholes designed for soakage in fractured rock areas

Pre-treatment devices are required to feed into the soak pits/soakholes and the type suitable will depend on the situation.  For example:

  • for paved areas of more than 20m² - sand filter, filter-strip and scoria trench apply (provide a high level of treatment)
  • for paved areas of less than 20m² - catch pit applies (provides minimal treatment)
  • for roof areas - settling chamber applies (removes coarse solids only)

Guidelines for soakage design including sizing are provided in Verification Method NZBC E1/VM1 or can be obtained from your local council.  The first step in designing a soak pit is to determine the soakage rate of the natural ground.  This is done by conducting a percolation test and, from the results of the test, determining the soakage rate.  As part of the building consent application calculations for soak pit sizing must be approved by the BCA.

7455 STORMWATER SOAKAGE SYSTEMS is available in Standard, Structural & Civil, Services, Landscapes and Basic.