The basic principles of rain and water penetration control have been well understood for many years. The application of those principles in real buildings has been less than ideal far too often. Controlling exposure to rain and water is a four step process:

• Deflection - the use of components and features to limit exposure of assemblies to rain. This includes overhangs and flashings with drip edges.
• Drainage - the use of drainage surfaces within cavities to redirect any water that enters the enclosure back to the exterior (rainscreen). The cavity space also stops capillary transport from the cladding to moisture-sensitive materials.
• Drying - the use of features that speed the drying of wet materials.
• Durability - the use of moisture tolerant assemblies and materials.