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Water-Shedding Details, Part II by Gordon Tully One of the chief concerns in detailing is to keep water from penetrating the building through cracks and joints. There are many ways to keep water out of a joint. I will discuss some of the most obvious: overlapping, layering, sealing, overhanging, and weeping. Overlapping Overlapping relies on the creation of a dam high enough that water cannot be driven over it by air pressure. The principle is used in (for example): siding roof shingles or tiles most flashing louvers ridge vents window and door sills In principle, it is possible to calculate the height of a safe overlap for any application, but simple calculation is defeated by other complexities, such as: 1. The air pressure difference that a joint or surface