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On a recent remodeling job, the asphalt roof shingles had just been replaced, and the roofers had conscientiously replaced the old chimney's lead step flashing, turning and caulking it into the face of the brickwork. However, due to the roughly 40-year age of the chimney, freeze-and-thaw cycling had taken its toll on the bricks, mortar, and concrete cap. The chimney still leaked, and a rebuild was called for, from the roofline up. Building a baseline. I began by covering the roof below the work area with a tarp to protect the shingles, and set up staging just below the chimney, which measured 28 inches across by 24 inches on the sloping side. I tore the old masonry down to just above the roofline, leaving the step flashing and the bricks that it covered in place (see Figure 1). The existing ceramic flue was a large, discontinued size but still in good condition, so I set it aside for reuse. Using a 7 1/4-inch-diameter diamond-abrasive circular saw blade and the plane of the roof as a guide, I cut through the lead and the brick, making repeated passes to cut as deeply as the blade allowed on the two sloping sides of the chimney. The offset of the saw's shoe kept the cutline about 2 inches above the actual roofline. The blade didn't cut all the way through the brick, but did go deep enough so that I could finish the cut with a chisel. Chiseling fractured a few of the bricks, so I replaced them to create an even, sloped plane.
Figure 1. Newly replaced stepped flashing won't stop the leaks in a failed chimney but will remain in place under the pan as a secondary barrier, to prevent wind-driven water penetration.