What four water butts actually catch off a shed roof

01

The arithmetic, which is simpler than it looks

One millimetre of rain falling on one square metre is one litre, because a millimetre deep over a square metre is a thousand cubic centimetres. The whole calculation is that one line, and once it is in your head the rest is measuring the roof and reading the gauge. It is the only sum on this site and it took me a while to trust it, because it seems too tidy.

My shed roof is a simple pitch. What matters for collection is the plan area, the footprint the roof covers when seen from above, not the sloping surface, because rain falls vertically and the shadow of the roof is what intercepts it. The footprint is 3.8 m by 3.0 m, so 11.4 square metres. A wet month here of 70 mm therefore ought to deliver about 800 litres, which would fill every butt I own.

The local annual rainfall over my six years has averaged 641 mm, which on that roof is about 7,300 litres a year. Put like that it sounds as though water should never be a problem, and that is the trap. The water does not arrive when it is wanted, the butts cannot store more than 800 litres at a time, and a great deal of what falls never reaches the butt at all.

• Roof footprint 3.8 m x 3.0 m, so 11.4 square metres
• 1 mm of rain on that roof is about 11.4 litres
• Four butts, 200 litres each, total storage 800 litres
• Six year mean rainfall here, 641 mm a year

02

Measuring the roof and the losses on it

I measured the footprint with a tape at ground level, taking the drip line of the eaves at each corner and squaring it off, which took ten minutes and is accurate to a few centimetres. The gutter only runs along one side of the shed, so in practice half the roof feeds the butts and the other half falls off the back onto the path. That single fact halved my expected yield and I had somehow never sat down and noticed it.

The next loss is wetting. A dry roof takes a certain amount of rain before anything runs off at all, and on felt that has warmed up in the sun it is surprisingly generous. I tested it crudely by watching the downpipe during light rain: on a dry roof, nothing came out of the pipe until about 1.2 mm had fallen according to the gauge. After that the flow tracked the rain closely.

Then there is overflow, which is the big one. A butt that is already full during a 20 mm November night sends the whole lot down the overflow and away. Over the winter of 2023 to 2024 I estimated, from the daily figures and the state of the butts, that more than 2,000 litres went past me because every butt was full from the first week of December onwards.

03

The figures for one real year

For 2024 the gauge recorded 688 mm. Applying the half roof that actually feeds the gutter, 5.7 square metres, that is roughly 3,900 litres reaching the downpipe before losses. Subtracting an estimated 1.2 mm of wetting per rain event across 122 separate rain days takes off something like 830 litres, leaving about 3,070.

Of that, I believe around 1,800 litres went out of the overflow in the winter half of the year, when everything was full and the ground did not need watering. What I actually used, judged by how often I emptied and refilled the butts through the summer, was between 1,100 and 1,300 litres. So the four butts delivered perhaps a third of what fell on the collecting half of the roof, and under a fifth of what fell on the whole shed.

The figures are not a disaster, but they are a long way from the impression you get from the tidy millimetres to litres calculation. The honest headline is that in a dry July, my 800 litres of storage is around three weeks of careful watering for about thirty square metres of thirsty crops, and after that I am on the standpipe like everyone else.

04

How many watering days a full butt buys

A ten litre can is what I actually carry, and a run down the beds in dry weather is eight cans, so 80 litres. In July 2022, which was the driest month in my record at 9.5 mm, I did that every second evening for three weeks. That is around 840 litres from 800 litres of storage plus what little fell, which is exactly why the butts were dry by the end of the month.

Knowing that number changed the way I water rather than the amount I store. Eight cans thrown at the whole plot does far less good than five cans put into the three beds that are carrying fruit, and a mulch of grass clippings 50 mm thick on those beds cut how often they needed it from every second evening to every fourth. That halving is worth more than another butt.

I did add a fifth butt in 2025, and it has made less difference than the mulch did. Storage only helps if there is rain between the dry spells to refill it, and in the driest month of my record there was 9.5 mm, which on the collecting half of the roof is 54 litres. You cannot store your way out of that.

05

Three changes that were worth making

The first was linking the butts properly with a pipe near the top so they fill in sequence and the overflow goes out of the last one, rather than each having its own overflow. That does nothing for total capacity but it means all four are full after a modest wet spell instead of one being full and the rest half empty.

The second was fitting a gutter to the second side of the roof, which I did in the spring of 2025 and which should have been the first job I ever did on the shed. It doubles the collecting area to the full 11.4 square metres. I do not yet have a full year on the new arrangement, but the butts refilled after 14 mm in early September 2025 where previously it took closer to 30 mm.

The third was a lid on every butt and a fine mesh over the inlet. Lids have nothing to do with water quantity. It is about mosquito larvae, a drowned mouse in 2021, and the green soup that a butt becomes when light gets in. Water that has gone anaerobic smells foul and I do not want it anywhere near seedlings.