The following resources are for chimney construction, design and maintenance. If you need to fill a large space around a chimney crown, try the following list of manufacturers who make cement stable silicone caulks. Also, ChimneySaver's CrownSeal, while not a project you can do at home, can help you with your chimney repair without having to start from scratch.
Chimneys. Every time my kids watch Mary Poppins, I think of chimneys. Imagine how many millions of chimneys there are in the USA: millions and millions. Heck, I've got two on my own house and I've seen houses with as many as four and five. Each chimney must be properly crowned with a gap between the flue liner and caulked with a cement stable sealant. You can also protect your chimney by installing a chimney cap to reduce weather damage.
Chimney failure can occur for various reasons. Small hairline cracks in chimneys need to be monitored. Concrete footers may not be adequate for the chimney. Chimney block and brick can also be defective. The chimney crown must be watertight. Chimney sweeps and structural engineers can inspect your chimney.
Chimney caps or crowns can stop damage from happening to your chimney. These chimney caps / crowns are simply roofs for your chimney. Water will be kept from entering the chimney and causing deterioration of the brick and masonry. Chimney flashing must also be installed to stop any water.
Hot water heaters and other fuel venting appliances must have vents sized according to code. Chimney venting may not be the right way to vent these appliances. Building inspectors can check your masonry chimney to make sure that none of the fuel exhaust will spill back into your home.