Last week I installed a water butt which collected rainwater off our outhouse roof using a clever bit of guttering bodging and plastic piping. We have been receiving a huge amount of rain over the past few days and as a result my water butt is full to bursting already. Its got me really interested in how much this little roof of mine can actually collect, so hunting the web I found various wierd and wonderful ways of calculation roof runoff. The most simplest being:
- Take the dimensions of the footprint in question, in this case my outhouse roof.
- Multiply the roof dimensions by the average rainfall
- Divide by 231 to get the number of gallons
So with this in mind then my calculation is as follows:
- 96 inches x 120 inches = 11520 sq inches
- 96 x 120 x 35 inches (Wiltshire’s yearly average rainfall)
- ((96 x 120) x 35)/231
So the answer to my question, how many gallons of water can my outhouse roof collect in an average year is…1745.45 gallons. And that really surprised me.
11:31 am on November 19th, 2009
But remember also that is just the run off - you’ll only collect that much if you empty the butt (excuse me) each time, you can always link butts together to make an overflow to ensure you don’t waste any precious water. There are four butts in Rode and because pumpkins and tomatoes are grown there, we always seem to run out at some point. The giant pumpkin alone drinks around 15-20L a day! We also only collect from the workshop roof, if the house was linked up I reckon that would produce a huge amount more.