One of the most commonly used materials in residential and commercial construction for exterior wall and soffit surfaces is cement plaster, or stucco. In use since Roman times, this combination of portland cement, water, lime, and sand provides a permanent, extremely hard, economical and beautiful finish to buildings and their appurtenances. In this age of high technology, stucco is "low-tech;" but, it's still one of the best, most reliable, and most predictable construction systems available.

Stucco is composed of the same basic constituents as concrete. For that reason, the two materials could be considered as first cousins, if not siblings, albeit one very strong and developed, and the other less so. To understand more about the nature of stucco, one is well advised to learn about concrete. Stucco on metal lath is essentially a reinforced concrete slab, only thinner.
While stucco is itself cement plaster, the term is also used to refer to a "cladding system" which includes not only the cement plaster, but also its reinforcing metal lath, its water-resistive barrier, its flashings, and trim pieces. Therefore, a "stucco leak" could indicate a problem with any of these components of the system.
Stucco as a system, with its barriers, flashings, weeps, and other accessories, always serves four functions in buildings, and sometimes five:
1. It keeps out the weather.
2. It provides a finished appearance.
3. It provides a permanent cladding.
4. It protects other parts of the building from damage, mechanical damage as well as damage from exposure to the elements.
5. It also provides lateral strength for the structure, sometimes as part of the structural design analysis, and sometimes gratuitously.
For a building to be weatherproof when it is finished, it must first be weatherproof in its "blacked-in" state, which is the term given to that point in the construction of a building after the water-resistive barrier and flashings are in place, prior to the application of the stucco. Since the stucco can be expected to crack somewhat, and since there will be some openings in the stucco, the stucco itself cannot be relied upon soley to keep out the weather.
Ultimately, keeping out the weather is the function of the water-resistive barrier, the black building wrap or paper behind the stucco, properly integrated with its flashing. This barrier collects any moisture that gets through the stucco in a prolonged rain, or through cracks or openings, and directs this moisture to the exterior. This weather barrier behind the stucco, with no openings allowed in it, except those needed for attachment, is a requirement of the building code.
However, because of the small openings in the barrier created by fasteners, the face of the stucco cladding itself should be sealed to eliminate the ability of water to enter into the cladding assembly.
The stucco, in turn, protects the water-resistive barrier from damage due to weathering, ultra-violet radiation, or physical abuse. Thus these two components act as a system, mutually dependent. Together, when properly done, they can last a lifetime. 
stucco as a water resistive barrier

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