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Large photo of a 44 gallon open drum water tank
Steel 44 gallon open drum water tank.   The inlet is a 65 mm PVC downpipe. and the overflow outlet is 25 mm irrigation pipe which connects to a wetland area.  The pipe probably should be larger but as the roof catchment area is fairly small this outlet usually copes. 

The roof catchment area is three quarters of a garage roof.  It usually takes around ten millimetres to fill the tank up form almost empty.