Cracking and staining below a chimney cap
The red arrow shows where water is getting through the hole in the chimney cap. The hole was created by the fastener used to hold the cap down. This is a common problem that we have fixed on 100′s of home along the Wasatch Front. The green arrows show the staining and cracking that have been caused by water getting in through the hole in the chimney cap. The roof is also letting water behind the stucco system. This is evident by the staining and cracking that is shown by the blue arrows. Most contractors would just paint or install new stucco finish over the chimney, but without fixing the problem water will continue to get behind the stucco and rot out the 2x framing. In fact the additional layer of paint or stucco used to cover up the symptoms will accelerate the rotting process. Since another layer of paint or stucco will only keep the water trapped behind the stucco.
The yellow arrows show the common method used for chimney cap installation. A fastener (nail) is placed through the cap, then black caulking is applied over the fastener. This caulking fails and then allows water to get through the hole that the fastener created.
The orange arrow shows where black caulking is placed around the flu pipe that goes through the chimney cap. This black caulking will eventually fail and allow water to pass through the chimney cap and into the fireplace.
Properly installed chimney cap
This picture show a chimney cap that was properly building installed. The fasteners were placed through the side of the cap and collar was welded to the chimney cap. No sealant is relied upon to keep water from getting through the cap.