SUMP SIZING When is a sump not a sump? When it doesn’t comply to code. APD outlines the requirements for Type 1, Type 2 and bubble-up surface water sumps. AUTHOR: ROSS MILLER
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TYPE 2 A Type 2 surface water sump needs to be 650mm x 460mm or 600mm dia, with 380mm to the base of the syphon trap. Often a concrete 450mm x 450mm sump is installed, creating a much bigger medium-term problem with faster silt build-up, as it is used for greater surface areas up to 40,000l/m2 than the Type 1.
Access caps and syphon traps The requirements for the use of removable access caps and properly-designed syphon traps are to ensure the long-term viability of the stormwater system installed. Being able to easily clean out sumps and associated pipework makes for ease of maintenance and collection of unwanted silts and debris in the system. The basic requirements outlined apply to both Type 1 and Type 2 sumps.
Rainfall requirements The Type 1 sump is deemed suitable for drainage of up to 4,500l/m2 and the Type 2 for an area up to 40,000l/m2, where the litre figure is the rainfall intensity for a storm with a 10% probability of occurring annually.
non-compliant sump is a pain for everyone: the person’s property on which it is installed, the tradesperson who has to replace it, and on occasion, the insurance agent who has to pay out on unnecessary damage. The Building Code is very specific and clear on sumps. The fact that you can get a cheaper product that ‘will do’, or ‘the inspector won’t notice’ isn’t an excuse, and often ends up making more work for everyone and a bad name for the installer. The requirements to comply are very clear in NZBC E1: TYPE 1 A Type 1 surface water sump must be 375mm dia wide with 300mm minimum from the base to syphon trap to ensure the silt has had time to drop out of solution. Often a 300mm dia sump is being found installed on sites, greatly reducing the capacity needed.
ABOVE: Figure 8 of NZBC Acceptable Solution E1/AS1 shows a Type 1 surface water sump, suitable for an area of up to 4,500l/m2.
Installing a Type 1 sump.
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ABOVE: Figure 9 of E1/AS1 shows a Type 2 surface water sump, suitable for an area of up to 40,000l/m2.
