Kazim, this is an ELECTRONICS site, not a plumbing site. We do use drains, in things called Field Effect Transistors, but they would not help you.However, as engineers we are taught to think through problems. The size of pipe you need would be influenced by:- the size of your roof- the amount of rainfall you would expect- Whether you have to route all the rainwater into the lake or just onto your land surrounding your house / building (if you have to join several pipes you'd need a larger one for the aggregated flows)Most domestic drainpipes I have seen would be about 100mm diameter or maybe 100x80 rectangular. For larger commercial properties I have seen up to 300 mm pipes.Your local building supplier may have someone who can advise you further.
@kazim: David Ashton was a lot more polite than I was going to be. My knee-jerk reaction would have been to ask: "What part of Electrical and Electronics Engineering Design Forum didn't you understand?" Fortunately, David answered you first.
