A.Steve Thomas responds: The key to leak-free details is to install "build-out boards" behind the trim. In our area, 1x6 cedar is commonly used for Tudor trim. We center the vertical and angled trim boards over a 1x4 build-out board (see illustration, below). The smaller width build-out board allows the two stucco base coats to be troweled in behind the 1x6. The finish stucco coat is run tight to the edge of the overlapping 1x6 trim board, which discourages water from working its way behind the base coats.
Unlike vertical trim, horizontal trim is held flush with the upper edges of the build-out boards, and a galvanized cap flashing is installed to prevent water infiltration.
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