As an ex Chicago Licensed Mason Contractor I can offer this.
It should protrude above the cap approx 4-6".
It looks better.
In all the years, I never saw a reason, just that's the way it was always
drawn on prints.
I really don't think there is a reason.
In fact I'm sure you have seen those terra cotta type flues with the fancy
shape and decorations on them that are used at the very top above the cap.
Some of them are 2 feet tall.
I think it's just an architectural detail.
If I remember right, the downwind side of an obstuction (roof) to a moving
fluid(wind) can generate enough pressure to reverse the air flow thru a
chimney and force the fumes back into the house (something to do with
rotating vortices). you usually want the chimmney above the roof line to
prevent that. There is a formula that says how high the chimney needs to be
based on distance from the roof line. Our new (back in the late
70's)subdivision had a familly killed from CO poisoning because of that. The
building inspectors never caught it. The builder than had to come around and
add 3 ft to everyones chimney.
I noticed that too. While we're answering the wrong question, I
thought it was 3' within a 10' circle. It's been while since I
did one though.
I don't think that the flue has to extend much more than a couple
of inches above the bricks. The couple of inches is to allow for
a cement cap that causes the water to drain off, not in, the flue.
If someone is saying that the flue needs to extend significantly
above the chimney by more than that, I'd be suspicious.
Thank you for all your replies.
We had to repair our chimney because the bricks were breaking apart in the
winter time when the water seeped in and froze.
We took all the cracked and damaged bricks of and rebuilt the chimney, we
ended up having to buy some extra bricks and decided to use then up and
added two more rows of bricks to the original height of the chimney but then
the terra cotta flue lining no longer sticks more the an inch or two above
the cap. We also made the cap thicker then before , hoping we do not have
the same problem as before with the water getting in and freezing.
Of course when we were done and really looked around we see that all
chimneys have the flue sticking out and we started to wonder if it was
necessary and why?
Thank you for all your advice, Joanne in Ontario
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