The purpose of a chimney chase cover is to prevent water from entering the chimney. A properly fabricated chimney chase cover should never pool water on the top of the pan. Pooling water on the top of the chase will cause inferior steel to rust more quickly. The cover should always shed water off the chimney. Water can sit on top of the chimney cover for days if the temperature is low and with no wind, which expedites the corrosion process.
This first example shows that the chimney cover is attached to the top of the chimney through the top of the chimney cover. You can see a clear line on the cover that is sending water directly to the nail head. When you drive a nail down through the top of the cover, that becomes the low spot on the cover. We all know water will travel downhill and in the path of least resistance. What happens over time is that the top of the chimney expands and contracts with the heat cycles and use of the fireplace in winter. This will cause the nail to pop out, creating a hole in the top of the chimney. If your chimney cover has nails going through the top of the cover, it should be replaced immediately. Fasteners should always go through the side skirt of the chimney cover.
This next example follows the same lines as the previous picture. However, this installer took it to the next level. This installer must have thought the chimney cover was going to blow away. Like we have previously stated, "Nails should not be used through the top of the cover." This amount of nails is not necessary. Even if the nails were driven through the side skirt, this amount of fasteners is way too many.
This next example has no nails going through the top of the chimney. This cover also has no support under the cover. When a chase cover is installed correctly, there is a piece of plywood under the cover with a hole cut in the wood large enough to maintain the proper clearance from the flue pipe. It rained 5 days before this picture was taken. That is how recessed the center of the cover was.
This next example is exactly how a chimney chase cover should not be installed. If you see anything used as a chase cover other than steel, replace it right away. This caused significant water damage to this customer's home. This home was relatively new, only about 10 years old. They had the leak start coming in 4 years ago and one day the bubble popped and they had a significant amount of water entering their home. This so-called chase cover was some type of roofing material, such as a big sheet of tar with asphalt sprinkled on it. The tar started to sag around the pipe and caused a steady drip going into the home. If you have any material on the top of your chimney chase other than steel, replace it immediately.
To cap this segment, if you see water pooling on the chimney cover, the general rule of thumb is to replace it right away.
Article Series: Why Is My Chimney Leaking?
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