This study aims to develop a novel method by which visually to simulate water flow paths on building facades. Rainwater runoff affects the designs of facades in terms of safety and aesthetics because runoff leaves dirt and stains on an aesthetic facade. The first step is to review flow simulation algorithms and identify its limitations in fluid visualization research on flow path simulations. The second step is to review an existing CFD program to identify its limitations, and then establish the goals of the newly proposed method. The third step is to identify the properties and behaviors of water on a surface, including basic fluid mechanics to establish the mechanical relationship between the water and the facade material. The fourth step involves an experiment with water flows to reveal their characteristics based on the literature. The fifth step is to develop an algorithm which visually simulates water flows on a building facade. As a result, "Rainflow01" and "Rainflow02" are developed based on the open-source Grasshopper component "Drainage Polysurface" and Rhino. Rainflow01 and Rainflow02, which work based on the angle variation and critical sliding angle of the water path, present an innovative approach for predictions of water paths over a facade.