Surface water entering the foul water system through gully dishes can cause problems for sewer network utility operators in flood-prone areas.
This information was confirmed as current in February 2016. It originally appeared in Codewords newsletters prior to January 2014. Changes to G13/AS2 3.3.1(a) consulted on in 2006 were not made.
Several parts of the Building Code and its Acceptable Solutions relate to this situation. Compliance with both E1 Surface Water and G13 Foul Water is required.
Building Code Clause E1 Surface Water
The Building Code Clause E1 Surface Water covers what is commonly called stormwater. There are two relevant parts:
- E1.3.1: Surface water, resulting from an event having a 10 percent probability of occurring annually and which is collected or concentrated by buildings or sitework, shall be disposed of in a way that avoids the likelihood of damage or nuisance to other property
- E1.3.2: Surface water, resulting from an event having a 2 percent probability of occurring annually, shall not enter buildings. Performance E1.3.2 shall apply only to housing, communal residential and communal non-residential buildings.
For convenience, in this article we will call:
- an event with a 10% probability of occurring annually the one in 10-year flood
- an event with a 2% probability of occurring annually the one in 50-year flood.
G13 Acceptable Solution
G13/AS2 states the following about surface water entering the foul water system:
3.3.1: All gully traps shall be constructed to prevent the ingress of surface water and foreign bodies likely to cause a blockage, shall be located within the legal boundary of the land on which the building is erected, and shall have:
- a) The overflow level of the gully dish no less than either:
- i) 25 mm above paved surfaces
- ii) or 100 mm above unpaved surfaces
Comment: It is imperative that the waste pipe connections to the gully trap remain watertight to prevent the ingress of ground/surface water.
In 2006 we consulted on, but did not incorporate, an amendment to G13/AS2 3.3.1(a) as follows:
- 3.3.1: (a) The overflow level of the gully dish no less than the height required to prevent the ingress of surface water, resulting from an event having 10% probability of occurring annually, from entering and either:
- i) 25mm above paved surfaces
- ii) or 100mm above unpaved surfaces.
- It is imperative that the waste pipe connections to the gully trap remain watertight to prevent the ingress of ground/surface water.
- Gully dish height is required above the 1 in 10 year event to protect the network utility operator's sewer system from surcharge problems.
All new building work must comply with the Building Code. In this case, both Clauses E1, Surface Water, and G13, Foul Water, apply. To achieve this:
- the floor level must be as given in E1/AS1, providing the location is not within a flood prone area
- if in a flood prone area, E1/VM1 applies. Depending on circumstances, this requires the floor level to be 150mm or 500mm above the one in 50-year flood level
- the top of the gully dish must be above the one in 10-year flood level to meet E1.3.1. In addition, the Network Utility Operator may have further requirements to avoid damage to its network (see E1.3.3(e)).