The most common symptoms of a failed chimney cap-spalling, cracking, and efflorescence-sometimes go unnoticed until the damage is extensive. THE RIGHT MATERIALS Of the errors made in cap construction, chimney repair people rank use of improper materials among the worst. According to the Brick Institute of America (BIA), chimney caps should be precast concrete, either factory-made or cast onsite on the ground. THE RIGHT DETAILING Regardless of materials used, caps should be seamless and finished so water will not collect on the top surface. The cap should be at least 4 inches thick to avoid premature cracking. BIA recommends that caps extend at least 2 1/2 inches beyond the chimney on all sides, and slope away from the flue on all sides. THE RIGHT WORKMANSHIP The quality of a chimney cap repair depends on how carefully you handle the job. Work slowly and cautiously in removing the old materials. The actual repair often takes less time than the preparation, but the extra time spent may be cheaper than an attorney's fees.