Why are so many chimneys built with the flue(so) that extend above the
To improve the chimney draw? I would think the small increase in height
would make little or no difference
To allow the chimney to meet code? I would think the chimney could just
have been built at the correct height in the first place.
AFAIK, the code requirement in most areas is that the flue must extend
at least 2" above the crown. YMMV
Picture wind gliding across a sloped crown with no flue extension. You
can almost imagine the wind sort of sealing the flue opening.
Now picture wind gliding across a sloped crown and hitting the wall of a
2" extension. The wind is forced upward instead of flowing across the
I cannot find anything on the web that indicates that the flue has to extend
2" beyond the crown.
The chimney has to be 3' beyond the portion of the roof where it emerges.
The chimney also has to be 2' higher than anything that is within 10'.
I found lot's of references to the flue being 2" above the crown but
none of them cite specific code references, other than "Check your local
code". This site says it the best, but I've included a few other
references for your reading pleasure.
Make sure the flue liner extends at least 2 inches beyond the top of the
chimney, but check local building codes for the requirements in your
area. This extension permits attachment of a rain hood and keeps ice and
snow that accumulates on the crown from dropping into the flue. To avoid
excessive cooling and condensation, dont project the flue liner more
than 6 inches above the crown.
Most flue liners extend above the chimney crown at least two inches. In
some chimneys the liner may extend higher than this depending on local
The crown should slope away from the flue liner, the flue liner should
protrude at least 4" from the crown and the joint between the flue liner
and the cap should be sealed with a flexible joint sealer.
The building code for my state does not specify that it must extend beyond
the crown, unless the crown is considered an "enclosing wall". Even then
there is no dimension is specified. Diagrams in the code do no specify it
Flue lining (installation): Flue liners shall extend from a point not less
than eight inches (203 mm) below the lowest inlet or as otherwise required
by 248 CMR or 527 CMR if applicable, or, in the case of fireplaces, from the
top of the smoke chamber, to a point above the enclosing walls.
In our area it must extend 4inches or more 6 is the norm
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