But conversely, maybe you want a heavier-than-rated line. That's because a lot of creek-type fly-casting is a short range deal. Often no more than 20 feet. For example, by putting a 5-weight line on your 4-weight-rated 7-foot rod, there enough line weight in the air at 20 feet to fully load the rod. At 30 feet, however, the full weight of the heavier line would be in the air and your little 4-weight would probably feel overloaded.