Flue Caps – Water Prevention

Water Penetration
All masonry chimney construction materials, except stone, will suffer accelerated deterioration as a result of prolonged contact with water. Masonry materials deteriorate quickly when exposed to the freeze/thaw process, in which moisture that has penetrated the materials periodically freezes and expands causing undue stress. Water in the chimney also causes rust in steel and cast iron, weakening or destroying the metal parts.

Note: While most stone is not affected by water penetration, large amounts of mortar are required to bond the stone together properly. Therefore, a stone chimney – just like a brick chimney – should be protected from the effects of water penetration.

Water penetration can cause interior and exterior damage to your home and masonry chimney including:

  • Rusted damper assemblies
  • Deteriorated metal or masonry firebox assemblies
  • Rusted fireplace accessories and glass doors
  • Rotting adjacent wood and ruined wall coverings
  • Water stained walls and ceiling
  • Clogged clean out area
  • Deteriorated central heating system
  • Stained chimney exterior
  • Decayed exterior mortar
  • Cracked or deteriorated flue lining system
  • Collapsed hearth support
  • Tilted or collapsed chimney structure
  • Chimney settlement

In addition, when water mixes with creosote in a wood burning chimney system, it will generate a highly disagreeable odor that can permeate a home. 

Preventing Water Damage
Chimney caps, also called rain covers, are probably the most inexpensive preventive measure that a homeowner can employ to prevent water penetration and damage to the chimney. Chimney caps have long been recognized as an important chimney safety and damage prevention component. Underwriters Laboratories (UL) specifies that any chimney lining system that is to be listed to their test standard must include a chimney cap. 

Chimneys have one or more large openings (flues) at the top that can collect rainwater and funnel it directly to the chimney interior. A commonly-sized flue has the potential to allow large amounts of rain or snow into the chimney during just one winter when freeze/thaw cycles are common.

Chimney caps also provide other benefits. A strong, well-designed cap will prevent birds and animals from entering and nesting in the chimney. Caps also function as spark arrestors, preventing sparks from landing on the roof or other nearby combustible material. 
A chimney cap should be easily removable to facilitate inspection and cleaning. For a long and effective service lifetime, a cap should be constructed of sturdy, durable and corrosion resistant material. Caps may be designed to cover a single flue, multiple flues, a large portion of the chimney or the entire chimney top. A full coverage chimney cap usually represents a larger initial investment. However, it is probably the best investment for long-term protection because of its ability to protect the entire chimney crown.

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