Effects of roof overhangs on wind-driven rain wetting of a low-rise cubic building: A numerical study

Wind-Driven Rain (WDR) is known to be a major source of moisture loads on building envelopes and is responsible for numerous cases of building envelope failure. Roof overhangs are traditionally used and have been shown to be effective in reducing the exposure of buildings to WDR in certain climates. However, their effects on WDR wetting of facades are not fully understood nor quantified. In this work, CFD-based numerical simulations are employed to investigate the effects of overhangs of various sizes on the wind-driven rain wetting of a low-rise building under various wind and rain conditions. The numerical simulations are validated against experimental and numerical data in the literature. The results provide insights on the protecting mechanisms of overhangs. It has been shown that the introduction of an overhang can significantly change both the amount and the pattern of wind-driven rain wetting of the facade, especially in the upper region. The performance of the overhang is highly dependent upon its size, wind speed and wind angle, while the influence of rainfall intensity is small. A new global measure is introduced to quantify the effectiveness of overhangs in protecting facades from wind-driven rain. This index is shown to give a meaningful measure of the effects of the overhang.