The peak acts like a wing if the wind blows in a certain direction and causes a downdraft into the chimney, something you don't want especially if you are burning wood.
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.
ftp://imgs.ebuild.com/woc/M930413.pdf
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, don?t project the flue liner more than 6 inches above the crown.
formatting link
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 building codes.
formatting link
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 either.
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
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.