Drains under slabs and G13/AS2: Key requirements

Where drains are installed under a concrete slab on ground, the key reference is G13/AS2. The Acceptable Solution specifies materials, gully trap heights, gradients, pipe diameters, jointing, access points, testing, and so on.

Where a drain passes under a floor, G13/AS2 section 5.8 requires that it shall:

  • be straight and of even gradient
  • be separated from the building foundation by at least 25 mm
  • be sleeved or wrapped in a durable and flexible material to allow for expansion and contraction when passing through concrete
  • have 50 mm clearance from the top of the pipe to the underside of the slab
  • have junctions beneath the building joining at an angle of not more than 45°.

A comment in the Acceptable Solution states that drains under buildings must meet the durability performance requirement of B2.3.1(a), that is, the life of the building being not less than 50 years. 

In addition, section 5.9 requires provision of access to the drain for cleaning where two or more soil fixtures are connected to a branch drain beneath the building. Access for cleaning shall be provided by a sealed floor level rodding point located downstream of the highest fixture connection to the branch drain.

Access points located within a building shall be in an area that complies with the isolation and ventilation requirements for spaces in which soil fixtures are located. They may be located in a space containing a soil fixture. 

From BRANZ Guideline, August 2016

Published in building.