Dear Tim,
I found the article "Cracks in New Chimney" helpful and I'm hoping not to make any mistakes in the construction of my chimney. I plan to build a concrete block chimney for a wood stove to be used in the basement of my house. The concrete block I refer to are square with a hole in the middle for the flue. I would like to build it up the outside of the south facing wall of the house. The stove pipe will exit the house through the concrete block foundation above grade and enter the chimney.
I'm not sure, however, what is required of the foundation to support the structure. I know that it needs to be about 42" deep, but I don't know how wide, or if rebar is required. I'd also like to know how high above the ridge it should be with respect to the peak of the roof and if an 8" flue is the proper size if the stove pipe is to be 8". Thanks for your help.
Sincerely,
Steve
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Steve,
I am familiar with that block. But keep in mind that a chimney needs a clay flue liner and the liner fits within the block. Flue sizes are a function of the capacity and output of the fuel burning appliance. The wood stove company often tells you the minimum flue cross section for the appliance. Get that information before you start.
As far as a footer, you need a fairly substantial one. I always pour fireplace footers one foot thick with 5/8 inch rebar spaced at one foot centers both directions. This steel needs to be placed so 4 inches of concrete is below it and 7 - 8 inches of concrete is above it. This odd height places the steel at the proper position to resist cracks in the footer.
The size of the footer is also critical. Assuming you have great soil, I would make it so that it is 16 inches larger on all sides than the actual chimney. If the chimney block were to measure 28 inches square, then the footer would be 60 inches square.
Extend the chimney 3 feet above the ridge. Follow your local codes as well for other critical clearances. This is vitally important!
Tim Carter
www.askthebuilder.com
W3ATB