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Thread: Calculating the area of a roof

  1. 2nd May 2016, 08:40 AM #1
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    Calculating the area of a roof

    I should have paid more attention in school I know but can anyone help me here as its confusing me. I want to work out how much water I get into my tanks when it rains. So I thought if I measure the roof (pitched) and then multiply it by the mm I get in the rain gauge - I should be able to work it out? Issue is - do I have to measure the roof itself - or can I just measure the floors and then add a bit for the eaves. I'm confused because I think its the area that the tin measures rather than the footprint of the house ? If that makes sense. Any help gratefully appreciated as usual.
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  2. 2nd May 2016, 08:45 AM #2
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    The pitch will make little to no difference.
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  3. 2nd May 2016, 08:58 AM #3
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    Ian is right. The pitch will make little or no difference, unless it is an extreme pitch, and the rain mostly comes from the side. The amount of rain you get will be determined by the area inside you gutters. If you really want to Know the area of the roof and you don't want to get up and measure the length from gutter to gable, then you need know the hight of the pitch. Then you can do some triangulation.
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  4. 2nd May 2016, 09:03 AM #4
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    As you have written, area of the house in square metres plus say 10% to15% to get a gross building catchment area allowing for eaves/verandahs multiplied by mms the the rain gauge.

    In practice, roof pitch is irrelevant.

    This is litres (if I've written it correctly ).

    eg. 200 sq m gross building area with 15mm in the gauge = 3000 litres

    There will be nominal losses in the system (allow 15%)
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  5. 2nd May 2016, 04:22 PM #5
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    As above. You need the plan area inside the gutter outside edge. Depending on how the downpipes are plumbed, I doubt losses would be as much as 15%, I would think closer to 5%.

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  6. 2nd May 2016, 04:33 PM #6
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    Beautiful. Thanks all.


    cheers,
    D.
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  7. 2nd May 2016, 07:00 PM #7
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    If you want to know how much rain you collect off the roof the conversion factor is 1L of rainwater from every 1m2 of roof per mm of rainfall.
    So a 1000m2 roof will collect 1000L water from a 1mm shower if my sums are correct.
    Andrew
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  8. 2nd May 2016, 07:20 PM #8
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    What Landy Andy says is correct
    Cheers

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  9. 2nd May 2016, 08:28 PM #9
    DiscoMick Guest
    You could ask if your local council can supply your house plan. Or just guess...

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  10. 2nd May 2016, 08:37 PM #10
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    Quote Originally Posted by DiscoMick View Post
    You could ask if your local council can supply your house plan. Or just guess...

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    or measure it
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