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Helping You Make Your Home Your Castle |
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Chimney Design and Construction
How Chimneys are Built
Overview: An improperly
constructed chimney can be a fire hazard if not
properly designed. The Chimney design and
construction for fireplaces and wood stoves must
meet building and fire construction code
requirements. If the chimney is constructed
properly and is cleaned periodically there is
little danger of a fire hazard.
In the olden days of the seventeenth century
chimneys came in to being as opposed to holes in
the roof to let out smoke from domestic fire pits.
These chimneys were massive with thick masonry
walls that were effective in preventing chimney
fires. They were called barrel chimneys and had
several flues and were integrated into the masonry
wall structure to provide fire protection. Since
the framing was not combustible the danger of fire
was reduced. Most homes today are wood frame
construction where a barrel chimney design does not
provide fire protection and is prohibited by
building code.
In the period of log cabin construction, chimneys
were designed and built at a slight angle to the
building and constructed on wooden supports. The
chimneys were constructed of mud and prone to
catching fire from the inside. When a chimney fire
occurred the homeowner would simply knock out the
supports for the chimney allowing it to fall down
and burn safely alongside the cabin rather than
inside the cabin.
In modern building construction of wood-framed
houses the building codes are more restricted to
prevent fire hazards from chimney fire. The code
requires a two inch air space around the masonry
chimney structure. Wood framing is not allowed to
be closer than three inches from the chimney and
insulation should not be placed in the space. The
chimney is supported within the framing of the
house by non-combustible materials such as metal or
masonry. The space between floors is filled with
cement (fire blocked) which prevents a draft effect
between floors in the two inch air cavity and
protects the structure form overheating.
To avoid fire hazards all combustible materials
should have a clearance of at least 2 inches from
the front faces and sides of a masonry woodstove or
fireplace chimney. For fireplace construction the
clearance between the fireplace structure and any
combustible material should be at least 4 inches
from the back and 6 inches from the fireplace
opening. The air space around the fireplace
structure should not be filled with anything except
for fire blocking.
The fire code requirements do not allow many
exceptions to these safety requirements however;
your local building codes may be more or less
constringent.
Combustible materials such as wood framing, trim,
and flooring may be placed around the sides and
hearth extensions but not the back of masonry
fireplaces. These types of materials must be at
least 12 inches from the inside surface of the
firebox.
Proper
chimney design and construction is critical to
avoid fire hazards for fireplace and woodstove
users. If you are a DIY home owner and intend to
build your own chimney, ensure that you check with
your local code officer before beginning the
project. In addition, chimney cleaning must be
accomplished between burning seasons to prevent
chimney fires.
Fireplace and Woodstove Safety
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