ROANOKE TIMES
Copyright (c) 1997, Roanoke Times
DATE: Sunday, January 26, 1997 TAG: 9701280109
SECTION: HOMES PAGE: D-1 EDITION: METRO
COLUMN: hints for homeowners
SERIES: First of a two-part series
SOURCE: STEVE ELDER
Building codes require chimney flues to be lined with a material that will resist the high temperatures of a potential chimney fire. However, a good number of old unlined chimneys are still in use. Whether or not these chimneys are safe depends on the thickness of their walls and the condition of the mortar.
Unlined chimney flues are still permitted by some building codes if the walls of the chimney are constructed with at least 8 inches of solid masonry. This is the equivalent of a double row of brick; indeed, many chimneys were built that way from colonial times on into the 20th century. What gives rise to problems with old chimneys is the type of brick and mortar used in their construction.
Brick is one of the world's oldest construction materials: it has been in use for more than 9,000 years. In the early American colonies, brick kilns were established at several locations, although the use of brick was generally restricted to the finer residences. However, the quality of the brick varied widely, depending on the quality of local clays and irregularities in the firing process. The main result of the variations was that most early bricks were softer than their modern counterparts. A reasonable standardization in brick making was not achieved until the 1870s when the dry-clay machine-made process was introduced.
The mortar generally used in older construction was a lime-based mortar, which is noticeably softer than a cement-based mortar. Portland cement, patented in 1824, was produced in this country since 1871, but its initial use was restricted to below-grade foundations or commercial construction, due to cost, short supply and the fact that many bricklayers preferred the ``feel'' of lime-based mortar. It did not come into general use in residential construction until the 1920s.
In addition to the building materials, another factor to be considered regarding chimney construction is workmanship. The current vogue seems to be automatic complaining about government regulation and restrictive requirements. While there may be regulatory excesses, it must be remembered that the original intent was to control the ``free spirits'' in the construction industry and provide some common safety standards for the rather questionable practice of lighting fires on the inside of your home. A number of the old chimneys draw poorly due to poor design, or were constructed of single-width brickwork, or were just poorly constructed.
The short sense of the history lesson is that old chimneys (pre-1920) should be inspected for safety. Old lime-based mortar softens with age and is apt to be in poor condition. The brick also may have deteriorated through years of exposure to moisture and the acids in creosote and soot.
An old chimney in poor condition will not contain a chimney fire adequately and must be considered a fire hazard. This is also true of old stone chimneys with deteriorated mortar. If creosote stain marking is visible on the room side of an old stone chimney, it means that so much creosote has accumulated on the inside that it has penetrated the old porous mortar. The risk to the inhabitants of the home is not primarily from evaporation of creosote or coal tar into the air; it is first and foremost from fire.
In addition to materials failure and suspect workmanship, the other safety problem with old unlined flues is the fact that they cannot be cleaned adequately. The rough and irregular surfaces of an unlined chimney easily trap soot and creosote, neither of which can be adequately removed by standard cleaning methods without causing some damage to the chimney. Condensed creosote and coal tars are flammable: if they are permitted to accumulate on the inside of a chimney flue and ignite from a spark, that chimney can turn into a Roman candle. If the chimney cannot safely contain a chimney fire, the consequences can be dire.
The popularity of the air-tight wood stove in the last decade has added another risk ingredient. When a wood stove is burning in the damped-down long-burn mode, the wood burns slowly - it actually ``cooks'' as well as burning. The smoke is lower in temperature and contains a higher percentage of organic vapors.
Because the smoke starts out cooler, it condenses on the inside of a flue much more rapidly. Just one season of regular wood stove operation can produce a significant buildup on a flue, for which reason wood stove flues should be cleaned annually without fail. If a wood stove is connected to an old unlined and uncleanable flue, most fire professionals consider it a recipe for disaster.
I normally recommend that old unlined flues be carefully inspected by a reputable chimney professional, preferably using a remote camera (one popular type is the Chim-Scan system). The usual advice is to have the chimney lined. If there is a possibility that a wood stove will be used, I consider chimney lining a must remediation.
Next: Retrofitted chimney linings
LENGTH: Medium: 88 linesby CNB