Fire Safety Strategies
As They Relate to American Chimneys Historically
2/18/12
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Carl Lowndsbury, Architectural Historian at Williamsburg, VA, reported in 1989 that they made an interesting discovery. The library at Historical Williamsburg had an extensive collection of photographs taken of log houses. The photographs were mostly taken in the 1920's to record these structures that were rapidly disappearing.
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The mud and wattle chimneys serving these old log houses - the ones that remained - were almost always leaning away from the house at a precarious angle. No big deal. Everyone just assumed the foundations were inadequate or that the mud had deteriorated near the ground causing the chimneys to tilt. Then they discovered a letter written by a young woman to her friend in Boston at Christmas time. "The chimney had caught |
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fire", she wrote, "and Father ran outside and kicked out the support so the chimney fell over and did not catch the house on fire." Of course! Now they understood why all those chimneys were leaning. They were built that way so they could be toppled over if they caught on fire. It was a fire protection strategy. The houses built in Williamsburg were more substantial than those pioneer log homes and they had brick chimneys rather than chimneys made of mud and wattle. The fire prevention strategies in Williamsburg, as we have already seen, included separating the chimneys from the roof, and the placement of ladders on the roof so it would be easy to get up on the roof and put the fire out if the roof did catch on fire. |
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During the 19th century when chimneys were almost always within the exterior walls of the house a different fire prevention strategy emerged - chimney mass and wall thickness. Chimney walls were made thicker, usually 8" thick instead of 4" thick, as they passed through the roof and floors. Sometimes these chimneys were called "barrel chimneys" because they featured a base (where they were thickened to pass safely though the roof) and narrowed to a shaft or barrel, corbeling out again for a drip edge at the top. Other 19th century chimneys were deemed safe because they were massive chimneys with several flues or were part of massive masonry walls. This strategy worked because these 19th century chimneys were exposed to the outdoors or plastered on the inside but not enclosed within combustible framing so that the heat from the flues would be absorbed in the mass and dissipate to the air around the chimney and would never get dangerously hot. Added mass, wall thickness and heat dissipation worked. |
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In modern times after WWII, when there was a great house building boom, the fire prevention strategies changed again. At this time most houses were framed with wood. Even if some houses were veneered with brick or stone, they were not generally solid masonry houses. Insulation added to the problem by trapping heat, especially if the chimney were enclosed within a framed and insulated chase. Masonry has a lot of thermal mass. Its takes a long time for it to heat up. But, as insulation, masonry is poor. Heated on the inside, eventually the outside of the chimney will get hot. |
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Codes have gradually emerged in the last fifty years that require fireplaces and chimneys to be separated from combustible materials by an air space. Today we can see that the result of these code changes, which worked pretty well for conventionally framed houses, can lead to confusion, misunderstanding, compromise and sometimes fire hazards when applied to older houses or other construction methods. Is it all combustible material that must be kept away from a chimney or just the framing? Is it okay for wood siding and interior trim like baseboards to touch the chimney? If not, how do you close off the air space? Not too many fireplaces are trimmed out with metal flashing. Can you insulate that air space? Sometimes, as the builder of this concrete masonry house found, it's just easier to install a listed manufactured fireplace and wooden chimney than to interpret a code that wasn't written with masonry walls in mind. |
 Masonry house with wooden chimney |
In an attempt to resolve some of the problems, the International Residential Code was recently amended:
1. Masonry fireplaces listed and labeled for use in contact with combustibles in accordance with UL 127, and installed in accordance with the manufacturer's installation instructions, are permitted to have combustible material in contact with their exterior surfaces. 2. Combustible materials, including framing, wood siding, flooring and trim, shall be permitted to abut the sides and hearth extensions, but not the backs, of masonry fireplaces, in accordance with FIGURE R1003.12, provided such combustible materials are a minimum of 12 inches (306 mm) from the inside surface of the nearest firebox lining.
3. Exposed combustible mantels or trim may be placed directly on the masonry fireplace front surrounding the fireplace opening provided such combustible materials shall not be placed within 6 inches (153 mm) of a fireplace opening. Combustible material within 12 inches (305 mm) of the fireplace opening shall not project more than 1/8 inch (3.2 mm) for each 1-inch (25 mm) distance from such opening.
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