I'm trying to sort out drainage for a large shed roof. It's 23 sqm. I'm going to stick gutters on it, but my nearest drain is the other end of my garden, 25m away.

My current plan is to drain the roof into a single, large (200 litre or similar) water butt behind the shed, then run 20mm MDPE pipe from there down the side of the garden and into the drain. Will this work? If my water butt is roughly a metre high and I put the drainage hole near the bottom, then I've only got a metre of head. I can raise the butt up, maybe half a metre.

I don't need any great pressure from the flow, just enough to reach the drain and to empty sufficiently between storms. I live in Leicester and the average rainfall is about 600mm per year.

I don't want to spend a fortune doing this.

I've looked around the net and on other forums, but no-one seems to have quite done the same thing. I asked a company about a similar setup but using a soaker hose instead of running to the drain, but they don't think there'd be enough pressure to force the water from the hose. I'd like to avoid using a water butt pump if possible - I don't want the maintenance, or the hassle for what is essentially waste water.

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    Key phrase: "rain barrel" – keshlam Jan 23 '17 at 23:38
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    @keshlam when searching online, "rain barrel" will get a lot more US hits, while "water butt" is the more common UK term. – Chris H Jan 24 '17 at 6:55
  • Just suggesting that websearch --for whichever term -- will produce many more good answers than asking here. – keshlam Jan 24 '17 at 14:18
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    I think the key feature to include in an installation of this kind is raising the butt off the ground (and if possible attaching the drain bung to the bottom of the butt). You don't really care how long it takes to empty the catch butt between storms but you should try to avoid a stagnant residue. Even a half metre of effective head when the butt is almost empty will insure complete draining. – A. I. Breveleri Feb 23 '17 at 16:54
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    @Alaska: The problem that OP was trying to solve was getting rid of the rush of water running off his shed during a rainstorm. He calculated that the volume would overwhelm any practicable drain, so he intended to use a catch butt (rain barrel) as a buffer. - If he saved the rain water that fell, the butt would not be available for flood control. – A. I. Breveleri Feb 24 '17 at 1:33

The annual rainfall isn't so much of an issue as how much you get in one go (before your water butt can drain). One millimetre of rain works out to one litre per square metre so 23 square metres of roof means you can collect 26mm of rainfall. I'm south west of you, but we probably get that in a few hours a few times a year.

Here's a calculator that should help you. Plugging your numbers and assuming the roughness value they prefill means you can drain 13 litres per minute (if that's 20mm bore, I'm not familiar with MDPE). That sounds to me like it would work even with some additional losses. But you would want to maximise the head, and consider what happens to any water that overflows, as this will be when the ground is already wet.

The soaker hose does need a bit of pressure, and it would dump the water onto wet ground - the advantage over just enough gutter to keep the rain away from the door probably isn't significant.

I would do it slightly differently - I'd take the pipe out near but not right at the top of the water butt. This would decrease the size of the buffer but increase the head (in a ground level pipe) and therefore the flow rate, while giving you some stored water to use in summer

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  • @AndyT I was perhaps a little too brief. Assuming the water butt is already full to the outlet, and you add a constant depth of water to it, the head will be more for a higher outlet -- you're effectively lifting the drainable stored water up with more water. It would never shift water faster overall than taking it out the bottom, but the extra head would mean that the flow started faster, offsetting the loss of buffer. – Chris H Feb 23 '17 at 16:39
  • @AndyT for me that's a significant advantage, which is why I mentioned it (I have two water butts for this purpose alone, so to some extent I treat the drainage as an extra). I'm about 200km (150mi) SW of the OP, with a similar climate, but my rainfall is skewed a little more towards winter so I need to water the garden in summer, sometimes quite a lot. I'm assuming an unbroken pipe to the same drain in both cases, so you can add the level of the outlet. – Chris H Feb 24 '17 at 9:34

So I've just noticed this in my history and decided to update with how it turned out.

What I proposed does work, and is still working. It's carried on working through heavy rain and snow over the winter. I haven't actually had any issues at all with this (unlike the guttering itself, which completely refuses to work in places...).

My setup in the end is much as described, with a 200 litre water butt sat on a platform roughly a metre off the ground. The 20mm MDPE is attached halfway down. This gives it roughly 1.5m of head, while not being at the bottom where it may collect all the rubbish that accumulates. There is still a tap on the butt, so the bottom half is available to use for my garden (were it not for the piles of stuff stacked in front of it).

All in all, the drain system cost me about 50 quid I estimate, with guttering extra. The only concern I have is that the pipe will clog with rubbish (foliage from nearby hedges) over time, especially with it being a small bore. So far this has been ok though, and hopefully the debris rots down quickly enough in the water to not become a problem. There's ways around that if it becomes an issue anyway.

Thank you very much to everyone that answered, you helped a lot and made it much easier for me to make a decision on this.

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Late answer, but I see you figured out the problem. I have a 207L ( 55 gal) butt barrel and a 65 gal (whatever L). They collect an amazing amount or leaves,needles, acorns, pine cones ,etc,etc. Then they grow mosquitoes. So pumping/draining water is a problem. Although the mosquitoes are great tropical fish food. I have tried a couple things to catch debris but have not found success. I do live in a temperate jungle with about twice your rainfall, so maybe you would not get as much debris.

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