Water and Your Masonry Chimney

The chimney is one of the most taken-for-granted parts of a home, yet it tends to receive neither the attention nor the concern usually accorded other household service systems. The fact that chimneys may do their job reasonably well, even when abused or neglected, contributes to this atmosphere of indifference. Chimneys are far from the passive black holes that most people assume them to be. They perform several vital functions, and their simple appearance belies their complex construction and performance requirements. A chimney, deteriorated by constant exposure to the weather can be a potential safety hazard. Weather-damaged lining systems, flue obstructions and loose masonry materials all present a threat to residents. Regular chimney maintenance is essential to prevent damage, deterioration and future high-cost chimney repairs.
Masonry Chimneys A masonry chimney is constructed of a variety of masonry and metal materials, including brick, mortar, concrete, concrete block, stone, flue tile, steel and cast iron. All masonry chimneys contain combinations of, or possibly all these materials, most of which are adversely affected by direct contact with water or water penetration.

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. Thus, 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 ceilings

Clogged clean out area

Deteriorated central heating system

Stained chimney exterior

Decayed exterior mortar

Cracked 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

Install a chimney cap

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 caps have long been recognized as an important chimney safety and damage prevention component. Underwriter's Laboratories (UL) specifies that any chimney lining system that is to be listed to their test standards must include a chimney cap.

Chimneys have one or more large openings (flues) at the top that can collect rain water and funnel it directly to the chimney interior. The most common flue size is 13 inches x 13 inches. An opening of this size 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 corrosive resistant material. Caps may be designed to cover a single flue, multiple flues, a larger 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.

Repair or replace a damaged chimney crown

The chimney crown (also referred to as the chimney wash) is the top element of a masonry chimney. It covers and seals the top of the chimney from the flue liner(s) to the chimney edge. Most masonry chimneys are built with an adequate crown constructed from common mortar mix, the same mixture used to lay the bricks of the chimney. This mortar is not designed for, and will not withstand years of weather abuse without cracking, chipping or deteriorating – situations that allow water to penetrate the chimney. In fact, most sand and mortar crowns crack almost immediately after installation because of shrinkage.

A proper chimney crown should be constructed of a portland cement-based mixture and cast or formed so it provides an overhang, or drip edge, projecting beyond all sides of the chimney by a minimum of two inches. The flue liner tile(s) should project above the crown a minimum of two inches. The crown should provide a downward slope that will direct the water from the flue to the edge of the crown. The over-hanging drip edge, by directing the run-off from the crown away from the sides of the chimney, helps prevent erosion of the brick and mortar in the chimney's vertical surfaces.

Repair deteriorated mortar joints

Deteriorated mortar joints are entry spots for water. Proper mortar joints have no gaps or missing mortar and are struck, or shaped, in a way that directs water out of the joint.

When mortar deteriorates from exposure to weather, it becomes much more absorbent. A common repair for improper or deteriorated mortar joints is called repointing or tuckpointing. In this process, the existing mortar joint is cut out to an appropriate depth and the joint is repacked with new mortar compound. The joint then is 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 rain water or snow melt from running down the chimney into living spaces where it can stain or damage ceilings and walls or wall paneling or cause rot in rafters, joists or other structural elements. In many cases, the flashing is a single L-shaped sheet of metal that is attached to the 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 own rates without breaking the waterproof seal in either area.

Install a cricket to stop or prevent leaks

If the chimney is located on the low side of the roof, where the flow of run-off is directed against the chimney, the installation of a cricket will afford additional protection against water leaking into the home. A cricket is a water deflector that serves to direct rainwater away from the chimney. Crickets are recommended on chimneys more than 30-inches wide and they are especially important on steep roofs.

Waterproof 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. Thus water that has penetrated the chimney is allowed to escape, while the waterproofing agent prevents water from entering from the outside. These products usually have a five-to-ten-year guarantee. Paint should never be used as a waterproofing agent because it will trap moisture inside the chimney.

Waterproofing is a preventive measure. When damage or deterioration (gaps, voids, cracks, missing mortar, etc.) already exist in a masonry structure, the chimney should be repaired before the waterproofing agent is applied. The chimney exterior may also need to be cleaned before the waterproofing material is applied.

A Word of Advice: Your chimney systems are an important part of your home heating system. For safe and efficient operation, your chimney should be checked annually by a CSIA Certified Chimney SweepŽ and cleaned as needed. Each year, unsafe chimneys cause significant numbers of injuries and deaths, and account for more than $200 million in property losses. Make chimney examinations a regular part of your home maintenance schedule. Don't become a statistic.