Underground Pipes

It’s important to make sure you are getting the right type of pipes and fittings that will be underground…..you don’t want to be digging them up in case of blockages.


Some plumbers will want to use, and bury, 90 mm diameter rainwater pipes, basically plastic downpipes.

You need to make sure you are getting as a minimum 100 mm PVC pipes rated as DWV.(Stands for Drainage, Waste and Vent).

The reasons are:

  • The 90 mm pipe is thin walled and can be easily deformed once buried. This means you lose capacity without realising it. The DWV is a much thicker and thus stronger pipe.
  • Although the increase in pipe diameter is fairly small the flow capacity of the larger pipe is over 40% higher. That makes a difference in storm conditions.

Pipes are normally marked at 1m intervals with the type, manufacturer, nominal diameter, and the Standards reference (AS/NZS1260) printed on the pipe every m (see below).



Bends are available in 4 different angles for DFW pipes as follows: 15 degrees, 30 degrees, 45 degrees and 90 degrees.

Although 90 degree bends are available, I would NOT install them underground due to the blockage risk….. If you need a 90 degree change of direction underground:

  • For a drain or a sewer use a junction pit.
  • For a charged (pressure) rainwater system use two 45 degree bends with an inspection ‘T’ in the middle.


  1. tim

    Is it illegal or against the BCA to bury 90mm pvc for a charged system

    1. Brian Ashworth (Post author)

      Not illegal but it doesn’t conform with BCA and Plumbing Regulations…………. But neither do a lot of plumbing installations.

      Based on the Plumbing Industries own statistics its likely that there are thousands of non-compliant installations every year. (See this article for an example http://www.smh.com.au/nsw/hidden-report-confirms-gutter-debacle-20110725-1hx55.html )


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