What Causes a Chimney Leak?

Standard gas fireplaces require a chimney or flue to address health and safety concerns.
An untreated leak might result in mold growth.
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  • Written By: S. Gonzales
  • Edited By: Bronwyn Harris
  • Last Modified Date: 21 November 2014
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One of the main causes of a chimney leak is deflective flashing. Problems with flashing can be due to wear and tear, corrosion, or improper installation. Flashing problems are usually verified by using a low pressure hose to spray water from around the chimney to the roof intersection. The presence of water indicates that repairs will be needed.

Ensuring that flashing is constructed of good material that is resistant to leakage is a way to protect leaks from occurring. Lead is the best material for flashing, but lead-coated copper and plain copper can help to deter leaks as well. Though inexpensive, aluminum has a high-corrosion rate and shouldn't be used for flashing. In addition, flashing should be properly installed in "step" fashion using 1 inch (2.54 cm) slots. Home owners should avoid the trend of gluing flashing on the brick because of this method's propensity for encouraging leaks.


Another problem that may lead to a chimney leak involves the top of the chimney itself. If the top of a chimney is cracked or loose, the abnormality may result in leaks, and gallons of water may accumulate inside of the chimney. During installation of the chimney, this can be avoided by requiring that bricklayers completely fill a chimney's interior with brick and mortar to avoid the formation cracks and crevices. When constructing the top 5 feet (1.52 m) of a chimney, brick masons can also be instructed to use nonporous hydraulic cement for mortar. If a home owner is experiencing a chimney leak with a chimney that has already been installed, he may consider installing a lead-coated copper sheet cover over the chimney to prevent water from falling into the chimney.

Crickets, or small, roof-like apparatuses, can also be installed into the chimney. Crickets divert the flow of water so that the chimney's structure and its flashing are not negatively impacted by the water's presence. While crickets can be installed at any time, home owners may be able to save money by having them installed while the chimney is being constructed.

A chimney that has wood or cedar siding can contribute to a chimney leak. Knots in these types of wood can become loose or will simply dislodge, opening up an area where water can cause a leak. If these materials split, a chimney leak can present itself and it may only be discovered after concentrated inspection of the underside of the chimney's siding.


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