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Drain pipe size for water tank.

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  1. #1
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    Default Drain pipe size for water tank.

    Hi,

    Just have a quick question. I'm currently installing a water tank that is approx 7000 litres in capacity, it is connected to a colourbond shed via 90mm pvc stormwater pipe. The shed is 6m x 6m in size and has a gable roof.

    Now with the drain pipe would I get away with 2" or 50mm pipe?

    Is there a formula to work this out?

    Thanks in advance.

  2. #2
    Tool Whore - 1K Club Member Vernonv's Avatar
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    Hi Com_VC,
    It really depends on whether is a small garden shed or a big 5 bay machinery shed.
    What is the roof area?
    Why do you want to use 50mm pipe?
    Can you use 2 or 3 x 50mm pipes?

    I would tend to think that 90mm is probably the minimum size for downpipes.
    Cheers.

    Vernon.
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  3. #3
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    It's a 6 metre by 6 metre gable roof with gutters at each end that will feed into the tank. Not sure what the roof area would be.

    Main reason for wanting to stick with the 50mm is I dont have much space to work with and I already have all the 50mm pipe and fittings here ready to go.

    It will be feeding into a 90mm stormwater pipe underground.

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    Tool Whore - 1K Club Member Vernonv's Avatar
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    6 x 6 = 36 sqm = 36L/mm of rain
    So each gutter would get half that = 18L/mm or rain
    If we assume a 200mm/hr maximum rainfall intensity (I couldn't find what Melbourne's was) = 0.055mm/sec, which on half of your roof (i.e. each gutter) = 1L/sec that each gutter would have to handle.

    If each gutter is serviced by a 50mm downpipe to an underground 90mm pipe, you may just be ok. I'm just not sure what the rated flow of a 50mm pipe is.
    Cheers.

    Vernon.
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  5. #5
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    Quote Originally Posted by Com_VC View Post
    Hi,

    Just have a quick question. I'm currently installing a water tank that is approx 7000 litres in capacity, it is connected to a colourbond shed via 90mm pvc stormwater pipe. The shed is 6m x 6m in size and has a gable roof.

    Now with the drain pipe would I get away with 2" or 50mm pipe?
    g'day Corn_VC,

    the "correct" answer is that you need to talk to your plumber. there are actually rules & regs that cover this.

    the short answer is that you shouldn't be "downsizing" any of the pipework.
    if you have 90mm pvc storm as a downpipe from the roof, you should also have a 90mm overflow from the tank..

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    There is a ruling on this but ill have to dig it up to find the exact wording, though if you have a 90mm inlet running flat out how would you expect 50mm outlet to remove surplus water? Do it in 90mm as its cheap to use and easy to hook into the existing 90mm you have.
    Plumbers were around long before Jesus was a carpenter

  7. #7
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    Quote Originally Posted by Vernonv View Post
    6 x 6 = 36 sqm = 36L/mm of rain
    So each gutter would get half that = 18L/mm or rain
    If we assume a 200mm/hr maximum rainfall intensity (I couldn't find what Melbourne's was) = 0.055mm/sec, which on half of your roof (i.e. each gutter) = 1L/sec that each gutter would have to handle.

    If each gutter is serviced by a 50mm downpipe to an underground 90mm pipe, you may just be ok. I'm just not sure what the rated flow of a 50mm pipe is.
    I've got a 1200 litre Waterwall slimline plastic tank made from the original mould and another the from the newer mould. The original mould had 50mm overflows and the newer mould has 90mm overflows (there are also other differences).

    Anyway I had about 20 sqm of roof running through a single downpipe (100x50 then 90mm) into the one with 50mm overflow and it did on one occasion overflow from the top (the mesh inlet) because the 50mm overflow wasn't capable of handling the quantity of overflow.

    I am in Melbourne. I'd fit a 90mm overflow if that was what's on the tank, but I still have the same old 50mm overflow on my first tank as it is a bit difficult to retrofit a 90mm overflow. Also it doesn't really matter if the overflow occasionally (once every few years) comes out the top instead of where it is supposed to go as it just lands on a concrete driveway. 200mm per hour is extremely heavy for Melbourne. What we think of as heavy rain is just considered a shower in other parts of Australia.

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    Quote Originally Posted by totoblue View Post
    I've got a 1200 litre Waterwall slimline plastic tank made from the original mould and another the from the newer mould. The original mould had 50mm overflows and the newer mould has 90mm overflows (there are also other differences).

    Anyway I had about 20 sqm of roof running through a single downpipe (100x50 then 90mm) into the one with 50mm overflow and it did on one occasion overflow from the top (the mesh inlet) because the 50mm overflow wasn't capable of handling the quantity of overflow.
    the downpour & hail localized to SE Melb suburbs this last Thursday (first time i've ever seen a 'black' the highest-end of the rain scale on the radar above our place) caused overflow from a 90mm downpipe with a rainhead on top.

    the rest of the ones we have in our place are all 100mm storm.

    only lasted a few minutes, but it was some serious rain!

    Quote Originally Posted by totoblue View Post
    I am in Melbourne. I'd fit a 90mm overflow if that was what's on the tank, but I still have the same old 50mm overflow on my first tank as it is a bit difficult to retrofit a 90mm overflow.
    our plumber saw the same thing on 2 tanks we had with 50mm overflows on them. he said that if you are going 90mm or 100mm into them then you need to go the same out of them.

    we just cut into them with a sabre saw & made a 100mm overflow from them.

    in our case, those two tanks are getting the majority (probably about 150m2) of roof surface area.

    i cut the 100mm overflow hole then put 100mm storm pipe in and silicon'd it up so it was sealed. i'd say the tank doesn't have any less strength as a result, the 100mm storm (its sewer pipe, the stuff that handles UV) is pretty damn strong.

