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Poly water tanks: Plumbing the overflow away from the tank

When installing poly water tanks, one area of concern is that when the tank fills up with water and overflows, where does the water go? We see many poly water tanks on houses and sheds with a downpipe or two going into the tank, but nothing coming out. The water then overflows out the inlet strainer, down the side of the tank, and onto the ground. Or as you see in the photo below, the overflow kit is installed in the side of this water tank but with no pipework connected. Pipework is needed to divert the water away from the tank. With poly water tanks like this one, the water would flow straight out onto the ground. This will cause erosion around the base of the rainwater tank.

Plumbing the overflow to prevent erosion around poly water tanks

Plumbing the overflow away from the tank is important to prevent erosion around the base of the tank. This is especially important if you are using crusher dust or sand as the base for your tank to sit on. Most poly water tanks come with a 90mm overflow fitting so you can plumb off with standard 90mm stormwater pipe. Poly water tanks like the one in the photo below, come with a 90mm overflow kit that is able to be installed anywhere around the tank and can be fitted through corrugations as well as slight curves in tank walls. It does not need a completely flat surface.

Three options for where the water should go to prevent flooding

  1. Into the stormwater line. This is applicable if you are in an urban area and have a stormwater line out to the street.
  2. Pipe the water at least two mitres away from the rainwater tank. This is applicable if you live out of town. With this option it is very important that it is diverted down hill and away from your house or shed. Look carefully at where the water will flow once it comes out the end of the pipe. You do not want to flood a garden, courtyard, or shed every time your tank overflows. Depending on where the water will flow, you may need to pipe the water further away from your rainwater tank.
  3. Get another tank to catch excess stormwater if your rainwater tank often overflows and if space permits. This gives you the added benefit extra water catchment, which can be very important if you’re not on town water.  See the Rapid Plas large tank range here.