I'm in a similar situation here in Washington, DC.
The key is to get the best bang for the buck. With the antenna so low,
the trick is to squeeze the biggest antenna that will safely fit. You
will have to compromise in one area, be it height or size and
performance. Never compromise in both areas.
I have an 6 ft roof tower with a C3-SS yagi mounted on top. The C3-SS
yagi has the largest turning radius/best performance ratio I could find
in an affordable (well, it was then!) yagi. It's a very nice
combination and works very well! (I can send you a photo of the
installation if you like).
As others have said, chimney's are designed for vertical (top down)
loads, not horizontal (side-to-side) loads. A VHF vertical will likely
be okay, but I'd never put anything rotatable on one.
73 & GL!
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