Most residential drainage systems start the same way. Roof runoff is collected at downspouts. That water is piped underground. Area drains are added where water ponds. Everything is sent to a ditch, swale, or low spot somewhere on the property or to the street. There’s nothing inherently wrong with that approach. In fact, it’s how most residential drainage systems should begin. Where things tend to fall apart is not in the pipe layout or the materials—it’s in the assumptions