Chimneys degrade from the top down, as the crown at the top of the chimney is gradually eroded by the weather. As the crown is subjected to rain, sun, and the freeze/thaw process, it gradually begins to erode, forming cracks on the surface. This is particularly true for us here in central Virginia; with warm days followed by frequent cold snaps (as we had this past winter), water gets into even small gaps in concrete or masonry, and expands as it freezes, damaging the material (you may have noticed the same problem affecting the roads).
Once the cracks appear, the process accelerates. Water gets into these cracks and eventually works its way down to the layer of bricks. Sometimes it can find its way into your home. After enough degradation, bricks can become loose and even fall off of the chimney completely, putting nearby people and animals at risk of being struck by falling masonry. It’s less costly to repair a wash before the process has gone too far than to restore a chimney that has experienced significant degradation. To protect your chimney, there are a number of possibilities, depending on budget and the level of protection you desire. A note about terms: a “wash” (sometimes called a crown) is a layer of concrete or mortar, applied over the top of the chimney, which slopes down to direct water away from your flue and onto your rooftop.
One possibility is a crown sealant, which is a plasticized coating applied to the crown to keep water from soaking into the masonry. It lasts approximately 10 years, and then needs to be reapplied. Depending on the size of your chimney, crown sealant usually costs no more than a few hundred dollars and protects your chimney crown for 10 years. The sealant cannot be applied if the crown is badly damaged. In that case, the crown will need to be restored, usually by removing the old cement and making a new crown. However, the crown sealant only protects the top of your chimney. If you do not have any sort of cap on your flue, your flue will be subjected to both the weather and critters. Critters can be an annoyance, but if your flue tiles degrade significantly, it can lead to the need for re-lining the chimney.
A single flue cap fits over the top of your flue to keep out snow, rain, and animals (birds’ nests are hard to remove without destroying the nest, and raccoons are hard to remove without damage to everybody involved). A stainless steel flue cap is the least expensive option for protecting your flues, generally less than $200 for a stainless steel cap with a lifetime warranty.
The best option, both for protecting your crown and your flue/s, is a full-coverage, multi-flue chimney cap, which extends past the edges of your chimney crown, sheltering your chimney top from the elements and reducing your long-term maintenance costs considerably. These caps come with a lifetime warranty, in either stainless steel (can be powder coated in a variety of colors) or copper. Cheaper painted steel chimney caps are available but they do not last a lifetime. The cost of these caps depends on the size of your chimney but typically run between $400 and $600.