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 chiminey 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.
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.
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. 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 fireplae 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 liner systems
- Collapsed hearth support
- Tilted or collapsed chimney structure
- Chimney settlement
Preventing Water Damage
Following are the main ways to prevent 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 damage prevention component. Underwriter's 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 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 benefites. A strong, well designed cap not only keeps this water out, but will also 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 the 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 inadequate 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. This drip edge directs the runoff from the crown away from the sides of the chimney, helping 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 rejointing 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 damage ceilings or walls, 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 - and L-shaped element extending up the chimney side and out onto the roof - is atached to the roof and sealed. The counter flashing, which overlaps the base flashing, is imbedded and sealed in the chimneys 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 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, or moisture that has originated from inside, is allowed to escape, while the waterproofing agent prevents water from entering from the outside. Paint, or any non vapor permeable water sealer, should never be used as a waterproofing agent because it will trap moisture inside the chimney, accelerating deterioration.
Waterproofing is a preventive measure. When damage or deterioration (gaps, voids, cracks, missing mortar, etc.) already exists 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 waterproofing material is applied.
Water damage to masonry chimneys is usually a slow, subtle process. The problem is often not evident until it has become quite serious.
Although these water prevention measures may cost a few dollars initially, they will save you the major expense of large masonry repairs or rebuilding of the entire chimney in the not too distant future, and as such represent a wise investment in your home.