When doing Racine home inspections, one of the things I see far more often than I should is improper chimney flashing or sometimes no flashing at all! That may be alright for the kids tree fort but you want to keep water out of your house no matter what and a bead of roofing sealant just isn’t going to do that in the long run. Before I get any further I want to comment on shingle roofs. Shingle roofs are NOT a waterproof system, instead they are made to SHED water.
Like any other place the roof runs into a vertical wall, chimney flashing is made up of two basic elements, step flashing and counter flashing. See the illustration below. Step flashing is an “L” shaped piece of sheet metal. There is a step flashing for each shingle. First a step flashing gets laid down. Then a shingle gets laid on top of the step flashing. Another step flashing goes above the last shingle, followed by another shingle, etc. This allows any water that gets under a shingle from the edge to run down the top of the step flashing and out onto the top surface of the shingle below preventing any water from getting through the roof.
There is one spot that sealant is commonly used on the step flashing. When a piece of step flashing comes to the corner of the chimney it is bent around the corner. The creates a small hole at the corner that needs to be filled with sealant.
Step flashing alone would be helpful but any water that hits the vertical wall will run down the wall and get in back of the step flashing and probably under the shingles. For this reason counter flashing is used. See the picture below. Counter flashing has a lip bent on the top of it. A groove is cut into the chimney where you want the top of the counter flashing. The lip of the counter flashing is stuck in the groove and a sealant applied in the groove. As water sheds down the chimney or wall it flows down the counter flashing which directs the water over the step flashing and out onto the shingles.
Because chimney areas are so prone to leakage, I recommend they be inspected every year or two. Look for loose or missing flashing. While you are up there is a good time to also inspect the chimney brick and crown. A sealant designed for masonry can be used on small cracks in the brick and crown. If you see anything major it is always best to call in a quality roofer or mason.
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