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.

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Starting Fireplace Fires

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