Because they are trained professionals with years of experience, chimney sweeps can spot potential problems that might be missed by the untrained eye. This is especially likely in the dark shaft of the chimney where creosote, like toxic tar, build up on the bricks. Proper chimney cleaning requires a lot of special equipment, which the professionals bring along as a matter of course and may be made part of the chimney repair.
Chimney repair is especially tricky for Do-It-Yourselfers. A chimney venting problem is often the reason for a chimney repair. The mechanical control of the vent flap stops operating due to corrosion, and there is no way to vent smoke through the flue and out of the room. Replacing this part usually means removing part of the masonry so that it is accessible. Proper installment of new masonry after the part has been replaced is a job best left to professionals.
Problems involving the mortar in the chimney may require a repair. Mortar gradually shrinks, which results in gaps between the chimney cap and the flue. These have to be sealed with mortar caulk and kept damp for several days so that the mortar caulk can cure properly.
Professional chimney repair ensures that the job is properly done. This prevents more costly replacements and repairs down the line when a DIY corrective measure actually exacerbates the problem and causes other defects to occur. It also means that homeowners do not have to move around on their roofs, falls from which can be deadly. Furthermore, cracks and holes in chimney flues and masonry create safety hazards, best assessed by professionals.
Obviously, some chimney repair, such as extending the height of the chimney and relining chimney flues, simply exceeds the grasp of most amateurs. The consequences of doing them incorrectly are so costly and potentially so dangerous that they are not worth the risk. The best bet for any sort of major chimney repair is to call in a professional.