A. Myron Ferguson, a drywall contractor in Galway, N.Y., responds: The best solution I've found for that sort of problem is a product called Trim-Tex's Magic Corner, which consists of two flanges joined by a rubbery, very flexible center section (trim-tex.com). I often use it for off-angle corners like those on Cape-style ceilings, and it should also work in the situation you describe (as long as the movement is not too extreme). The product attaches with a combination of spray adhesive and divergent-leg staples. The damage you see now may have been caused by lumber shrinkage as well as settling of the chimney. Given the age of the house, there's a good chance that future seasonal movement will be less pronounced.
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