After years and years of exposure to the elements, the mortar and wash cap around your tile chimney top can start to crack and deteriorate. These types of defects should be noted by your home inspector at the time of your home inspection.
Cracks in your chimney cap should be caulked or sealed in order to prevent damage form freeze/that cycles during the harsh Rochester MN weather. When water is trapped in the masonry or mortar joints, freezing literally starts to break the chimney apart.
Small cracks can be sealed with masonry caulk, or silicone. The best choice would be a urethane or one part epoxy product.
After inspecting your chimney mortar and cap for cracks and deterioration, clean the cracks with a brush or air spray. Fill the cracks with caulk and/or sealant. Caulk should only be used if the cracks are relatively small. You should inspect your chimney cap at least once a year. If it is too high or dangerous for you to attempt yourself, contact a local Rochester MN chimney contractor to perform an inspection and repairs.
Larger cracks or major chimney repairs should be handled by professional masons or certified chimney sweeps. If the chimney cap is in real bad shape, the long term solution would be to have the cap replaced with poured concrete. The concrete mix used should be formulated to withstand the weather extremes and moisture. Caps should have adequate expansion joints between the concrete and the clay chimney flue tile. This expansion joint should be sealed after the chimney cap is cured. Pre cast chimney caps may also be available in various sizes from you local chimney contractor.