Chimneys can leak for a variety of reasons, not all because of deterioration in the condition of the chimney.
Masonry chimneys are essentially porous, so given enough rain (like recently in Portland), any chimney can leak.
If a chimney is leaking we take a systematic approach. Any repairs should be addressed, cracked or worn mortar joints, loose bricks, etc., should be repaired. Rain covers should be installed, keeping the water out of the inside of the chimney (the flue). A custom cap covers the whole top of the chimney, preventing seepage from the top down. Usually chimneys deteriorate from the top down, due to water penetration and subsequent freeze damage. When wet chimneys freeze, they crack, allowing more and more water to enter.
Water and freeze damage over time (or suddenly if there is a freeze after a period of heavy rain) is a primary reason we repair chimneys.
We like to waterproof each chimney we work on with a water repellent that lasts 10 years. If water can’t penetrate and freeze the chimney should be in pretty much the same condition in 10 years.
A water repellent is preferable to a compound that puts a “skin” (like paint) over the surface as it allows the chimney to breathe, not allowing moisture to be trapped where it can freeze.
Chimneys have to be pretty dry when sealed.
We also wrap chimneys with plastic or shrink wrap temporarily until repairs or waterproofing can be done.
In general, if a chimney is in good repair, flashings are installed correctly, is waterproofed and has rain caps, it will not leak even in the worst weather.
When wood is burned the by-products of combustion go up the chimney. These include smoke and water vapor, as well as unburned wood particles and other things. As these hit the relatively cooler chimney, condensation occurs, like steam on a mirror. This creates a residue which gets deposited on the inside of the chimney walls.
This is known as creosote and is flammable.
Cleaning the chimney removed the creosote build-up so it can’t start a chimney fire.
There are several factors which contribute more or less to creosote build-up. Burning wet wood will create more creosote than burning dry wood. Hard woods create less generally than soft woods, as they burn hotter and increase the temperature in the flue and reduce condensation.
A wood stove or fireplace insert (wood stove in a fireplace) is normally burned differently than an open fireplace. In a normal fireplace there isn’t usually a way to control the rate of burn, so the wood burns rapidly, heating the chimney.
A wood stove or insert is normally used for heat, and there is usually an effort to conserve fuel. Plus a metal box weighing several hundred pounds can get pretty hot with a fire inside, so people understandably “damp” them down, causing the temperature in the inside of the chimney (“flue”) to be lower than in an open fireplace. This increases condensation and creosote build-up.
People often ask how often a chimney should be cleaned. There is no easy answer, as it depends on how much it was used, what kind of wood, etc.
We recommend that chimneys be inspected once a year for safety and structural reasons. If one is used regularly it could need to be cleaned yearly. We have some customers who burn a stove more or less around the clock in winter and have us clean it twice during the season.
Hope this has been helpful.
Oregon Chimney Repair and Cleaning, Inc.