Water & Your Masonry Chimney

There are approximately 40 million masonry chimneys in North America and the most common problem with these structures is water penetration. Water causes over 1 billion dollars in damage annually in the form staining, loss of insulation value, freeze thaw damage, deterioration, and ultimately structural damage. All masonry chimney construction materials 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 penetration can caue 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 
• Deteriorated central heating system
• Stained chimney exterior
• Decayed exterior mortar
• Cracked flue lining system
• Collapsed hearth support
• Tilted or collapsed chimney structure

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 are probably the most inexpensive preventive measure that a homeowner can employ to prevent water penetration and damage to the chimney. Chimneys have one or more large openings (flues) at the top that can collect rainwater and funnel it directly to the chimney interior.

Repair or replace a damaged chimney crown

The chimney crown is the top element of a masonry chimney. It covers and seals the top of the chimney from the flue liners to the chimney edge. Most masonry chimneys are built with an inadequate crown constructed from common mortar mix. This mortar is not designed for and will not withstand years of weather abuse without cracking, chipping ofr deteriorating – situations that allow water to penetrate the chimney. In fact most sand and mortar crowns crack almost immediately after installation because of shrinkage.

Repair deteriorated mortar joints:

Deteriorated mortar joints are entry spots for water. Proper mortar joints have no gaps or missing mortar and struck, or shaped, in a way that directs water out of joint. A common repair for improper or deteriorated mortar joints is called rejointing or tuckpointing. In this process, the existing morta joint is repacked with new mortar compound. The joint is then struck to form a concave surface that will direct water out of the joint. A good repointing job, using proper materials, will give the chimney a much longer life span, and often will enhance its appearance.

Repair or replace flashing

Flashing is the seal between the roofing material and the chimney. Flashing prevents rainwater or snow melt from running down the chimney into living spaces where it can stain or damage ceiling and walls or wall paneling or cause rot in rafters, joist or other structural elements. In many cases, the flashing is a singled L shaped sheet of metal that is attached to side of the chimney and the roof. The most effective flashing is made up of two elements, the flashing and the counter flashing. The flashing or base flashing – an L- shaped element extending up the chimney side and out onto the roof – is attached to the roof and sealed The counter flashing, which overlaps the base flashing, is imbedded and sealed in the chimney’s masonry joints. This two element flashing allows both the roof and the chimney to expand or contract at their rates without breaking the waterproof seal. 

Waterproofing your chimney

Most masonry materials are porous and will absorb large amounts of water. Common brick is like a sponge, absorbing water and wicking moisture to the chimney interior. Defective mortar joints or the use of improper mortar or brick can greatly increase the tendency to absorb and convey water to the interior of the masonry chimney structure.
Several products have been developed specifically for use as waterproofing agents on masonry chimneys. These formulas are vapor permeable, which means that they allow the chimney to breathe out, but not in. Therefore, water that has penetrated the chimney is is allowed to escape, while the waterproofing agent prevents water from entering from outside. These products usually have a 5 to 10 year guarantee. Paint should never be used as a waterproofing agent because it will trap moisture inside the chimney.

Chimney caps

Chimney caps help eliminate moisture from entering your chimney. Moisture entering your chimney erodes masonry and mortar joints and will cause damper and firebox rust- out that leads to expensive repairs. Chimney caps also prevent birds and animals from entering your chimney and nesting or entering your home. Caps protect against airborne embers and sparks thus reducing the risk of house fire, caused by airborne embers and sparks. Chimney caps also prevent leaves and debris from entering the chimney. Leaves and other debris can cause flue blockage, which in turn can lead to fire, smoke damage, and even carbon monoxide poisoning.