wallaby1 wrote: I did use bath water and washing water many years ago when there was a dorught and hosepipe ban, that problem seems to be not so bad now, and I live elsewhere, my only reservations on using grey water on a constant basis would be the build up of detergents.
I use my barrels mainly for all the pots I have with plants in, in the summer it can be dry and water can run out but now with more barrels it doesn't take many small downpours to have enough until the next one, but I do think I could put another one next to the house for pots I have there. A good downpour and water goes over the end of the guttering where two different angled rooves meet with a gully between.
The existing barrels which take water from rooves do fill very quickly, the garage roof has been rebuilt to join with the shed roof, so it only has one slope which is around 16' x 9', the shed is a similar size. I have thought of putting another barrel next to the ones there with an overflow, it seems such a waste when water is need when there isn't much coming from the skies! I do use some water even over winter in the greenhouses, late winter into spring is sometimes dry too and some of the pots outside need watering, I know I was watering late in autumn to winter last year.
We rarely get even an inch of rain at one time, so considering your roof sizes and the rain you get, plus using it on the ground, you will need the biggest storage containers you can get your hands on. I would like to be able to hook up a hose pipe too but would need more space, larger containers.
The calculations would be a tricky one of how much rain you would get, rain gauges are normally fairly narrow tubes so how that would compute to shed rooves I have no idea! Now if you could measure what volume you had in a certain amount of time, no that wouldn't work either, rain falls at different rates of 'heaviness', think I'll pass on that one!
You should be able to get diverters from a DIY store, we have B&Q here where you can get all manner of DIY goods, building materials, garden items, kitchen units, flooring, tools, plumbing, you name it. As you have already got a downspout you would require a diverter, without a downspout it's just a matter of what we have done, making it go in the right direction.
In the pic the barrel next to the garage, the other two can just be seen, the black top only of the other side in view, the one at other side of the greehouse has a home made wooden top as it had no top. It was a purpose made water butt from the tip, with a very slow running plastic tap, and a leak in the bottom! A thick blob of silicone sealant fixed the leak.
The windows in the rebuilt garage were free too from our neighbour who does building jobs, they are old aluminium patio doors. Well one does need light for the plants to grow in there!