Water butts & volume of rain coming off a shed

Anna33

Ok, possibly a bit subjective depending on how much rain we're getting at any one time, but that aside - I need to re-route guttering on our existing shed (approx 10' x 8', at a guess, with apex roof). The current set up is that the guttering from the left hand side is funnelled round the back of the shed through downpipes into a water butt on the right hand side. Pretty standard set up.

I need to remove that funnelling section of pipes (don't ask, long story!), so there will basically be standalone guttering on each side of the shed, unconnected to each other. The shed is about 1' - 1.5' from the house, on a concrete base laid on paving. We didn't install any of this.

I want to set up water butts to collect the water from the left hand side of the shed (and the right hand side, but that's not so tricky as better access/nearer soil for run off etc), but the only place for water butts is on paving slabs rather than soil, and is next to the house. I was going to get two linked water butts to allow for maximum rain capture/minimum run off onto paving when full, but I'm not sure on sizing.

My question (finally) - how much rain water typically comes off a one side of a shed roof of that size/design?? If I got 2x 200l water butts, that would be 400l capacity - how quickly would they take to completely fill up? Imagine a typical downpour, or a normal (!) winter's load of rain. I'm just thinking about run off when they're full.  Are those sizes about right, or massively underestimating how much water we'll collect???

PS would provide photos, but that area is temporarily being used to store loads of [email protected] whilst we've had work done elsewhere in the garden.

owd potter

Definitely a string theory question, as in how long is a piece.
Well your collecting surface area is about 5m2 so at an average rainfall where you are of about 750mm? (mostly at this time of year I would assume) you should collect about 3500l p.a, or about 18 x 200l butts.
So I would say 2-3 weeks to fill one, but that is a very rough guess.  
You could always start with 1 x 200l and see how you go, you have the option to connect a further one later if needed.
 

JennyJ

I can't answer the question about what capacity you'll need, but you can get water butt taps that you can attach a hose to with the hozelock-type fittings. Might be worth considering in case you do ever need to drain off the butts onto a soil area.

raisingirl

you could quite easily fill one 200l butt in an afternoon, or one decent thunderstorm. Probably 2 or 3 days of more normal rainy weather. We've got about 1400litres of storage for rain from our garage roof. It usually takes a couple of weeks to fill up in the autumn.

The other question though is how fast you use it. If you have a greenhouse, you'll continue to use water in the winter, but if not, the butts will fill up after the first week of rainy weather in late autumn and then will just overflow for the rest of winter until you start watering stuff again in the spring. 

My advice would be to put in as much capacity as you can fit/afford as it'll never be too much, and make provision for it to overflow for quite a lot of the time in winter - i.e. channel/slope it away from your house. 

BobTheGardener

All the water butts I've seen have an overflow hole marked near the top (which you have to drill-out yourself.)  If you have several butts at the same height, you can link them using 15mm PVC pipe, or even a piece of hose (which doesn't have to be horizontal.)  When the first one connected to the downpipe fills, water will flow into the next butt, and so on.

If one of the connected butts is in a more convenient place to dispose of the run-off when they are all full, drill another hole at the top of that one and use it to route the run-off to a drain etc.

Biglad

I can't really add too much to the discussion (in terms of an answer), but I put in a 100L butt collecting from half our shed roof (approx. 2.1m x 1.4m) last summer. It's nowhere near big enough for our climate/situation. 

The main reason for getting it was to try and reduce the waterlogging issue in that area but I don't have any run-off facility for the excess (other than into the waterlogged area I'm trying to dry out ). The end result is that I periodically fill a few buckets from the butt and pour them down the drain Not ideal!

Once some building work has been completed later this year, I'll probably get another butt or storage receptacle (as big as they come) to put at the side of the house. I'd fill that with any excess from the shed butt.

Anna33

Thank you all, this is so helpful. I appreciate there's no definitive answer as it depends on far too many variables, but I think from what you guys have said, that I definitely need to get as big a capacity as fits, and that I need to have a plan for how to deal with the overflow. Knowing there's the options to put a hose attachment on some taps is ideal, I hadn't realised that was a possibility. I think I will also be chucking a load of water away periodically, but at least capturing some is still way better than capturing none at all...!

Really appreciate the responses, thank you.

mikeymustard

I think I had this question in my maths gcse 1978!

JennyJ

I once had a maths question about the angle of raindrops hitting the side of a moving vehicle

Biglad

Surely it was an O' Level in those olden times, @mikeymustard  

JennyJ

@Anna33 , if you do decide to use a different tap from the one that come with the butt (and I think you'd only need to be able to use a hose to drain off the last one, if the others are set to overflow from one to the next), it's much better to fit them straight off while the butt is new, clean and dry inside. It's a messy job to fit a tap later when it's already got wet and mucky (been there, done that ).

(I rewrote that sentence because it could have been miscontrued the way I wrote it initially, talking about wet and mucky butts )