People sometimes call them chimney ‘caps’, which can be confusing even though, in a way, it is accurate. A chimney crown ‘caps’ the chase, which is what many people think of as the chimney. A real “chimney cap” tops the flue (which is inside the chase except at the very top), and it typically looks like a little hat.
It is not at all surprising that most homeowners are unfamiliar with the many different components of a chimney. We do not know anything about plumbing, and we cannot begin to name the components of a home’s water-carrying systems. “Pipes” are what we know about plumbing, and “chimneys” are what many homeowners know about smoke-carrying systems.
In the same way that drain covers prevent water from moving into the pipes, crowns keep it out of the ‘chimney’. A chimney crown seals the chase, keeping water from getting into the bricks that enclose your ‘smoke pipe’. Its design more closely resembles a cape than a cap, as it serves to slope water off and away from the chimney (all of it!).
Water is a chimney’s worst enemy, so preventing its entry through any opening is absolutely critical to the safe use of your fireplace. Since you cannot really slope bricks, and since they are porous, you have to top them with something else. Chimney crowns, often made from concrete or stainless steel, higher at center than edge and overhanging the chase, do this critical job.
Unfortunately, crowns are often installed that are not properly sloped and do not provide sufficient overhang. The vast majority of them lack beveled drip edges. Basically, there is room for a lot of improvement with most chimney crowns. Now that you are sure what they are and what they are supposed to do, do you need a plumber or a chimney guy to prevent leaks?