Do you have enough roof?

I was talking to a dear friend the other day about my hopes and dare I use the term, plans, for getting our house running entirely off rainwater and my dreams for 40,000L-50,000L of water tank capacity. She brought up a very valid point, one I had not yet considered.

Do you have enough roof to collect that much water and do you get enough rain?

Got enough roof?

Good point. 😉

She put her husband on the phone, a man who never ceases to astound me with his ability to take a glance at an idea and then see the bigger picture. I see rain water tanks connected to down pipes and he sees roof space, location of down pipes, gravity feed, pumps, capacity and local rainfall. He’s pretty amazing really. 🙂 He gave me the mathematical formula to figure out whether or not we can fill 50,000L and explained what it really means too. Thank you! 😀

Don’t panic!

Now before you flinch or run away from the word “mathematics” do not worry. This is a very easy formula, particularly for those with lovely square (or rectangular) houses. 🙂 To work out if you have sufficient roof space to collect enough water to fill a tank you need to work out the square meterage of your roof space and then multiply it by your annual average rainfall. The thing is you’re not measuring how much roof you have but how much horizontal space it takes up. 🙂 The easiest way to do this is to measure your floor space of your house and any eaves too.

The formula is this:

Length of the house x Width of the house x annual rainfall = potential Capacity in Litres

L x W x AR = C

Jasper and I grabbed the biggest tape measure we own (50m thank goodness) and headed around the house to measure up. Our house is 9.6m wide excluding the small bay window area which was too tricky to work the maths for (which would be a few square metres at most) and 15m long. Our annual rainfall was a little harder to find for accuracy. The Ballan (Fiskville) figures run from 1928 until 1969 which isn’t all that recent these days (44 years of data missing) and otherwise there is Bacchus Marsh which is a lot closer to sea level than we are, or Ballarat which is quite a way away. I decided to base my figures on the old data of Ballan. 🙂

So, our formula works out to be:

9.6 x 15 x 571.2 = 82252.8L

Yes, we have enough roof to collect 50,000L. 😀

12 thoughts on “Do you have enough roof?

  1. says:

    Excellent information Jess. Thanks for sharing:)

    Sent from my iPhone


  2. Linne says:

    Awesome, Jess. I hope to be able to use this one day. I’m saving that formula. ~ Linne

  3. Lynda says:

    Im not going to bother measuring. If your house is big enough, then so is mine. Hmmmm Where to put 50,000L worth of tanks?

    • Ah, but what is your annual rainfall? I know for a fact that you’re on the dry side of town (attended a talk by City West Water back in 2008). It’s not just about the size of the roof but the rainfall too.

  4. narf77 says:

    I agree with Lynda (even though, as an accountant, I have a sneaking suspicion that she knows her square roots and her integers better than I know how to make kefir out of sesame milk 😉 ). We have a rectangular house roof. We also have a rectangular shed roof…a rectangular Mumbly Cumumbus shelter roof (prior to repurposing it was the wood shed), a rectangular chook shed roof and a small shed that strangely enough has a rectangular roof and ALL of them can be used to harness rainwater. That means that I should probably start saving up and buying several more enormous rainwater tanks if I wish to harness the water swooshing power of my roof and NO MATHS INVOLVED BWAHAHAHAHA! I LOVE it when a plan comes together :). By the way, can I borrow your good friends husband over the coming year whenever we come up against a mathematical problem in our studies? 😉

    • If he was mine to loan out I would lend his mind to all my friends. He’s a very clever man. 🙂
      We have our 3000L tank collecting from the shed, about 1800L collecting from the greenhouse and also providing thermal mass in the greenhouse, 1000L hooked to the chook and goat sheds and I’m tempted to hook up a tank to the kids cubby house to stop them draining my tank on the greenhouse. It should collect about 1000L! 🙂 The possibilities, as you always say, are endless!

      • narf77 says:

        And it is our job to extract and wring out every ounce of possible goodness from all of those delicious possibilities that we are faced with and have an absolute ball researching how to do it the cheapest, most sustainable way 🙂 Yah gotta LOVE Permaculture 🙂

        • Permaculture is the ONLY way forward in a post peak oil world. Permaculture applies to every single aspect of our lives. EVERYTHING can have more than 1 use. A bench top water filter can help purify water AND add humidity to the air. I love it for homeschooling too. I can turn a lesson in gardening into maths, cooking and of course horticulture. 😉
          A choko vine on a greenhouse will use the greenhouse for climbing, inside AND the outside of it for food production and the verdent leafy growth as shade as well as beauty. 4 in 1! 😀
          I see the coming changes to our society as huge, potentially overwhelming and quite sad at how har it will be BUT most of all as an incredible opportunity which inspires all the best aspects of human nature. We will need comrdeship and community, generosity, sharing, frugality and most of all, ingenuity. 😀 BRING IT ON!

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