Why 1-inch overlap works
The Construction Details sections of The NRCA Roofing Manual: Membrane Roof Systems—2007 and The NRCA Spray Polyurethane Foam-based Roofing Manual—2006 recommend edge metal—such as parapet caps (copings), fascias and gravel stops—overlap the joint at the top of a wall and wood nailer a minimum of 1 inch. Since the early 1970s, NRCA has recommended edge metal overlapping wood nailers and tops of walls. In 1995, NRCA began recommending a 1-inch minimum overlap. Additionally, NRCA does not recommend using sealant between edge metal's drip edge and a wall.
The 1-inch minimum overlap recommendation is based on how far water will move upward under pressure. One equation and one mathematical relationship can explain this relationship.
The equation P = 0.00256 x V2 depicts the relationship between pressure (P); fluid pressure (in this case, the constant 0.00256 is used for the fluid pressure of air); and velocity (V) of that fluid pressure (wind speed).
The mathematical relationship is 1 pound per square foot (lb/ft2) = 0.1922 inches of water. This mathematical relationship is based on pressure differential and the weight of water. Essentially, this relationship means 1 lb/ft2 of pressure will move water upward 0.1922 inches.
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