Cracking the Code: How Flat Is Flat?
A: The code does have a tolerance for the brown (basecoat) coat of Portland cement plaster (stucco). The International Building Code section 2512.5 has an allowable tolerance for basecoat to no variation greater than 1/4-inch (6.4mm) in any direction under a 5-foot (1524mm) straight edge. This is a far cry from 1/8-inch in 10 feet, which seems to appear in many project specifications as the expected tolerance. While this tolerance is possible, it is unrealistic to expect this perfection. It must be remembered that plastering is a handcrafted application done onsite and not a factory or machined finish created inside an enclosed building under perfect environmental conditions. Some imperfections and allowable tolerances must be allowed.
What Should We Expect?Judging the acceptable level of tolerance for out-of-plane stucco work is one of the most difficult tasks I have had to perform. There are so many factors that come into play making the decision or judgment of acceptability. While most industry experts have settled on 1/4-inch in 10 feet as a tight, achievable and reasonable tolerance for stucco basecoat work, one should look at the bigger picture. The term I have used for years was one should not be able to see eye-catching discrepancies in the stucco work.
The visual importance of a finished stucco project cannot be overlooked and should not be subject to a set of simple mathematical numbers. Plastering is part art, craft, science and certainly reliant on good planning. For example, architects that desire a “smooth finish” should be a warning sign to any contractor that trouble may be on the horizon. They are most likely anticipating a very flat wall with no imperfections. This becomes very apparent when the suggestion by the plasterer to add some sand to the finish for a little texture is flatly rejected.
LET THERE BE LIGHTEven the finest plaster work will look non-compliant with a true and flat plane when subjected to harsh side lighting. This fact has led to another favorite phrase, “Judging should not be limited to short periods of time as the light casting across the surface can make surfaces well within the most stringent tolerance appear unacceptable.”
Good planning by the plastering contractor should also take into account any sconce or up lighting that will artificially create the same harsh conditions as natural critical side lighting. W&C
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