Downpipe sizes and number has always been a grey area. The standard gutter for cottages is 125mm wide. The ave size of a downpipe is 100 50 mm. It is generally accepted a 110 50 downpipe will carry the water off 70 m2 of roof. Work out m2 of roof area , divide by 70 = no. of dp’s needed. Australian Standard HB39 – 1992 has some info eg. the cross section of a downpipe should be 1/2 the cross section of the gutter. But nothing on how many needed. The newer code AS3500 has some info but it was not included in 1995.