I am planning a more general post about water preps shortly but as this project was just completed, I thought I’d post about it now……

I was recently looking to add a water butt to my garden but then I found the getcomposting website where local authorities have teamed up to offer discounts on water butts and composters.  They had a buy one get one half price deal so I have just added two 190L water butts to garden instead of one.  As far as my unknowing other half is concerned, they are purely for watering the garden as ‘rain water is better for the plants than tap water’.  In reality, whilst that reason is true and holds up, the butts are primarily there to augment my water preps.  They give me 380L of combined filterable and/or treatable water supply should I need it.

Locations

I have a shed and a summer house in the garden so I decided to add a butt to each one.  Each butt came with a stand, tap and diverter kit to install in the downpipe.  Obviously it is important to decide on the best location around your garden buildings.  They need to be easily accessible but they are not the most attractive element of your garden design so having them out of site might be important to you.  I installed guttering to the back length of each building and fed into the butts located just out of view from further up the garden.

 

Calculating Rainfall Collection

I was fairly confident that the roofs of the buildings would result in an adequate rate of rainfall collection but it didn’t hurt to do some calculations to check.  The basic formula for calculating water run-off for a roof is:

Area of roof (sq.m) x depth of rainfall (mm) = Maximum water run-off in litres.

I had two roofs to calculate for and they are different designs.  The shed was a ‘pent’ or ‘shed’ roof and was 4.3m long and 2.5m wide.  The summer house was a ‘hip’ roof and was 4.7m long and 2.5m wide

Pent Roof

Pent Roof

Hip Roof

Hip Roof

 

 

 

 

 

 

 

 

The calculation for the shed roof was straightforward.  If I assumed a rainfall of 10mm:

4.3m x 2.5m * 10mm = 107.5 Litres

Some of the water will stay on the roof or in the gutter and evaporate. So, not all of that run-off will make it into the water butt.  Being conservative, say that 10% is lost which gives an adjusted figure of 96.75 litres for every 10mm of rainfall.

The calculation for the hip roof is a little more involved.  I didn’t actually climb on the roof to measure how much of the available roof was lost to the side ‘hips’.  I started the calculation with half of the total roof area (0.5). Then, again being conservative I allowed a 25% overall reduction due to the hips and evaporation (0.75).  So, for the summer house, the calculation is:

(4.7m x 2.5m * 0.5) * 10mm *0.75 = 88.13 litres for every 10mm of rainfall

Assuming those figures are correct, I get a combined renewable rainwater store of 184.88 litres per 10mm of rainfall.

 

Final Notes

This renewable water supply is useful in normal day-to-day life. Should the need arise it can all be used as it is for sanitation or treated, filtered or boiled for drinking and other uses. The addition of extra  butts or by tarpaulin collectors to feed the same butts quicker increases the capacity.  As part of anyones prepping, water butts are an extremely useful addition.