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How to Prevent Roof Leaks around Chimneys and Plumbing Vents

Updated on December 23, 2012
Grouping roof penetrations leaves inadequate room for proper flashing.
Grouping roof penetrations leaves inadequate room for proper flashing. | Source
Plumbing stack flashing issues.
Plumbing stack flashing issues. | Source
Chimney step flashing diagram
Chimney step flashing diagram | Source

When performing improvements to a building, it is very important to make sure that if you are going to be adding plumbing or a heat source, that you plan for a proper exhaust system. In many cases, the exhaust goes through the roof. However, when retrofitting an exhaust port or plumbing stack, you need to be conscience of where other penetrations are located. Flashing of these penetrations is extremely important to prevent water and air infiltration. Grouping multiple exhaust ports into one area can have a detrimental affect on the integrity of the roof. The picture to the right shows proper and improper plumbing stack flashing. Many times after years of movement, some of these ailments can rear their ugly head. The closer that these penetrations are to each other, the weaker the structure around them is and the more movement that is likely to occur. While there is no real code to regulate this, it is appropriate to keep all roof penetrations a minimum of 36 inches from one another to allow for proper flashing.

Plumbing vent flashings expand a great deal because they are made from plastic and rubber. Seasonal changes can expose small gaps in the flashing and overtime, collapse the to of the flashing boot that creates the protective seal around the plumbing vent. If the vent has collapsed, water sits in on top of the flashing and will work its way down the pipe and into the house.

Chimney flashing is a combination of step flashing and cap flashing. The flashings overlap each other on the roof surface to prevent water from backing up under the flashing. The cap flashing is embedded into the mortar of the chimney or is pushed into a cut line in the masonry. Typical maintenance of the flashings involves using a silicone sealant on the cap flashing every 10 years as the cap flashing will loosen from the mortar during expansion and contraction.


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