Collecting all the water that falls on the barn roof might prove cheaper than buying water of the local water company but only if you can store enough of the water and clean it enough to do anything other than flush the toilet with. You still need to filter it before you can do that.
IBC on a wooden pallet
You can store 1000 litres of water in an IBC (Industrial Bulk Container) you need a strong platform to mount it on as it will weigh over a ton when full. It’s recommended that you don’t pump greywater or rainwater so it needs to be mounted as high as possible, although you should try and filter the water before it enters the main storage tank. If you intend to use slow sand filtration the water needs to pass through at least one metre of fine sand before entering the main storage container, and ideally it should pass through a settlement tank and some filter to catch floating items before that. An empty 1000 litre IBC container weights approximately 62Kg, and the dimensions of a 1000 litre IBC Liquid water pallet are Height 1,160mm, Width 1,000mm and Depth 1,200mm
You can check the label on an IBC to see what has been stored in it last, Group Z is for low risk items like foodstuffs.
Extreme Hazard
Group I
Pack Group X
Moderate Hazard
Group II
Pack Group Y
Low Hazard
Group III
Pack Group Z
IBC Liquid containers are also available in include 640 litres, 820 litres and 1250 litres, they can all be moved with a fork lift just as you would move any wooden pallet. The 1250 litre IBC’s are slightly higher than the more common 1000 litre containers.
120 Litre plastic barrels
A 1 metre high 120 litre food storage container barrel could be used as the housing of the slow sand filter, there are plenty of these on ebay for £10-£15 each.