The Wisconsin Uniform Dwelling Code states that: "...Chimney caps shall have a minimum thickness of 2 inches, shall slope outwards away from the flue, and shall provide a one-inch overhang and drip edge on all sides....."
We pour our crowns approx. 4" thick and lace them with rebar (similar in thickness to what a typical driveway is). Keep in mind, all the crown at the top of your chimney needs to do is waterproof the top of the chimney and shed the water so that it does not immediately run down the brick/stone surface. Some of my competition likes to sell consumers on having a 5" thick crown. I think they do this solely to look like they are doing that much more for your money. Well unless they have to support a car on top of your chimney, there is no use for it. Furthermore, in my opinion, the 5" thick crown looks industrial. I hate the looks of a 5" thick crown. I too could pour you a crown that thick, but would not do so because it does not fit residential archiecture.
General advice. Contact licensed chimney expert for your particular situation.
For years, people throughout the greater Milwaukee and Waukesha areas have been turning to Smoke Stacks, Inc. for their chimney repair and relining needs. At Smoke Stacks, Inc. we believe that attention to detail, professional work, courteous service and honest prices are the qualities that make the difference.