On Sat, 4 Sep 2010 09:28:05 +0100, Stan The Man wrote:

a normal hose will likely be sub-optimal as it requires more than
gravity pressure to send the water down its full length ...

Eh? My 50m hose works feed by gravity alone and from an outside tap
on a long convoluted 15mm pipe run from the tank. OK the tank is in
the roof space so about 20' of head compared to only a foot or so for
a butt but water will find its level.

Are there any figures on the leakage rate for leaky hoses and at what
pressure and unit lenght? Just a maths excercise after that if you
know the area of the roof that is being drained by the down pipe and
assume say 1mm/hr rain fall.

--
Cheers
Dave.