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    Enemy of mediocrity corbs's Avatar
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    We had a dodgy plumber install our tank with 90mm going in and 50mm going out (out was directly opposite to the in). As more could get in than out, overflow came back out the inlet side and eroded that side of the tank which then meant that no water went out the correct side. Was a pain in the a$$ to fix but its now 90mm in and 90mm out. Do it properly and you won't regret it. I still have to fix the float valve he put in too.
    I'd rather be a has been than a never was.

  10. #10
    House Husband - 1K Club Member
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    To calculate roof area you also need to take in the pitch. www.blocklayer.com has handy calculators for this sort of thing... There are also piping calculators avail on tradelink's website for Iplex http://www.iplex.com.au/iplex/calcul...essurecalc.asp

    I would use these as an interest thing as theoretical and practical are two different things. Thats where an experienced plumber can help.
    But the calculators are a bit of fun to plug in different senarios.

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    the downpour & hail localized to SE Melb suburbs this last Thursday (first time i've ever seen a 'black' the highest-end of the rain scale on the radar above our place) caused overflow from a 90mm downpipe with a rainhead on top.
    I went into Bayswater Mitre10 where I bought my 2100 litre tank and complained that it keeps overflowing! I cant seem to use more than a couple of hundred mm out of of it before it overflows again!
    Actually I was buying timber, you should have see the sad looks when I said I wanted to complain about the tank.
    And then the looks of relief

  12. #12
    Tool Whore - 1K Club Member Vernonv's Avatar
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    Quote Originally Posted by sundancewfs View Post
    To calculate roof area you also need to take in the pitch...
    Pitch doesn't really come into it when calculating the amount of rain that falls onto a roof ... the same amount of rain will be caught on a flat roof, as it would on a 45 degree pitched roof assuming the rain is falling vertically (or near enough to).
    Cheers.

    Vernon.
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    Quote Originally Posted by Vernonv View Post
    Pitch doesn't really come into it when calculating the amount of rain that falls onto a roof ... the same amount of rain will be caught on a flat roof, as it would on a 45 degree pitched roof assuming the rain is falling vertically (or near enough to).
    actually, technically speaking, any pitch will mean you get less water.

    e.g. if you had 100m2 of roof "surface" on a flat roof and 100m2 of roof "surface" on a pitched roof, the pitched roof will have less 'collecting' area.

    it probably makes sense to look at it as the area underneath the roof.

  14. #14
    Tool Whore - 1K Club Member Vernonv's Avatar
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    Quote Originally Posted by president_ltd View Post
    actually, technically speaking, any pitch will mean you get less water.
    Technically speaking a pitched roof or a flat roof on the same sized house doesn't make a cracker of a difference to the amount of water captured.
    So if you know the outside dimensions of your house/shed/whatever then you can easily work out how much water you will capture per mm of rain. There is no need to do any sort of complex calculations to take into account roof pitch to work out actual roof surface area.

    it probably makes sense to look at it as the area underneath the roof.
    If you look at the original calculations they were based on the building area (6m x 6m) and not the roof surface area.
    Last edited by Vernonv; 1st Dec 2008 at 12:25 PM. Reason: Added more info
    Cheers.

    Vernon.
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    Quote Originally Posted by Com_VC View Post
    Hi,

    Just have a quick question. I'm currently installing a water tank that is approx 7000 litres in capacity, it is connected to a colourbond shed via 90mm pvc stormwater pipe. The shed is 6m x 6m in size and has a gable roof.

    Now with the drain pipe would I get away with 2" or 50mm pipe?

    Is there a formula to work this out?

    Thanks in advance.

    In a downpour the 90mm pipe can deliver a lot of water, and when the tank is full the overflow must let that water escape, or backflood somewhere. A lot of pressure may be created.

    The cross section area of 90mm is 6,362 mm2.
    The cross section area of 50mm is 1,963 mm2 = less than a third of the 90mm cross section.

    The outlet should be larger than the inlet (to allow a safety margin) ; you could use 4x50mm or one 100mm pipe.

    Cheers

    Graeme

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    Cheers for the advice, makes sense when you think about it a little. Decided to go with the 90mm stormwater pipe. Bunnings have it for just under $15 at the moment for 6 metre lengths.
    <O</O
    I've just got another question. What fall should the pipe running from the gutters have. I will have a 6m pipe running horizontally from each gutter of the shed and then Tee'd into the tank. Should it be 10 degrees?<O
    <O
    Thanks<O

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    Also just one other thing I thought of, would any brackets be needed to support that length of 6 metre pipe or will it be ok without?

    Thanks

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    Minimum fall for stormwater up to 100mm is 1:100 or 10mm every metre and a bracket every 1200, so five saddles for every 6m length.
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    Cheers mate, any idea what that would be in degrees? I've got an adjustable level and that would make things a bit easier Somehow I have 10 degrees in my head?

    Ok with these brackets, going by your measurements it means I can get 3 into the structual part of the shed, the rest would have to go into the colourbond sheeting, I would have thought that would be a bit weak?

    What sort of brackets are they?

    Thanks


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