A rainwater butt in Joan's garden fills from a flat roof of an equivalent area to 25 square metres. The butt holds 200 litres and is full when Joan starts using it in the summer. During the gardening season, 160 litres falls on each square metre of roof. Only half of this is collected as the rest evaporates. Joan uses 100 litres from the butt to top up her garden pond each week for 25 weeks. If there is not enough water in the butt, she completes the job with mains water which is rather expensive. During the summer months there is never enough rain to overfill the butt.