Today, conventional systems are commonly used for roof rainwater drainage. In recent years, the use of siphonic drainage systems has become widespread, based on the principle that the pipes are full flow. With the full flow principle in the siphonic system, pipe diameters are reduced compared to traditional (conventional) systems. Thanks to a number advantageous features, especially in industrial and office buildings, this system has started to be used more in widely. In this study, a design example of a siphonic rainwater drainage system was evaluated. The same system has traditionally been calculated and the results compared in terms of cost.