When building exterior walls with stone, stucco, brick cedar or fiber cement siding, it is important to use an effective rain screen system that provides drainage and ventilation. Without this protection, moisture finds its way into the wall systems via cracked mortar joints, gaps or cracks in the surface of the material. Natural absorption of the edges of wood or fiber cement board should also be considered. Once moisture penetrates the outer wall surface it becomes trapped in the wall system. Without any way of escaping, the trapped moisture creates a perfect environment for toxic mold growth and possible structural failure.