What is Cap Flashing?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 08 April 2020
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Cap flashing is a form of flashing which is designed to provide horizontal coverage with the goal of minimizing water intrusion. This type of flashing is available in a number of materials, with copper being very popular, and it is installed around things like windows, doors, and chimneys. It is a good idea to periodically inspect openings in a home to make sure that the flashing and other weatherproofing materials are still in place, preferably before signs of damage start to appear.

Flashing is a weatherproofing material which is impervious to water. It is used in any area where water might be able to slip under, around, or through the cladding or roofing on a structure, so that the water will be repelled before it can reach the underlying framing and other materials. This prevents mold, mildew, and rot, keeping a structure sound and pleasant to be in. Flashing is installed under the cladding, so it should be difficult to spot in a finished building.


In the case of cap flashing, the flashing is designed in an L shape. The flashing lies flat against the structure, with the corner of the L butting up against the side of the framing for an opening such as a chimney or window. When water hits the flashing, it is directed away from the inside of the structure, and the L shape ensures that water does not collect in the crack between the opening and the structure. Some cap flashing is sold already shaped, while in other cases, it must be bent into place.

Copper is a good choice because it naturally forms a patina. The patina acts as a protective layer to protect the copper from corrosion and other problems, ensuring that the cap flashing will remain sound. Since the only way to fully inspect flashing is to pull part of the cladding away, it's important to make sure that sound materials are used from the start, so that people will not have to wonder about whether or not water intrusion is going on where it can't be seen.

Some signs that cap flashing may need to be replaced include signs of water intrusion on the inside, such as mold, mildew, or bubbling paint around a window, along with cracks or buckles in the flashing which could allow water to seep past. It is important to be aware that cap flashing is only one among several layers of protective material, all of which must be installed to create a durable weatherproofing system.


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