1. Crowning achievement
The crown, or top, is one of its important elements of the chimney. Compare it to the roof of your home. Your roof is designed to keep the inside of your house dry. The crown of a chimney is supposed to keep the chimney dry. Water that seeps into a chimney can cause all sorts of damage, especially if the chimney is exposed to cold weather. The water that soaks into the masonry freezes. Water expands as it freezes, and this pressure works to break the bonds of all the mortar used to construct the chimney.
2. Building it right
Many bricklayers build chimney crowns the wrong way. Some chimney tops are just made from the same mortar used to lay the brick or stone. There may be no overhang, no reinforcing steel in the top and no flashing under the crown. It's very rare to come across a chimney crown built the right way.
3. The overhang
For starters, a chimney crown needs an overhang. Most roofs on houses overhang the outside wall. That's to minimize water flowing down the sides of the house. You don't want water flowing off the top of the chimney down its sides. Granted, chimneys, by their very nature, are much more exposed to windblown rain than a house is. But you might as well minimize that exposure as much as you can.
4. Concrete answer
The chimney crown should be made using real concrete, not the mortar mix used for the brick or stone. The mortar mix rarely has any stones in it. The stones in concrete are what give it strength.
5. Protecting the flue liners
If the chimney has clay flue liners, the crown or top concrete material should never touch them. The flue liners get hot from wood fires, furnaces, boilers, water heaters and so forth below, and they expand. This expansion can crack the top or crown. The contractor needs to wrap the flue liners in thin foam about 3/8 inch thick to isolate the flue liners from the concrete mix. Once the crown is hard, they cut away some of the foam and caulk the gap with special masonry caulk.
6. Sloping the crown to shed water
Reinforcing steel or mesh needs to be placed in the concrete mix to help prevent large cracks from forming. The top of the crown needs to be sloped so it sheds water. A groove needs to be made in the underside of the crown overhang all the way around the crown. This groove is a drip edge that stops water from rolling down the sides of the chimney. It should be at least 1/4 inch wide and 3/8 inch deep.
7. Flashing: the last line of defense
Last but not least, a flashing should be placed on top of all the brick masonry that separates the chimney top or crown from the masonry used to build the chimney. This flashing is the last line of defense should water make it through the chimney crown. The flashing can be made from asphaltic-rubberized membranes or metal that is soldered to make one solid piece. The flashing should extend past the face of the chimney on all four sides and be turned down at a 45-degree angle so water that runs off drips out past the face of the chimney.