How much roof area can you drain through one 90 mm stormwater pipe?

Discussion in 'Adding Value' started by brendio, 19th Feb, 2011.

  1. brendio

    brendio Member

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    I was just wondering if there is a maximum area of roof that you can discharge through a single stormwater pipe out of the curb. Our plumber, instead of piping the downpipe on the front corner of our house out to the curb 10 m or so away, has instead piped the stormwater (uphill slightly) all the way around the house to the opposite rear corner (about 35 m) to join up with the rest of the downpipes and has the entire house draining through a single 90 mm pipe to the curb on our side boundary (corner block). The area of the house would be just under 300 sqm. Just under half of that goes into a tank, but if we get a lot of rain (like recently) and the tank is overflowing into the stormwater pipe, that seems like a lot of water to be going through a little pipe and I am worried the gutters will overflow. The house was previous serviced by three stormwater outlets before we moved it.

    Any plumbers out there know if there is a rule of thumb for how much area a single stormwater pipe can handle?
     
  2. Pa1nter

    Pa1nter Member

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    I have around 330m2 of roof on my PPOR ,I have 2 x 100mm stormwater and when we get decent rain it cant keep up.

    Only one thing can happen is your gutters will back up and water can flow over the back of the gutters into the eves and into the walls of your house

    I had a similar problem with a lazy plumber,and after he finished ,I fitted rainhoods,so excess water flows over them instead of the gutters.
    You can see in the photo,simple but effective.
     

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  3. tiger97

    tiger97 Member

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    I think the code is generally a downpipe no greater than every 12meters.

    Hope this helps.

    Scott.
     
  4. RumpledElf

    RumpledElf all fun in the big city!

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    The stormwater plan the builder has given us has a good half dozen downpipes ... that all join together into one at the back, so effectively 150sqm going into one 90mm pipe. Probably need a decent rainhead for the back, but its not on the plan, and this is allegedly drawn up by an engineer. The rainwater tank itself only has one 90mm inlet and outlet.

    If there's standards, not sure what they are ...
     
  5. Rockstar

    Rockstar Member

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    I am currently building a 2 bed home which has been engineered for stormwater detention. Prior to the water reaching the detention tank there are 5 x 90mm downpipes which join into 2 x 100mm pipes as soon as they reach the ground - and then feed into the tank. The roof area is approx 100sqm and the gutter size is 150mm quad. The system is designed to handle most storm events (except 1 in 20yr event)
     
  6. brendio

    brendio Member

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    Thanks for the responses guys. I might have to look into those rain heads.

    I did a bit of googling and came up with this site with calculators for required downpipes and storm water outlets. Having a play with it (if I've put in the right numbers), it seems to indicate that one 90 mm stormwater outlet is enough (though it would be gushing 19 L/sec in a 1 in 20 year storm), but we need a lot more downpipes, like about 14. There are currently only about 3 or 4 on the main roof, and a couple more on window awnings.

    That said, a condition of our subdivision is that we are to provide two points of connection in the kerb for roofwater discharge (galvanised steel adapters). So there is obviously allowance there for more than one outlet. Don't know why our plumber thinks we only need one for such a big house and big block.
     
  7. Gools

    Gools Member

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    Brendio, what about the ground storm water too. Does that go into some grates and use the same pipe to the street as the down pipes?
     
  8. brendio

    brendio Member

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    No, not at the moment. It has a couple of acres to soak into. Once we subdivide though, there will be interallotment drainage for the ground stormwater to drain into so it doesn't cross neighbouring boundaries.