90mm vs 100mm PVC for stormwater underground

• 

  Hazzard on 27/03/2019 - 18:59

  +2

  however 100mm PVC has higher flow rate

  Not as straight forward as you may think. Slope of the pipe also affects the flow rate.

  Based on what we do, we only propose 90mm pipe for vertical installments ie as downpipes. If it is installed underground, minimum 150mm and increase if higher flow capacity needed.

• 

  Ocker on 27/03/2019 - 19:19

  The price difference between 90mm and 100mm is sfa. So why not cater for the "1 in a 100 year event" and go for the larger size?

  • 

    Hazzard on 27/03/2019 - 19:36

    Referred to as 1% AEP (Annual exceedance probability) now and not 1 in 100 yr.

• 

  Switchblade88 on 27/03/2019 - 19:47

  -1

  Why not both??

  Seriously. 90mm INSIDE 100mm will provide an additional level of protection, especially where used underground. This ensures integrity if the outer layer is broken by a misplaced mower blade or car tyre… 10mm extra diameter is negligible for flow rate anyway.

  Only issue is it'll be the more expensive solution, as you're paying for X m of both sizes instead of one or the other.

  • 

    [Deactivated] on 27/03/2019 - 20:01

    +2

    I have not heard of anyone doing this.

    The solution of the mower or or a tyre hitting the pipe is digging deeper.

    • 

      Switchblade88 on 27/03/2019 - 21:23

      That'll work if you've got the gradient, yes. However in my neck of the woods we've not got enough road gutter depth to make that work - there's probably about 1m of fall across the entire town!

    • 

      [Deactivated] on 27/03/2019 - 23:30

      +1

      That's because it is not actually that big a deal when a stormwater drain pipe breaks underground and doesn't require this level of protection.

      They do do this on fuel tanks where it is expensive and bad for the environment.

      • 

        [Deactivated] on 28/03/2019 - 17:32

        That's really good thinking.

        It's essentially a protective conduit for a pipe. I like my estate to be temper proof but I'm not sure this would have any benefit for water pipes.

        I have a commercial grade size zero turn. PVC is like butter.

• 

  GangGang on 27/03/2019 - 20:12

  The bigger the better.

  • 

    AlienC on 27/03/2019 - 21:23

    That's what she said.

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  frostface on 27/03/2019 - 22:02

  It all depends on how much water it's catching, where you live (rain intensity) and the risk of flooding in your dwelling due to overflowing. 100mm has about 20% more capacity than 90mm. I would recommend this site: https://www.roof-gutter-design.com.au/ to get a basic understanding of what you need. The website looks like it was created in 1996 but it's actually really handy (and is up to date with plumbing codes).

  The important thing also is to ask for sewer grade pvc (also known as drain, waste & vent pipe), as it's much, much stronger.

  • 

    Euphemistic on 27/03/2019 - 22:54

    Wot ‘e said. No point using 100mm if 90mm is already capable of excess capacity.

    Work out how much water you need to get rid of then choose the pipe. If you can’t work out how big the pipe needs to be, flip a coin or work out how many eneloops you can buy with the difference in cost and go from there.

• 

  borntospeed on 28/03/2019 - 07:56

  +1

  AS3500.3 states that you need to design for a AEP 1% for box gutters and 5% for eaves gutters. If this is your stormwater drainage for a house, there's a very high chance it's going to be eaves gutter.
  Google BOM IFD and input the coordinates of your building. Multiply the AEP (mm/hr) by the roof area and divide by 3600 to get your total flow rate for the roof. Once you calculated that, you can then select the correct size pipe. You also need 600mm of cover over your pipe if it's a trafficable area.
  If you are worried about the mower, you can always use HDPE. It cost more than PVC but it's virtually indestructible. You can even drive a truck over it and it will not break, just deform.

• 

  nugley on 28/03/2019 - 18:24

  Some really good information here. Thank you for you input. Appreciate it.

  It's definitely helped me get a better understanding of what I need to do from here.